WO2004042906A1

WO2004042906A1 - Current controlled flyback converter

Info

Publication number: WO2004042906A1
Application number: PCT/SG2002/000230
Authority: WO
Inventors: Manfred Bernhard Gasteier; Poh Keong Ng
Original assignee: Perkinelmer Singapore Pte Ltd
Priority date: 2002-10-07
Filing date: 2002-10-07
Publication date: 2004-05-21
Also published as: AU2002368321A1

Abstract

The present invention provides a flyback converter (10) comprising: a switching means (140), a flyback transformer (120), coupled to the switching means for converting a current from a power source into an oscillating current for charging the capacitive load (190); the flyback transformer having a primary coil (122) and a secondary coil (124); a primary current sensor (144), coupled to the switching means and the primary coil (122); the primary current sensor (144) for sensing a predetermined primary current amplitude from the primary coil (122) and for providing an OFF signal; a switching control (150), coupled to the primary current sensor (144), the secondary current sensor (130) and to the switching means, the switching control (150) for receiving the OFF signal from the primary current sensor (144) and for switching OFF the switching means, whereby the switching control (150) further for providing a time delay before switching OFF the switching means.

Description

CURRENT CONTROLLED FLYBACK CONVERTER

FIELD OF THE INVENTION

The present invention relates in general to electronic power converters. In particular, this invention relates to electronic Flyback converter circuits for charging capacitive loads.

BACKGROUND OF THE INVENTION

The use of Flyback converters to supply a continuous chain of current pulses for charging capacitive loads is well known. Flyback converters are typically used in devices which require the charging of a capacitor which subsequently discharges to supply a high voltage to a load. Some of these devices can be found in flash lamps, strobing lights, and defibrillators.

A Flyback converter typically includes a Flyback transformer. A power switch or switching means switches ON to supply a primary current to the Flyback transformer from a DC power source. The Flyback transformer then stores the primary current as magnetic flux in the Flyback transformer. When the power switch or switching means is switched OFF, the primary current supply is cut off and the Flyback transformer starts to discharge the magnetic flux as a secondary current. It is this rapid switching ON and OFF of the switching means coupled to the Flyback transformer that results in the continuous chain of current pulses that are used for charging of capacitive loads. The period of time that the power switch is ON is known as ON time and the period of time that the power switch is OFF as OFF time. This cycle of switching ON and OFF is repeated until the capacitor is fully charged. In some prior art Flyback converters, the On time and the Off time are both predetermined and held constant. This simplifies the operation of the Flyback converter and also ensures that the capacitor is charged quickly. However, a problem arises when magnetic flux in a Flyback transformer is not sufficiently discharged during the OFF time. In the subsequent ON time, the amplitude of the primary current begins at the same level as the secondary current, multiplied by the Flyback transformer turn ratio, when the Flyback transformer stopped discharging. If the secondary current was still high when the Flyback transformer stopped discharging after the OFF time, the primary current would be even higher in the subsequent ON time. Since primary and secondary current in the Flyback transformer and the magnetic flux stored are linearly proportional to each other, this will result in accumulation of magnetic flux in the Flyback transformer. This inability of the Flyback transformer to discharge the magnetic flux to a capacitor fast enough during the OFF time results in high primary current amplitudes during the ON time. In addition, ringing due to parasitic effects caused by the inherent characteristics of the components used results in high current peaks during the initial period of the ON time. Together, the inability of the Flyback transformer to discharge the magnetic flux rapidly and the ringing effects causes a phenomena known as inrush current.

The effect of this inrush peak current results in the overall increase of the maximum current amplitude in the circuits of these devices using such a Flyback converter. This is undesirable and disadvantageous with equipment that requires low current levels such as digital devices, that shares the same power source.

A method to get around the inrush current problem is to simply increase the fixed OFF time of the Flyback converter to allow all the magnetic flux stored in the Flyback transformer to be discharged. This is however, not advantageous as such an increase in OFF time may not necessarily result in a fast charging Flyback converter. Generally, the approach is to have a variable ON and OFF time that would discharge the Flyback transformer to a predetermined level before charging it again. This predetermined level of discharge would be low enough to prevent the effects of inrush current from exceeding the maximum primary current level, allowable in the circuits, yet right to allow a relatively short ON time for charging the Flyback converter.

