CA1128139A - Induction heating apparatus with negative feedback controlled pulse generation - Google Patents
Induction heating apparatus with negative feedback controlled pulse generationInfo
- Publication number
- CA1128139A CA1128139A CA330,657A CA330657A CA1128139A CA 1128139 A CA1128139 A CA 1128139A CA 330657 A CA330657 A CA 330657A CA 1128139 A CA1128139 A CA 1128139A
- Authority
- CA
- Canada
- Prior art keywords
- voltage
- transistor
- induction heating
- heating apparatus
- pulse
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/02—Induction heating
- H05B6/06—Control, e.g. of temperature, of power
- H05B6/062—Control, e.g. of temperature, of power for cooking plates or the like
Abstract
ABSTRACT OF THE DISCLOSURE
An induction heating apparatus includes a DC power source, a transistor for generating a trigger current in response to a base drive pulse applied thereto, and a resonant load circuit including a work coil and a capacitor for generating in response to the trigger current an oscillating resonating current when the switching device is turned off, and which current is passed through a diode connected in inverse parallel relation with the transistor. A current transformer is provided to detect the load current. A low voltage detector senses a nearly zero voltage at the collector of the transistor and triggers a ramp generator to generate a ramp voltage which is compared with a reference voltage to generate a train of base drive pulses for application to the transistor. To control the base drive pulses at a variable duration in accordance with an inductive load, the reference voltage is varied as an inverse function of the sensed load current.
An induction heating apparatus includes a DC power source, a transistor for generating a trigger current in response to a base drive pulse applied thereto, and a resonant load circuit including a work coil and a capacitor for generating in response to the trigger current an oscillating resonating current when the switching device is turned off, and which current is passed through a diode connected in inverse parallel relation with the transistor. A current transformer is provided to detect the load current. A low voltage detector senses a nearly zero voltage at the collector of the transistor and triggers a ramp generator to generate a ramp voltage which is compared with a reference voltage to generate a train of base drive pulses for application to the transistor. To control the base drive pulses at a variable duration in accordance with an inductive load, the reference voltage is varied as an inverse function of the sensed load current.
Description
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BACKGROUND OF THE INVENTION
The present invention relates to an induction heating apparatus in which a power representativ~ signal is negatlvely fed back to render the oscillation frequency variable in accordance with the inductive load to provide constant power delivery to the load.
Conventional induction heating apparatus is relatively complicated in circuit configuration to provide various features for safe operation of the inverter circuit under varying magnitude of utensil loads, resultin~ in an increase in cost which has prevented the extensive use of the induction heating apparatus.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an induction heating apparatus which provides constant - power regardless of variations in external power source or loadings with a minimum of power los -Another object is to provide an induction heating apparatus which achieves a high frequency conversion efficiency.
A further object of the invention is to provide a low cost, reliable induction heating apparatus parti- -cularly for cooking purposes.
According to the present invention there is provided an induction heating apparatus comprising a DC power .~ .
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source, a transistor connected to receive power from the DC power source to generate a trigger current in the presence of a control pulse applied thereto, a diode connected in inverse parallel relation with the tran-sistor, a resonant load circui-t inclùding an induction heating work coil and a capacitor for generating in response to the trigger current an oscillating current, means for detecting the magnitude of an electrical quantity in the load circuit, means for detecting a predetermined voltage level of the potential across the transistor, and negative-feedback-controlled pulse generating means responsive to an output signal from -~` the voltage detecting means for generating an output pulse with a duration which varies as an inverse function of the detected electrical quantity for application tothe transistor as the control pulse.
Preferably, the pulse generating means comprises a ramp generator responsive to an output signal from the low voltage detecting means to generate a ramp voltage and a comparator for comparing the ramp voltage with a reference voltage which is varied as an inverse function of the detected electrical quantity of the load clrcuit so that the output of the comparator is a train of pulses whose duration is controlled bY the negativelv feed-back-controlled reference voltage. In order to further stabilize the inverter operation, signals from the low !
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voltage detecting means is inhibited by the outpu-t signal from the comparator to prevent the low voltage detecting means from delivering unwanted signals.
- BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described in detail with reference to the accompanying drawings, in which:
Fig. 1 is a general schematic diagram of the induction heating apparatus of the invention;
Fig. 2 shows details of the base drive control circuit of the apparatus of Fig. 1, Figs. 3a to 3g are a waveform diagram associated with the apparatus Figs. 1 and 2;
Fig. 4 is a modification of the circuit of Fig. 2;
and Fig. 5 is a modified clrcuit diagram of Fig. 1.
DETAILED DESC~IPTION
The induction heating apparatus according to the invention is shown in Fig. 1 as comprising a filter capacitor network 12 connected to the input power lines 10, 11 and a full-wave rectifier 13 which converts a low fre~uency alternating current from a source 1~ in~o a full-wave rectified sinusoidal halfwave pulses which appear across positive power line I5 and negative power line 16 which is grounded. A semiconductor switching unit, comprising an inversely parallel connected power ~ 3 --3~
transistor 17 and a diode 18, is connected between the power lines 15 and 16, with the collector of transistor 17 connected to the posi-tive line 15. A resonant load circuit, including a p~rallel-connected induction heating work coil 19 and a capacitor 20, is connected in the power line 15 in series with a filter inductor 21 and the semiconductor switching unit. The transistor 17 receives power from the rectifier 13 to generate in response to a base drive pulse supplied from a control circuit 22, a trigger current that passes through the load circuit, the latter acting as a source of an ~
oscillating current which is generated during the time when the transistor 17 is turned off, the period o~ the oscillating current being a function of the resonant - 15 frequency of the load circuit which is in the ultrasonic range. A filter capacitor 23 is connec~ed between the power lines 15, 16 to allow the high frequency current to circulate through the inverter circuit comprised by ` the load circuit and the switching unit.
The base drive control circuit 22 receives power ~rom the positive power line 15 through terminal 22a to supply a base drive pulse to the transistor 17 through terminal 22b in response to the high frequency voltage reducing to a nearly zero voltage level by sensing the voltage across the switching unit through terminal 22c. To _ ~ _ ~:
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determine the interval of ~he based drive pulse to transis-tor 17, the con-trol circuit 22 also receives a current from a current transformer 24 associated with the input power line 11 through terminals 22d and 22e.
Details of -the control circuit 22 are illust-rated in Fig. 2. The control circuit 22 inclùdes a DC power supply circuit having a voltage regulator 31 and a voltage comparator 32 which receives power from terminal 22a to charge a storage capacitor 33 via diode 3~ to develop a filtered DC voltage across the capacitor 33 which is in shunt with a circuit including a resistor 35 and a second storage capacitor 36. The voltage developed across the capacitor 36 :is maintained at a constant level by the voltage reguLator 31 of a con-ventional design and fed into one terminal of the com-parator 32 for comparison with a reference voltage applied to another input thereof and also to a power supply terminal 30a. This reference voltage corresponds to an operating voltage level to permit the comparator 32 to generate a gate control pulse for an inhibit gate 37 when the voltage across the capacitor 36 is lower than the reference to prevent the induction heating apparatus from generatiny high frequency current when the input DC voltage is not high enough during the initial period $ollowing the power switch-in of the appara~-us.
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A power control circuit 38 is provided for inter-mittently supplying gate drive pulses to the transistor 17. This circuit comprises a ramp generator 39 and a voltage comparakor 40 which compares the ramp voltage with a user-controlled voltage reference from a voltage : divider 41 -to generate a train of constant frequency pulses whose duration is a function of the-user's setting level to provide different ratio of active to inactive --:
periods so that its active period has a longer duration when a relativel~ high power level is desired than it has for a lower desired power level. The pulses from the power control circuit 38 occur at a much lower fre-quency than the ultrasonic frequency of the inverter circuit and serves as an enable signal for an ~ND gate -~ 15 42 which is adapted to pass gate trigger pulses to a pulse ampli~ier circuit 43 and thence to a pulse trans-~ormer 44.
The gate trigger pulse is derived from a circuit : including a low voltage detector 4S connected to the input terminal 22c, an inhibit gate 46, a ramp generator 47 which operates as a free-running oscillator in the absence of trigger pulses and a voltage comparator 48.
The low voltage detector 45 detects when the high frequenc~
voltage at the collector o~ switching transistor:l7.drops.
to nearly zero and feeds a trigger impulse through the , . .
,, 3~
inhibit gate 46 to -the ramp generator 47 to cause it to generate a ramp voltage which is then compared in the comparator 48 with a reference voltage. This reference voltage is derived from a variable reference setting circu.it 50 which includes a rectifier-filter circuit 51 connected to receive current from the current transformer 24, a storage capacitor 52 connected by a diode 53 to the rectifi.er-filter 51 to develop a voltage for the inverting input of a differential amplifier 54 for making a comparison with a reference voltage provided by a voltaye divider 55 corresponding to a rated value of power input. Further included is a clamping circuit or limi-ter 60 which comprises a Eirst circu.it leg including resistors 56 and 57 connec-ted in series between voltage 5Upply terminal 30a and ground to define a low threshold voltage VL at a circuit node 63, and a second circuit le~ .-formed by series-connected resistors 58 and 59 connec~ea in parallel with the first circuit leg to define a high threshold voltage VH at a circuit node 64. A transistor 65 is provided having its base connected to the node 63 of resistors 56 and 57 and its emitter connected to the node 64 o~ resistors 58 and 59, the latter node 6~
being coupled by a diode 61 to the output of the dif-ferential amplifier 53 and also to ground by a storage capacitor 62. ~hen the voltage across the capacitor 62 is lower than the low -threshold voltage Vl at node 63, the transistor 60 is rendered conductive to charge the capacitor until it develops a voltage e~ual to VL, so that the capacitor voltage is clamped to VL, and if the diode 61 is nonconductive, the capacitor 62 is charged to the voltage level of VH at the circuit node 64.
Therefore, when the input current is lower than the rated value, the output signal from the di-fferential ampllfier 54 is negative to render the diode 61 non-conductive, so that the capacitor 62 is char~ed to VH,and when the input current is higher than rated value, the diode 61 becomes conductive to discharge the capacitor ! 62 by an amount proportional to the differential voltage, so that the capacitor 62 will develop a voltage inversely proportional to the input-current. Since the magnitude of the input current varies as a function o the power delivered to the load circuit including an inductive utensil placed over the work coil l9, the voltage across the capacitor 62 varies as an inverse function of the amount of power delivered to the utensil.
The voltage developed in capacitor 52 is m~onitored by a voltage comparator 68 which generates an inhibik signal when the monitored voltage is lower than a specified level determined by a voltage divider 69 and feeds it to the inhibit gate 37.
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Assuming that the voltage across the capacitor 52 has reached the normal operating level and the reference voltage across capacitor 62 is likewise at a suitable level, the output from -the ramp generator 47,.which is ' initially operating as a free-running oscillator,-drives the comparator 48 to a high output state..:A high level output signal ~rom the power control'oscillator 38 enables ' the AND gate'42-to'apply the output from the comparator 48 to the pulse ampliier 43 and thence to the pulse trans-former 44.
The pulse ampliEier 43 includes a pair of transistors 70, 71 of opposite conductivity types having their emitters connected to~ether to the outpu~ of the AND gate 42 through series-connected resistors 72, 73 and 74 and to the base o~ a switching power transistor 75 through ' parallel-connected resistor 76 and capacitor 77. -The transistors 70 and 71 receive power ~rom the voltage supply terminal 30a to generate current there*hrough in response to the base drive derived from the junction between resistors 73 and 74. The transistor:,70 is turned on in response to the base drive pulse to turn on.the .-transistor 75 charging the capacitor 77, The reverse bias on the capacitor 77 biases the transistor 71 on to produce a reverse current through resistor 78 connected ~S between the base of trans,istor 75 and ~round to turn it off, - 9 _ .. .
providing a fast switching action of transistor 75.
This results in a rapid rise in voltage in the primary of the transfor~er 44 to generate a base drive control pulse in the secondary winding which is applied to the base of power transistor 17 through resistor 79. An inter-media-te point of the secondary of the transfonner 44 is connected to ground and the end of the secondary winding opposite to the terminal connected to the base of tran-sistor 17 is connected by a diode B0 to the voltage supply terminal 30a to charge the capacitor 36 with part of the energy generated in response to the b~ase drive pulse in the secondary winding. A snubber circui-t formed by series-connected capacitor 81 and resistor 82 is con-nected across the collector of switching transistor 75 and ground to minimize the effect of a rapid change in the collector voltage of the transistor 75 upon turning it again in the absence of its base drive.