In further prior art, implementation of current sensing circuits allows detection of the levels of the primary current and the secondary current which are used as the indications of the levels of magnetic flux stored or discharged in the Flyback transformer. The current sensing circuits are further coupled to controllers and switching means. The current sensing circuits detect the levels in the primary and the secondary current and provide the switching means with the signals to switch ON or OFF when the predetermined current levels corresponding to the magnetic flux are reached.

In US Patent No. 5,485,361 (issued Jan 16 1996 to Nathan O. Sokal), a Flyback converter is described comprising a controller for current sensing. Both the primary current and the secondary current in the Flyback converter are detected and used to determine the On time and the Off time of the Flyback converter. In the disclosed circuit, current sensing is achieved by using resistors in series to sense both the primary current and the secondary current. A comparator is thus required to sense the predetermined current amplitudes from the resistors and generate the required switching signals. This may be disadvantageous as a comparator would further increase the cost of the circuit as well as the overall footprint. Furthermore, in this prior art invention, the problem with inrush current, especially due to parasitic effect, is not addressed at all. The conventional method of using a resistor to sense such a primary current is also less efficient. The current sensing circuits in many of these Flyback converters further require the use of comparators, micro-controllers and timers. These components are costly and also add to the overall footprint of the Flyback converter.

There is thus a need for a Flyback converter utilizing less components and employing a lower maximum overall current amplitude or level in its circuitry by reducing effects of inrush current.

SUMMARY OF THE INVENTION

The present invention seeks to provide a current controlled Flyback converter utilizing fewer components.

Accordingly, the invention provides a Flyback converter circuit for charging a capacitive load, the Flyback converter circuit has: a switching means, a Flyback transformer, coupled to the switching means for converting a current from a power source into an oscillating current for charging the capacitive load; the Flyback transformer further having a primary coil and a secondary coil; a primary current sensor, coupled to the switching means and the primary coil, the primary current sensor for sensing a predetermined primary current amplitude from the primary coil and for providing an OFF signal; a switching control, coupled to the primary current sensor, to the secondary current sensor and to the switching means, the switching control for receiving the OFF signal from the primary current sensor and for switching OFF the switching means, whereby the switching control further for providing a time delay before switching OFF the switching means. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the waveforms of magnetic flux density, primary current and secondary current of a prior art Flyback converter.

FIG. 2 shows a functional block diagram of a Flyback converter in accordance with the present invention.

FIG. 3 shows a schematic diagram of FIG. 2.

FIG. 4 shows the waveforms of magnetic flux density, primary current and secondary current of FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, the waveforms shown are the magnetic flux ( ) stored in the Flyback transformer, the primary current (l_p) during the ON time and the secondary current (l_s) during the OFF time of a prior art Flyback converter. The ON time and the OFF time of such a prior art Flyback converter circuit does not consider maximum current values reached by the circuit. FIG. 1 shows the effects of not controlling the OFF time to achieve sufficient discharge of stored magnetic flux in the Flyback transformer. During the ON time, current is supplied to the Flyback transformer and the primary current increases corresponding with the stored magnetic flux in the Flyback transformer 120. A designated maximum current amplitude for the circuit is designated Max Current 5.

When the primary current is cut off, the secondary current starts to decrease correspondingly with the discharge of magnetic flux from the Flyback transformer. However, if the OFF time is not regulated as is the case in FIG. 1, the magnetic flux (φ) stored in the Flyback transformer will not discharge sufficiently. This results in an accumulation of the magnetic flux stored in the Flyback transformer. Thus the consequent ON time would result in the primary current (l_P) in the Flyback transformer exceeding Max Current 5. This is because the same period of ON time would results in the same increase in current as in the preceding ON time, but at a higher starting point, thereby increasing the primary current above the Max Current 5. As the accumulation of stored magnetic flux increases, so too the primary current. Furthermore, with the addition of the ringing effect, the overall current amplitude would increase much higher than Max Current 5.

Decreasing the ON time to control this increase in the primary current would not be effective because of the ringing effect, which decreases the current rise time drastically. This can be seen in Fig. 1 where the accumulation of magnetic flux results in Cycle 2, 3 and 4 experiencing primary current l_p and secondary current l_s in excess of Max Current 5.