By virtue of the feedback current to the capacitor 36 from the secondary of the pulse transformer d4, the resistance value of the coupling resistor 35 can be held to a minimum, so that the amount of power loss therein is minimized. If without the energy feedback operation, a low-frequency transformer would be required to generate the DC power.
The operation of the circuit of Fig. 2 is visualized 3~
with reference to waveforms shown in Figs. 3a to 3g.
~lith the output of the comparator 48 being initially at a high voltage level (Fig. 3g), a base drive pulse bl (Fig. 3b) is generated b~ the positive edge of an output pulse al (Fig. 3a) of -the power control oscillator 38 to turn on transistor 17 at time tl, causing capacitor 20 to discharge producing a positive spike Cl and sub-sequen-tly a current C2 (Fig. 3c) through the transistor 17 and the load circuit and causing the voltage at the collector of transistor 17 to drop to nearly zero (Fig.
3d). A trigger pulse el is generated from the low voltage detector 45 to allow the ramp generator 47 to generate a ramp voltage fl (Fig. 3E) and when this ramp voltage reaches the reference level Vr supplied from the capacitor ~2 of variable reference circuit 50, the comparator 48 is switched to a low voltage output st~ate - at time t2 (Fig. 3g), thus terminating the drive pulse bl. This results in a negative going pulse b2 in tbe secondary of transformer 44 to quickly turn off the transistor 17. The turn-off of transistor 17 causes its collector potential to rise again producing a positive halfwave voltage dl (Fig. 3d) which decays to nearly zero at time t4 and at the same time permits the load clrcuit to produce an oscillating current c3 during the interval from t3 to t4, which interval is primarily determined ~Z~3~
by the resonant frequency of the load circuit~
Responsive to the voltage dl reducing to ~he nearly zero voltage level at time t4, the low voltage detector ~5 produces a trigger pulse e2 which resets the ramp generator A7 to produce a second ramp voltage f2 which causes a second base drive pulse b3 to occur. Con-currently, the diode 18 is rendered conductive due to the absence of positive high potential dl at time t4 and allows the negative current c3 to pass therethrough as current c4 until time t5, whereupon the transistor 17 is rendered conductive to generate a positive current c5 which passes through transistor 17 and the load circuit.
This process is repeated so long as the enable pulse al . is present.
`~ 15 Since the reference voltage Vr supplied to the ;~ comparator 48 is inversely proportional to the current : flow in the load circuit, the width of the pulse delivered from the comparator ~8 is controlled in a negative feedback fashion in response to the amount of the input current - 20 so that the interval between successive halfwave pulses dl and d2 is substantially held constant ~or a given load. Otherwise stated, the oscilla-tion period is ; variable in accordance with the loading so long as the reference voltage Vr is within the range between upper and lower limits VH and VL, so that the amount of power .
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delivered to the load during the active period of the power control circuit 38 is held constant.
~` Therefore, the volta~e across the transistor 17 is held constant within safe limits, ensuring a safe operation oE the inverter circuit under varying loads.
With this negative feedback pulse width control, the negative load current c3, c4 increases to a maximum value while the transistor 17 current c5 decreases to a-minimum value, soa substantial amount of power savings is achieved during the no load condition since the negative current represents negative power and the reduction of -the positive current reduces the power dissipation of the transistor 17.
The above-described negative i~eedback pulse width control is inhibited during the inactive period o~ the power control oscillator 48 by increasing the voltage across the capacitor 52 with a charging current supplied through diode 66a from the inverter 66 connected from the output of the inhibit gate 37. The high voltage on capacitor 52 drives the differential amplifier 5~ to-a negative voltage level at its output to cause the diode 61 to be rendered conductive to discharge the capacitor 62 until the voltage thereacross reaches the minimum threshold level VL which appears at the junction 63.
There~ore, during the inactive period of the oscillator 38, 3~
the comparator 48 outPut has a minimum pulse dura-tion, and this minimum pulse duratlon exists for a certain period set by the time cons-tant value of capacitor 52 and resistor 67 immediately following the positive edge of the pulse 84 of the oscillator 38. This short-- duration tri~ger pulse drives the transistor 17 with a small amount of power and assures safe operation of the inverter circuit during the initial period of the enablement by the power con-trol circuit 38. The voltage across the capacitor 52 will be subse~uently charged with the current supplied from the filter 51 so that it takes on a value representative of the laad current to resume the ne~ative feedback pulse wid-th con-trol.
In the presence of the ou-tput pulse from the comparator 48 the inhibit gate 46 is activated to prevent the passage of any trigger pulse which migh-t~
occur due to the spurious high frequency components of the oscillation current throu~h the inverter circuit. The resistance values of resistors 56 and 57 of the clampin~ circuit 50 are so chosen that the minimum threshold VL corresponds to the inherent turn~off time of transistor 17. The minimum threshold VL
thus sets an upper limit to the oscillation frequency and ensures against the failure of -turn-o-Ef of transistor 17 which might occur when the load size is e~cessively large.
When the size o~ a load is very small such as fork or spoon which is inadvertently placed over the work coil, a very small current will flow in the input circuit ll and the comparator 68 senses this condition by detect-ing a small voltage developed in the storage capacitor 52 and provides an inhibit signal to the gate 37 to disable the inverter circuit.
The time constant value of the storage capacitor 52 and resistor 67 is so selected that charge on capacitor 52 is representative of an average value of the input current through lead 11~ This provides an advantage in that there is a minimum amount of input current variations due to the difference in material between different inductive loads. Capacitor 62 on the other hand serves as a damping circuit for purposes of absor~ing transient Yariations of reference voltage which might occur in response to a rapid ~ariation of load such as replacement of a utensil during cooking operation.
Since it is known that an inductive load utensil of non-magnetic stainless material produces a larger input current and a s~aller oscillation ~oltage then in the case of an inductive load of ferrous material, .
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it is preferable to utilize the peak oscillation voltage as a negative feedback signal in addition to the input current value. This is accomplished by a peak detector lO0 as illustrated in Fig. 4 which has its input connected S to the terminal 22c and its output connected via diode lOl to the differential amplifier 54 to which the capacitor 52 and resistor 67 are connected. The output of the rectifier-filter 51 is also connected to the differential amplifier 54 through a diodè 102 which forms with the diode 101 a comparing circuit which supplies a higher voltage to the capacitor 52. The peak detector lO0 es~
; sentially comprises a voltage divider formed by resistors 103 and 104 with the junction there~etween ~eing connected by a diode 105 to a capacitor 106. The capacitor 106 is charged through the diode 105 when the latter is forward biased. A resistor 107 is connected in parallel with th,e capacîtor 106 to disc~arge'ît when the diode 105 is ~ackward biased so that the voltage developed across the capacitor 106 is repres~ntative of the peak value of the voltage at the collector of transistor 17. With the, negati~e feed~ack operation, ~oth input current and oscillatîon voltage are maintained constant and hence the power le~el is held constant for a given user r S
sett~ng regardless of t~e size of load to be heated.
Furthermore, an overvoltage at the collector of transis-tor 3~
17 can be effectively prevented by the negative feedback signal derived from the peak detector lO0.
The circuit of Fig. 1 can be modified as shown in Fig. S in which the con~utating capacitor 20 is connected across the diode 18. However, the parallel connectîon of work coil 19 and capacitor 20 as illustrated in Fig. l is preferred since it reduces the current passing through the capacitor 23, so that a small capacitance value is required for the capacitor 23.
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BACKGROUND OF THE INVENTION
The present invention relates to an induction heating apparatus in which a power representativ~ signal is negatlvely fed back to render the oscillation frequency variable in accordance with the inductive load to provide constant power delivery to the load.
Conventional induction heating apparatus is relatively complicated in circuit configuration to provide various features for safe operation of the inverter circuit under varying magnitude of utensil loads, resultin~ in an increase in cost which has prevented the extensive use of the induction heating apparatus.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an induction heating apparatus which provides constant - power regardless of variations in external power source or loadings with a minimum of power los -Another object is to provide an induction heating apparatus which achieves a high frequency conversion efficiency.
A further object of the invention is to provide a low cost, reliable induction heating apparatus parti- -cularly for cooking purposes.
According to the present invention there is provided an induction heating apparatus comprising a DC power .~ .
.
.,, ~
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source, a transistor connected to receive power from the DC power source to generate a trigger current in the presence of a control pulse applied thereto, a diode connected in inverse parallel relation with the tran-sistor, a resonant load circui-t inclùding an induction heating work coil and a capacitor for generating in response to the trigger current an oscillating current, means for detecting the magnitude of an electrical quantity in the load circuit, means for detecting a predetermined voltage level of the potential across the transistor, and negative-feedback-controlled pulse generating means responsive to an output signal from -~` the voltage detecting means for generating an output pulse with a duration which varies as an inverse function of the detected electrical quantity for application tothe transistor as the control pulse.
Preferably, the pulse generating means comprises a ramp generator responsive to an output signal from the low voltage detecting means to generate a ramp voltage and a comparator for comparing the ramp voltage with a reference voltage which is varied as an inverse function of the detected electrical quantity of the load clrcuit so that the output of the comparator is a train of pulses whose duration is controlled bY the negativelv feed-back-controlled reference voltage. In order to further stabilize the inverter operation, signals from the low !
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voltage detecting means is inhibited by the outpu-t signal from the comparator to prevent the low voltage detecting means from delivering unwanted signals.
- BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described in detail with reference to the accompanying drawings, in which:
Fig. 1 is a general schematic diagram of the induction heating apparatus of the invention;
Fig. 2 shows details of the base drive control circuit of the apparatus of Fig. 1, Figs. 3a to 3g are a waveform diagram associated with the apparatus Figs. 1 and 2;
Fig. 4 is a modification of the circuit of Fig. 2;
and Fig. 5 is a modified clrcuit diagram of Fig. 1.
DETAILED DESC~IPTION
The induction heating apparatus according to the invention is shown in Fig. 1 as comprising a filter capacitor network 12 connected to the input power lines 10, 11 and a full-wave rectifier 13 which converts a low fre~uency alternating current from a source 1~ in~o a full-wave rectified sinusoidal halfwave pulses which appear across positive power line I5 and negative power line 16 which is grounded. A semiconductor switching unit, comprising an inversely parallel connected power ~ 3 --3~
transistor 17 and a diode 18, is connected between the power lines 15 and 16, with the collector of transistor 17 connected to the posi-tive line 15. A resonant load circuit, including a p~rallel-connected induction heating work coil 19 and a capacitor 20, is connected in the power line 15 in series with a filter inductor 21 and the semiconductor switching unit. The transistor 17 receives power from the rectifier 13 to generate in response to a base drive pulse supplied from a control circuit 22, a trigger current that passes through the load circuit, the latter acting as a source of an ~
oscillating current which is generated during the time when the transistor 17 is turned off, the period o~ the oscillating current being a function of the resonant - 15 frequency of the load circuit which is in the ultrasonic range. A filter capacitor 23 is connec~ed between the power lines 15, 16 to allow the high frequency current to circulate through the inverter circuit comprised by ` the load circuit and the switching unit.
The base drive control circuit 22 receives power ~rom the positive power line 15 through terminal 22a to supply a base drive pulse to the transistor 17 through terminal 22b in response to the high frequency voltage reducing to a nearly zero voltage level by sensing the voltage across the switching unit through terminal 22c. To _ ~ _ ~:
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determine the interval of ~he based drive pulse to transis-tor 17, the con-trol circuit 22 also receives a current from a current transformer 24 associated with the input power line 11 through terminals 22d and 22e.
Details of -the control circuit 22 are illust-rated in Fig. 2. The control circuit 22 inclùdes a DC power supply circuit having a voltage regulator 31 and a voltage comparator 32 which receives power from terminal 22a to charge a storage capacitor 33 via diode 3~ to develop a filtered DC voltage across the capacitor 33 which is in shunt with a circuit including a resistor 35 and a second storage capacitor 36. The voltage developed across the capacitor 36 :is maintained at a constant level by the voltage reguLator 31 of a con-ventional design and fed into one terminal of the com-parator 32 for comparison with a reference voltage applied to another input thereof and also to a power supply terminal 30a. This reference voltage corresponds to an operating voltage level to permit the comparator 32 to generate a gate control pulse for an inhibit gate 37 when the voltage across the capacitor 36 is lower than the reference to prevent the induction heating apparatus from generatiny high frequency current when the input DC voltage is not high enough during the initial period $ollowing the power switch-in of the appara~-us.