This is the effect of inrush current discussed earlier which is disadvantageous for digital devices as the peaks in the primary current raises the overall current amplitude of the circuit above the allowed specifications.

Referring to FIG. 2, the Flyback converter comprises of a Flyback transformer 120, a primary current sensor 144, a secondary current sensor 130, a switching control 150, and a voltage regulator 170. The Flyback converter further comprises a power switch or a switching means which in accordance with the present invention is a Field Effect Transistor (FET) 140. The switching control 150 is coupled to both the primary current sensor 144 and the secondary current sensor 130. The Flyback transformer 120 comprises a primary coil 122 and a secondary coil 124. The primary current sensor 144 for detecting a predetermined primary current amplitude from the primary coil 122 is coupled to the primary coil 122. Upon detecting the predetermined primary current amplitude, the primary current sensor 144 sends an OFF signal 146 to the switching control 150. The secondary current sensor 130 for sensing a predetermined secondary current amplitude from the secondary coil 124 is coupled to the secondary coil 124. Upon detecting the predetermined secondary current amplitude, the secondary current sensor sends an ON signal 134 to the switching control 150. The switching control 150 is coupled to the FET 140 and the FET 140 is further coupled to both the primary coil 122 and the primary current sensor 144.

In operation, a charge signal 112 is sent to the switching control 150. The switching control 150 then switches the FET 140 to an ON state. This enables a primary current from power source Vin 105 to flow through the primary coil 122 of the Flyback transformer 120. The primary current is converted into energy and stored as magnetic flux in the Flyback transformer 120. As more magnetic flux is stored in the Flyback transformer 120, the primary current correspondingly increases. When the primary current reaches a predetermined amplitude corresponding to magnetic flux stored in the primary coil, the primary current sensor 144 sends the OFF signal 146 to the switching control 150. The switching control 150 then switches the FET to an OFF state. This stops the primary current from flowing in the primary coil 122. The magnetic flux stored in the Flyback transformer 120 now discharges through the secondary coil 124 as a secondary current. The secondary current is used to charge a capacitor 190 which is coupled to the secondary coil 124 via a charging diode 180. As the magnetic flux discharge from the Flyback transformer 120, the secondary current correspondingly decreases. When the secondary current reaches the predetermined secondary amplitude, the secondary current sensor 130 sends the ON signal 134 to the switching driver 150 which then switches the FET 140 to an ON state. This cycle repeats itself until the capacitor 190 is fully charged. The Flyback converter further comprises a voltage regulator 170. The voltage regulator 170 senses voltage across the capacitor 190. When the capacitor 190 is fully charged and the voltage across the capacitor 190 reaches a predetermined amplitude, the voltage regulator 170 will force the FET 140 to an OFF state. This stops the cycle from repeating itself once the capacitor 190 is fully charged. The primary current sensor 144, the secondary current sensor 130, and the switching control 150 are all coupled to a power source which is designated here as Vcc 110.

Referring to FIG. 3, the Flyback converter is shown here with the details of the primary current sensor 144, the secondary current sensor 130, and the switching control 150 shown.

The secondary current sensor 130 comprises a sensing resistor 230 or sometimes referred to as a current sensing resistor. The sensing resistor 230 is coupled between the secondary coil 124 of the Flyback transformer 120 and the switching control 150 and is also connected to Vcc 110.

The switching control 150 comprises a switching transistor 270, a first delay capacitor 280, a second delay capacitor 260, a following transistor 220, a first resistor 262, a second resistor 264, and a sensing transistor 210. The first delay capacitor 280 being coupled between an emitter 272 and a base 274 of the switching transistor 270. The emitter 272 of the switching transistor 270 is coupled to the secondary coil 124 and a collector 276 of the switching transistor 270 coupled to the primary current sensor 144. The collector 212 of the sensing transistor 210 is coupled to the second resistor 264 which in turn is coupled to the first resistor 262. An emitter 216 of the sensing transistor 210 is coupled to GND 107 via a regulating resistor 218. The first resistor 262 is further coupled to Vcc 110. A base 224 of the following transistor 220 is coupled to a point between the first resistor 262 and the second resistor 264. A collector 226 of the following transistor 220 is coupled to the voltage regulator 170 and to a input contact for receiving a charge 112 signal via a resistor 266. An emitter 222 of the following transistor 220 is coupled to both the voltage regulator 170 and to Vcc 110. The second delay capacitor 260 being coupled parallel to the second resistor 264 and between the base 224 of transistor following 220 and the collector 212 of the sensing transistor 210.