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A power control circuit 38 is provided for inter-mittently supplying gate drive pulses to the transistor 17. This circuit comprises a ramp generator 39 and a voltage comparakor 40 which compares the ramp voltage with a user-controlled voltage reference from a voltage : divider 41 -to generate a train of constant frequency pulses whose duration is a function of the-user's setting level to provide different ratio of active to inactive --:
periods so that its active period has a longer duration when a relativel~ high power level is desired than it has for a lower desired power level. The pulses from the power control circuit 38 occur at a much lower fre-quency than the ultrasonic frequency of the inverter circuit and serves as an enable signal for an ~ND gate -~ 15 42 which is adapted to pass gate trigger pulses to a pulse ampli~ier circuit 43 and thence to a pulse trans-~ormer 44.
The gate trigger pulse is derived from a circuit : including a low voltage detector 4S connected to the input terminal 22c, an inhibit gate 46, a ramp generator 47 which operates as a free-running oscillator in the absence of trigger pulses and a voltage comparator 48.
The low voltage detector 45 detects when the high frequenc~
voltage at the collector o~ switching transistor:l7.drops.
to nearly zero and feeds a trigger impulse through the , . .
,, 3~
inhibit gate 46 to -the ramp generator 47 to cause it to generate a ramp voltage which is then compared in the comparator 48 with a reference voltage. This reference voltage is derived from a variable reference setting circu.it 50 which includes a rectifier-filter circuit 51 connected to receive current from the current transformer 24, a storage capacitor 52 connected by a diode 53 to the rectifi.er-filter 51 to develop a voltage for the inverting input of a differential amplifier 54 for making a comparison with a reference voltage provided by a voltaye divider 55 corresponding to a rated value of power input. Further included is a clamping circuit or limi-ter 60 which comprises a Eirst circu.it leg including resistors 56 and 57 connec-ted in series between voltage 5Upply terminal 30a and ground to define a low threshold voltage VL at a circuit node 63, and a second circuit le~ .-formed by series-connected resistors 58 and 59 connec~ea in parallel with the first circuit leg to define a high threshold voltage VH at a circuit node 64. A transistor 65 is provided having its base connected to the node 63 of resistors 56 and 57 and its emitter connected to the node 64 o~ resistors 58 and 59, the latter node 6~
being coupled by a diode 61 to the output of the dif-ferential amplifier 53 and also to ground by a storage capacitor 62. ~hen the voltage across the capacitor 62 is lower than the low -threshold voltage Vl at node 63, the transistor 60 is rendered conductive to charge the capacitor until it develops a voltage e~ual to VL, so that the capacitor voltage is clamped to VL, and if the diode 61 is nonconductive, the capacitor 62 is charged to the voltage level of VH at the circuit node 64.
Therefore, when the input current is lower than the rated value, the output signal from the di-fferential ampllfier 54 is negative to render the diode 61 non-conductive, so that the capacitor 62 is char~ed to VH,and when the input current is higher than rated value, the diode 61 becomes conductive to discharge the capacitor ! 62 by an amount proportional to the differential voltage, so that the capacitor 62 will develop a voltage inversely proportional to the input-current. Since the magnitude of the input current varies as a function o the power delivered to the load circuit including an inductive utensil placed over the work coil l9, the voltage across the capacitor 62 varies as an inverse function of the amount of power delivered to the utensil.
The voltage developed in capacitor 52 is m~onitored by a voltage comparator 68 which generates an inhibik signal when the monitored voltage is lower than a specified level determined by a voltage divider 69 and feeds it to the inhibit gate 37.
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Assuming that the voltage across the capacitor 52 has reached the normal operating level and the reference voltage across capacitor 62 is likewise at a suitable level, the output from -the ramp generator 47,.which is ' initially operating as a free-running oscillator,-drives the comparator 48 to a high output state..:A high level output signal ~rom the power control'oscillator 38 enables ' the AND gate'42-to'apply the output from the comparator 48 to the pulse ampliier 43 and thence to the pulse trans-former 44.
The pulse ampliEier 43 includes a pair of transistors 70, 71 of opposite conductivity types having their emitters connected to~ether to the outpu~ of the AND gate 42 through series-connected resistors 72, 73 and 74 and to the base o~ a switching power transistor 75 through ' parallel-connected resistor 76 and capacitor 77. -The transistors 70 and 71 receive power ~rom the voltage supply terminal 30a to generate current there*hrough in response to the base drive derived from the junction between resistors 73 and 74. The transistor:,70 is turned on in response to the base drive pulse to turn on.the .-transistor 75 charging the capacitor 77, The reverse bias on the capacitor 77 biases the transistor 71 on to produce a reverse current through resistor 78 connected ~S between the base of trans,istor 75 and ~round to turn it off, - 9 _ .. .
providing a fast switching action of transistor 75.
This results in a rapid rise in voltage in the primary of the transfor~er 44 to generate a base drive control pulse in the secondary winding which is applied to the base of power transistor 17 through resistor 79. An inter-media-te point of the secondary of the transfonner 44 is connected to ground and the end of the secondary winding opposite to the terminal connected to the base of tran-sistor 17 is connected by a diode B0 to the voltage supply terminal 30a to charge the capacitor 36 with part of the energy generated in response to the b~ase drive pulse in the secondary winding. A snubber circui-t formed by series-connected capacitor 81 and resistor 82 is con-nected across the collector of switching transistor 75 and ground to minimize the effect of a rapid change in the collector voltage of the transistor 75 upon turning it again in the absence of its base drive.
By virtue of the feedback current to the capacitor 36 from the secondary of the pulse transformer d4, the resistance value of the coupling resistor 35 can be held to a minimum, so that the amount of power loss therein is minimized. If without the energy feedback operation, a low-frequency transformer would be required to generate the DC power.
The operation of the circuit of Fig. 2 is visualized 3~
with reference to waveforms shown in Figs. 3a to 3g.
~lith the output of the comparator 48 being initially at a high voltage level (Fig. 3g), a base drive pulse bl (Fig. 3b) is generated b~ the positive edge of an output pulse al (Fig. 3a) of -the power control oscillator 38 to turn on transistor 17 at time tl, causing capacitor 20 to discharge producing a positive spike Cl and sub-sequen-tly a current C2 (Fig. 3c) through the transistor 17 and the load circuit and causing the voltage at the collector of transistor 17 to drop to nearly zero (Fig.
3d). A trigger pulse el is generated from the low voltage detector 45 to allow the ramp generator 47 to generate a ramp voltage fl (Fig. 3E) and when this ramp voltage reaches the reference level Vr supplied from the capacitor ~2 of variable reference circuit 50, the comparator 48 is switched to a low voltage output st~ate - at time t2 (Fig. 3g), thus terminating the drive pulse bl. This results in a negative going pulse b2 in tbe secondary of transformer 44 to quickly turn off the transistor 17. The turn-off of transistor 17 causes its collector potential to rise again producing a positive halfwave voltage dl (Fig. 3d) which decays to nearly zero at time t4 and at the same time permits the load clrcuit to produce an oscillating current c3 during the interval from t3 to t4, which interval is primarily determined ~Z~3~
by the resonant frequency of the load circuit~
Responsive to the voltage dl reducing to ~he nearly zero voltage level at time t4, the low voltage detector ~5 produces a trigger pulse e2 which resets the ramp generator A7 to produce a second ramp voltage f2 which causes a second base drive pulse b3 to occur. Con-currently, the diode 18 is rendered conductive due to the absence of positive high potential dl at time t4 and allows the negative current c3 to pass therethrough as current c4 until time t5, whereupon the transistor 17 is rendered conductive to generate a positive current c5 which passes through transistor 17 and the load circuit.
This process is repeated so long as the enable pulse al . is present.
`~ 15 Since the reference voltage Vr supplied to the ;~ comparator 48 is inversely proportional to the current : flow in the load circuit, the width of the pulse delivered from the comparator ~8 is controlled in a negative feedback fashion in response to the amount of the input current - 20 so that the interval between successive halfwave pulses dl and d2 is substantially held constant ~or a given load. Otherwise stated, the oscilla-tion period is ; variable in accordance with the loading so long as the reference voltage Vr is within the range between upper and lower limits VH and VL, so that the amount of power .
.
- .
. ~ .
.
3~
delivered to the load during the active period of the power control circuit 38 is held constant.
~` Therefore, the volta~e across the transistor 17 is held constant within safe limits, ensuring a safe operation oE the inverter circuit under varying loads.
With this negative feedback pulse width control, the negative load current c3, c4 increases to a maximum value while the transistor 17 current c5 decreases to a-minimum value, soa substantial amount of power savings is achieved during the no load condition since the negative current represents negative power and the reduction of -the positive current reduces the power dissipation of the transistor 17.
The above-described negative i~eedback pulse width control is inhibited during the inactive period o~ the power control oscillator 48 by increasing the voltage across the capacitor 52 with a charging current supplied through diode 66a from the inverter 66 connected from the output of the inhibit gate 37. The high voltage on capacitor 52 drives the differential amplifier 5~ to-a negative voltage level at its output to cause the diode 61 to be rendered conductive to discharge the capacitor 62 until the voltage thereacross reaches the minimum threshold level VL which appears at the junction 63.
There~ore, during the inactive period of the oscillator 38, 3~
the comparator 48 outPut has a minimum pulse dura-tion, and this minimum pulse duratlon exists for a certain period set by the time cons-tant value of capacitor 52 and resistor 67 immediately following the positive edge of the pulse 84 of the oscillator 38. This short-- duration tri~ger pulse drives the transistor 17 with a small amount of power and assures safe operation of the inverter circuit during the initial period of the enablement by the power con-trol circuit 38. The voltage across the capacitor 52 will be subse~uently charged with the current supplied from the filter 51 so that it takes on a value representative of the laad current to resume the ne~ative feedback pulse wid-th con-trol.
In the presence of the ou-tput pulse from the comparator 48 the inhibit gate 46 is activated to prevent the passage of any trigger pulse which migh-t~
occur due to the spurious high frequency components of the oscillation current throu~h the inverter circuit. The resistance values of resistors 56 and 57 of the clampin~ circuit 50 are so chosen that the minimum threshold VL corresponds to the inherent turn~off time of transistor 17. The minimum threshold VL
thus sets an upper limit to the oscillation frequency and ensures against the failure of -turn-o-Ef of transistor 17 which might occur when the load size is e~cessively large.
When the size o~ a load is very small such as fork or spoon which is inadvertently placed over the work coil, a very small current will flow in the input circuit ll and the comparator 68 senses this condition by detect-ing a small voltage developed in the storage capacitor 52 and provides an inhibit signal to the gate 37 to disable the inverter circuit.
The time constant value of the storage capacitor 52 and resistor 67 is so selected that charge on capacitor 52 is representative of an average value of the input current through lead 11~ This provides an advantage in that there is a minimum amount of input current variations due to the difference in material between different inductive loads. Capacitor 62 on the other hand serves as a damping circuit for purposes of absor~ing transient Yariations of reference voltage which might occur in response to a rapid ~ariation of load such as replacement of a utensil during cooking operation.
Since it is known that an inductive load utensil of non-magnetic stainless material produces a larger input current and a s~aller oscillation ~oltage then in the case of an inductive load of ferrous material, .
'. .
it is preferable to utilize the peak oscillation voltage as a negative feedback signal in addition to the input current value. This is accomplished by a peak detector lO0 as illustrated in Fig. 4 which has its input connected S to the terminal 22c and its output connected via diode lOl to the differential amplifier 54 to which the capacitor 52 and resistor 67 are connected. The output of the rectifier-filter 51 is also connected to the differential amplifier 54 through a diodè 102 which forms with the diode 101 a comparing circuit which supplies a higher voltage to the capacitor 52. The peak detector lO0 es~
; sentially comprises a voltage divider formed by resistors 103 and 104 with the junction there~etween ~eing connected by a diode 105 to a capacitor 106. The capacitor 106 is charged through the diode 105 when the latter is forward biased. A resistor 107 is connected in parallel with th,e capacîtor 106 to disc~arge'ît when the diode 105 is ~ackward biased so that the voltage developed across the capacitor 106 is repres~ntative of the peak value of the voltage at the collector of transistor 17. With the, negati~e feed~ack operation, ~oth input current and oscillatîon voltage are maintained constant and hence the power le~el is held constant for a given user r S
sett~ng regardless of t~e size of load to be heated.