The primary current sensor 144 comprises: a blocking diode 144 coupled to an offset diode 250, the offset diode 252 further coupled to a high ohmic resistor 256 and a potentiometer 254 coupled in parallel with the offset diode 252. The collector 276 of the switching transistor 270 of the switching control 150 is coupled to a gate 244 of the FET 240. The high ohmic resistor 256 is also coupled to the gate 244 and to a grounding resistor 258 which is connected to ground (GND) 107. A source 246 of the FET 240 is also coupled to GND 107. The high ohmic resistor 256 is further connected to the offset diode 252 which is further connected to the blocking diode 250. The blocking diode 250 is further connected to a drain 242 of the FET 240. The potentiometer 254 is connected across the offset diode 252 such that the potentiometer 254 is in parallel with the offset diode 252. A variable contact 255 of the potentiometer 254 is coupled to a base 214 of the sensing transistor 210.

In operation, a charge signal 112 is first sent to the switching control

150. The switching control 150 switches the transistor 270 to an ON state.

Thereafter, the FET 240 is also switched ON. This enables the primary current from power source Vin 105 to flow through the primary coil 122 of the Flyback transformer 120. As the primary current increases corresponding to the magnetic flux stored in the Flyback transformer, the voltage across the FET 240 also increases but the resistance of the FET 240 remains constant.

Since the transistor 270 is in an ON state, current flows from Vcc 110 through the transistor to the primary current sensor 144. A voltage is experienced across the offset diode 252 and across the blocking diode 250.

The total voltage across the FET 240, the blocking diode 250, the offset diode 252 is then reduced by the potentiometer 254 and placed at the base 214 of the sensing transistor 210 via the variable contact 255. This total voltage is initially insufficient to switch ON the sensing transistor 210. However, as the voltage across the FET 240 increases with the increasing primary current, this total voltage also increases until the sensing transistor 210 switches ON. Thereafter, the following transistor 220 of the switching control 150 is also switched ON causing current from Vcc 110 to stop flowing into the base 274 of transistor 270 and therefore the transistor 270 switches OFF. The second delay capacitor 260 functions to increase time delay before the following transistor 220 is switched ON.

Thereafter, the FET 240 is also switched OFF. This stops the primary current from flowing in the primary coil 122. The magnetic flux stored in the Flyback transformer 120 now discharges through the secondary coil 124 as the secondary current. The voltage across the sensing resistor 230 of the secondary current sensor 130 results in the base-emitter voltage of the transistor 270 being reverse bias, as such the transistor 270 remains in an OFF state. When the secondary current drops to the predetermined amplitude, the transistor 270 switches to an ON state as the base-emitter voltage is now forward bias. The delay capacitor 280 adds to the time taken for the base-emitter voltage of the transistor 270 to become forward-bias which consequently switches the transistor 270 to an ON state. Thereafter, current flows from the transistor 270 to the FET 240 and also switches the FET 240 ON. This enables the primary current from power source Vin 105 to flow through the primary coil 122 of the Flyback transformer 120. This cycle thus repeats itself until the capacitor 190 is fully charged.

The FET 240 in the present invention is a switching means by which current is supplied and cut off to the Flyback transformer 120 thereby producing the chain of current pulses that are used for charging of capacitive loads. The FET 240 here also works in combination with the primary current sensor 144 for detecting a predetermined primary current amplitude from the primary coil 122. The blocking diode 250 of the primary current sensor 144 is coupled to the gate 242 of the FET 240. This allows current sensing directly from the gate 242 of the FET 240. This is because the FET 240 drain-source junction exhibits a constant resistance as long as the FET 240 is in an ON state. The use of the FET 240 thus advantageously eliminates the use of a current sensing resistor while at the same time providing the function of a switching means.

Referring to FIG. 4, the waveforms shown are the magnetic flux (φ) stored in the Flyback transformer, the primary current (l_p) during the ON time and the secondary current (l_s)during the OFF time of a Flyback converter in accordance with the present invention.