Furthermore, an overvoltage at the collector of transis-tor 3~
17 can be effectively prevented by the negative feedback signal derived from the peak detector lO0.
The circuit of Fig. 1 can be modified as shown in Fig. S in which the con~utating capacitor 20 is connected across the diode 18. However, the parallel connectîon of work coil 19 and capacitor 20 as illustrated in Fig. l is preferred since it reduces the current passing through the capacitor 23, so that a small capacitance value is required for the capacitor 23.
~` .
.
.
'` .
.
s ~. - .
~; ,
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An induction heating apparatus comprising a DC
power source, a transistor connected to receive power from said DC power source to generate a trigger current in the presence of a control pulse applied thereto, a diode connected in inverse parallel relation with said transistor, a resonant load circuit including an induction heating work coil and a capacitor for generating in response to said trigger current an oscillating current, means for detecting the magnitude of an electrical quantity in said load circuit, means for detecting a predetermined voltage level of the potential across said transistor, and negative-feedback-controlled pulse generating means responsive to an output signal from said voltage detecting means for generating an output pulse with a duration which varies as an inverse function of said detected electrical quantity for application to said transistor as said control pulse.
power source, a transistor connected to receive power from said DC power source to generate a trigger current in the presence of a control pulse applied thereto, a diode connected in inverse parallel relation with said transistor, a resonant load circuit including an induction heating work coil and a capacitor for generating in response to said trigger current an oscillating current, means for detecting the magnitude of an electrical quantity in said load circuit, means for detecting a predetermined voltage level of the potential across said transistor, and negative-feedback-controlled pulse generating means responsive to an output signal from said voltage detecting means for generating an output pulse with a duration which varies as an inverse function of said detected electrical quantity for application to said transistor as said control pulse.
2. An induction heating apparatus as claimed in Claim 1, further comprising an inhibit gate connected between said voltage detecting means and said pulse generating means for preventing the application of an output signal from said voltage detecting means to said pulse generating means in the presence of said output pulse of said pulse generating means.
3. An induction heating apparatus as claimed in Claim 1, further comprising power control means including means for setting a user's desired power level and means for periodically permitting and preventing the application of said control pulses to said transistor at a frequency much lower than the frequency of said control pulses for respective periods with a ratio depending on said desired power level.
4. An induction heating apparatus as claimed in Claim 1, wherein said electrical quantity detecting means comprises means for generating a reference voltage which varies as an inverse function of the detected electrical quantity and wherein said pulse generating means comprises a ramp generator for generating a ramp voltage in response to an output signal from said voltage detecting means and a comparator for generating an output pulse when said ramp voltage is below said reference voltage.
5. An induction heating apparatus as claimed in Claim 3, further comprising means for reducing the interval of said control pulse for a certain interval immediately following the initiation of the period in which said control pulses are permitted to be applied to said transistor.
6. An induction heating apparatus as claimed in Claim 5, wherein said interval reducing means comprises an RC network connected to said electrical quantity detecting means for developing an average value of the detected electrical quantity, a first reference setting circuit for establishing a low reference voltage, a second reference setting circuit for establishing a high reference voltage, VH a storage capacitor connected to said second reference setting circuit, switching means for charging said storage capacitor to said low reference voltage when the voltage across said storage capacitor is below said low reference voltage, and discharging means including a differential amplifier for generating an output signal representative of the deviation of said average value of the detected electrical quantity from a predetermined rated value, and a diode connected between the output of said differential amplifier and said storage capacitor for discharging same by an amount proportional to the amount of said deviation, and means for charging the capacitor of said DC network during the period in which said control pulses are prevented from being applied to said transistor.
7. An induction heating apparatus as claimed in Claim 4, further comprising a second DC power source including a storage capacitor serving as a voltage source, a diode connected to the first-mentioned DC power source for charging said storage capacitor, and a pulse trans-former having a primary winding connected to be respon-sive to the output signal of said comparator and a secondary winding connected to the control terminal of said transistor, said secondary winding being further connected to said storage capacitor for charging same with part of the energy induced in said secondary winding.
8. An induction heating apparatus as claimed in Claim 1, wherein said induction heating work coil is connected in parallel with said commutating capacitor and in series with said inversely parallel-connected transistor and diode between the terminals of said DC
power source.
power source.
9. An induction heating apparatus as claimed in Claim 1, wherein said electrical quantity detecting means comprises a current transformer for generating a signal representative of the current in said load circuit to represent said electrical quantity.
10. An induction heating apparatus as claimed in Claim 1 or 9, wherein said electrical quantity detecting means further comprises a peak detector connected to be responsive to the voltage developed across said semi-conductor switching device to represent said electrical quantity.
11. An induction heating apparatus as claimed in Claim 10, further comprising means for selectively coupling the one of said current representative signal and said voltage detected by said peak detector which is greater than the other to a circuit point to represent said electrical quantity.
12. An induction heating apparatus as claimed in Claim 4, wherein said pulse generating means further comprises a pair of first and second transistors of opposite conductivity types having their base electrodes connected together to be responsive to the output of said comparator, a parallel-connected circuit including a capacitor and a resistor, a third transistor having the base electrode thereof being connected through said parallel-connected circuit to the emitters of said first and second transistors, and a pulse transformer having a primary winding connected at one end to a voltage source and at the other end to ground through the collector-emitter path of said third transistor and a secondary winding for generating said control pulses, the collector-emitter paths of said first and second transistors being connected in series between a voltage source and ground.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP53076869A JPS5949795B2 (en) | 1978-06-23 | 1978-06-23 | Inverter device |
| JP53-76869 | 1978-06-23 | ||
| JP53-84472 | 1978-07-10 | ||
| JP8447278A JPS5511653A (en) | 1978-07-10 | 1978-07-10 | Frequency converting device |
| JP53-94846 | 1978-08-02 | ||
| JP53094846A JPS5821792B2 (en) | 1978-08-02 | 1978-08-02 | induction heating device |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1128139A true CA1128139A (en) | 1982-07-20 |
Family
ID=27302275
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA330,657A Expired CA1128139A (en) | 1978-06-23 | 1979-06-22 | Induction heating apparatus with negative feedback controlled pulse generation |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US4277667A (en) |
| CA (1) | CA1128139A (en) |
| DE (1) | DE2925308C2 (en) |
| GB (1) | GB2025094B (en) |
Families Citing this family (25)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3049863C2 (en) * | 1979-09-17 | 1985-02-28 | Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka | Induction heater for heating metal cooking vessels |
| GB2062985B (en) * | 1979-11-12 | 1983-11-02 | Matsushita Electric Ind Co Ltd | Small load detection by comparison between input and output parameters of an induction heat cooking apparatus |
| US4617442A (en) * | 1982-01-12 | 1986-10-14 | Sanyo Electric Co., Ltd. | Induction heating apparatus with controlled switching device for improved efficiency |
| US4595814A (en) * | 1982-08-19 | 1986-06-17 | Matsushita Electric Industrial Co., Ltd. | Induction heating apparatus utilizing output energy for powering switching operation |
| JPS6134884A (en) * | 1984-07-26 | 1986-02-19 | 株式会社東芝 | Induction heating cooking device |
| US4864083A (en) * | 1988-04-15 | 1989-09-05 | Lucerne Products, Inc. | Reversing switch |
| US5111014A (en) * | 1988-06-14 | 1992-05-05 | Kabushiki Kaisha Toshiba | Electromagnetic cooker including load control |
| US4972309A (en) * | 1989-03-14 | 1990-11-20 | Hughes Aircraft Company | N-phase sinewave converter |
| US5134265A (en) * | 1990-02-16 | 1992-07-28 | Metcal, Inc. | Rapid heating, uniform, highly efficient griddle |
| US5227597A (en) * | 1990-02-16 | 1993-07-13 | Electric Power Research Institute | Rapid heating, uniform, highly efficient griddle |
| AU1467501A (en) | 1999-11-03 | 2001-05-14 | Nexicor Llc | Hand held induction tool |
| DE10304505A1 (en) * | 2003-02-05 | 2004-08-26 | Abb Patent Gmbh | Process for feeding an induction furnace or inductor |
| US7034263B2 (en) | 2003-07-02 | 2006-04-25 | Itherm Technologies, Lp | Apparatus and method for inductive heating |
| US7279665B2 (en) * | 2003-07-02 | 2007-10-09 | Itherm Technologies, Lp | Method for delivering harmonic inductive power |
| US6943329B2 (en) * | 2003-09-25 | 2005-09-13 | 3M Innovative Properties Company | Induction heating system for reduced switch stress |
| US6943330B2 (en) * | 2003-09-25 | 2005-09-13 | 3M Innovative Properties Company | Induction heating system with resonance detection |
| DE102005050038A1 (en) * | 2005-10-14 | 2007-05-24 | E.G.O. Elektro-Gerätebau GmbH | Method for operating an induction heater |
| DE102008015036A1 (en) * | 2008-03-14 | 2009-09-17 | E.G.O. Elektro-Gerätebau GmbH | Apparatus and method for controlling induction heating of an induction hob |
| CN102711298B (en) * | 2012-05-23 | 2014-01-01 | 美的集团股份有限公司 | Heating control device of induction cooker and control method |
| US9155130B2 (en) * | 2012-05-29 | 2015-10-06 | General Electric Company | Method to detect a position of a cookware utensil in an induction cooktop system |
| WO2015038010A1 (en) * | 2013-09-12 | 2015-03-19 | Auckland Uniservices Limited | Resonant power supply with self tuning |
| EP2999303B1 (en) * | 2014-09-18 | 2018-11-14 | Electrolux Appliances Aktiebolag | Induction hob and method for operating an induction hob |
| CN108289350B (en) * | 2017-01-09 | 2021-03-19 | 佛山市顺德区美的电热电器制造有限公司 | Electromagnetic heating control method and electromagnetic heating equipment |
| US11140751B2 (en) | 2018-04-23 | 2021-10-05 | Whirlpool Corporation | System and method for controlling quasi-resonant induction heating devices |
| WO2023172211A1 (en) * | 2021-07-05 | 2023-09-14 | Mamur Teknoloji Sistemleri San. A.S. | Load sensing method for a single switch partial resonance inverter circuit |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3733540A (en) * | 1972-02-03 | 1973-05-15 | Motorola Inc | Switching regulator sweep starting protection circuit |
| US3798531A (en) * | 1972-06-05 | 1974-03-19 | Instrumentation Specialties Co | Switching-mode power supply |
| US3846694A (en) * | 1973-03-30 | 1974-11-05 | Fonseca J | Constant power supply employing a variable frequency inverter |
| SE408518B (en) * | 1974-05-17 | 1979-06-11 | Matsushita Electric Ind Co Ltd | INDUCTION HEATING DEVICE |
| JPS5820226B2 (en) * | 1976-01-14 | 1983-04-22 | 松下電器産業株式会社 | static power converter |
| US4115676A (en) * | 1976-02-10 | 1978-09-19 | Tokyo Shibaura Electric Co., Ltd. | Induction heating apparatus |
| US4112286A (en) * | 1976-06-28 | 1978-09-05 | Firing Circuits, Inc. | Power circuit for induction heating |
-
1979
- 1979-06-21 US US06/050,757 patent/US4277667A/en not_active Expired - Lifetime
- 1979-06-22 GB GB7921772A patent/GB2025094B/en not_active Expired
- 1979-06-22 CA CA330,657A patent/CA1128139A/en not_active Expired
- 1979-06-22 DE DE2925308A patent/DE2925308C2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US4277667A (en) | 1981-07-07 |
| GB2025094B (en) | 1982-10-13 |
| DE2925308A1 (en) | 1980-01-24 |
| GB2025094A (en) | 1980-01-16 |
| DE2925308C2 (en) | 1983-04-07 |
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