The ON time in Cycle 2 and Cycle 3 are appreciatively shorter than the

ON time in Cycle 1. This shorter ON time prevents current amplitude of the Flyback converter from exceeding the designated Max Current 5. The varying ON times as determined by the Flyback converter ensures that magnetic flux stored in the Flyback transformer does not result in high levels of secondary current which would exceed Max Current 5.

Claims

1. A Flyback converter circuit for charging a capacitive load, said Flyback converter comprising: a switching means, a Flyback transformer, coupled to said switching means for converting a current from a power source into an oscillating current for charging said capacitive load; said Flyback transformer further having a primary coil and a secondary coil; a primary current sensor, coupled to said switching means and said primary coil, said primary current sensor for sensing a predetermined primary current amplitude from said primary coil and for providing an OFF signal; a switching control, coupled to said primary current sensor and to said switching means, said switching control for receiving said OFF signal from said primary current sensor and for switching OFF said switching means, whereby said switching control further for providing a time delay before switching OFF said switching means.

2. A Flyback converter in accordance with claim 1, wherein said Flyback converter further comprises a secondary current sensor coupled to said secondary coil for sensing a predetermined secondary current amplitude from said secondary coil and for providing an ON signal;

3. A Flyback converter in accordance with claim 2, wherein said switching control for receiving said ON signal from said secondary current sensor and for switching ON said switching means;

4. A Flyback converter circuit in accordance with claim 2, wherein said secondary sensor comprises a current sensing resistor.

5. A Flyback converter in accordance with claim 1 , wherein said Flyback converter further comprises of a voltage regulator.

6. A Flyback converter in accordance with claim 1, wherein said primary current sensor further comprises: a blocking diode coupled to an offset diode, said offset diode further coupled to a high ohmic resistor and a potentiometer coupled in parallel with said offset diode.

7. A Flyback converter circuit in accordance with claim 1 , wherein said switching control further comprises a switching transistor, a delay capacitor, a following transistor, a first resistor, a second resistor, and a sensing transistor.

8. A Flyback converter circuit in accordance with claim 1, wherein said switching means is coupled to said primary coil of said Flyback transformer.

9. A Flyback converter circuit in accordance with claim 1, wherein said switching means comprises of a field effect transistor.

10. A Flyback converter circuit for charging a capacitive load, said Flyback converter comprising: a Field Effect Transistor, a Flyback transformer, coupled to said Field Effect Transistor for converting a current from a power source into an oscillating current for charging said capacitive load; said Flyback transformer further having a primary coil and a secondary coil; a primary current sensor, coupled to said Field Effect Transistor and said primary coil, said primary current sensor for sensing a predetermined primary current amplitude from said primary coil and for providing an OFF signal; a switching control, coupled to said primary current sensor, said secondary current sensor and said Field Effect Transistor, said switching control for receiving said OFF signal from said primary current sensor and for switching OFF said Field Effect Transistor, whereby said switching control further for providing a time delay before switching OFF said Field Effect Transistor.

11. A Flyback converter in accordance with claim 10, wherein said Flyback converter further comprises a secondary current sensor coupled to said secondary coil for sensing a predetermined secondary current amplitude from said secondary coil and for providing an ON signal;

12. A Flyback converter in accordance with claim 11, wherein said switching control for receiving said ON signal from said secondary current sensor and for switching ON said Field Effect Transistor;

13. A Flyback converter circuit in accordance with claim 11 , wherein said secondary sensor comprises a current sensing resistor.

14. A Flyback converter in accordance with claim 10, wherein said Flyback converter further comprises of a voltage regulator.

15. A Flyback converter in accordance with claim 10, wherein said primary current sensor further comprises: a blocking diode coupled to an offset diode, said offset diode further coupled to a high ohmic resistor and a potentiometer coupled in parallel with said offset diode.

16. A Flyback converter circuit in accordance with claim 10, wherein said switching control further comprises a switching transistor, a delay capacitor, a following transistor, a first resistor, a second resistor, and a sensing transistor.

17. A Flyback converter circuit in accordance with claim 10, wherein said Field Effect Transistor is coupled to said primary coil of said Flyback transformer.