Flyback power converter having a constant voltage and a constant current output under primary-side PWM control

• US 6,862,194 B2
• Filed: 06/18/2003
• Issued: 03/01/2005
• Est. Priority Date: 06/18/2003
• Status: Expired due to Fees

• Current Assignee: Fairchild Taiwan Corp (ON Semiconductor Corporation)
• Original Assignee: System General Corporation (ON Semiconductor Corporation)
• Inventors: Yang, Ta-yung, Chen, Chern-Lin, Lin, Song-Yi, Lin, Jenn-yu G.
• Primary Examiner(s): Patel, Rajnikant B.
• Application Number:
  US10/465,467
• Publication Number:
  US 20040257833A1
• Time in Patent Office: 622 Days
• Field of Search: 363/16, 363/17, 363/21.1, 363/41, 363/95, 363/89, 363/20, 363/49, 363/80
• US Class Current: 363/16
• CPC Class Codes: H02M 1/0006 Arrangements for supplying an adequate voltage to the control circuit of converters
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence

Abstract

A primary-side flyback power converter supplies a constant voltage and a constant current output. To generate a well-regulated output voltage under varying load conditions, the power converter includes a PWM controller. The PWM controller generates a PWM signal to control a switching transistor in response to a flyback voltage detected from the first primary winding of the power supply transformer. To reduce power consumption, the flyback energy of the first primary winding is used as a DC power source for the PWM controller. The flyback voltage is sampled following a delay time to reduce interference from the inductance leakage of the transformer. To generate a more accurate DC output voltage, a bias current is pulled from the detection input to form a voltage drop across a detection resistor for compensating for the voltage drop of the output rectifying diode.

Claims

1. A flyback power converter, comprising:

a transformer having a secondary winding, a first primary winding, and a second primary winding, said first primary winding and said second primary winding each having a first terminal and a second terminal respectively, wherein said second terminal of said first primary winding is connected to said first terminal of said second primary winding, wherein said second terminal of said second primary winding is connected to a ground reference;

an output rectifier connected from said secondary winding to an output of the flyback power converter;

a switching transistor for controlling the voltage across the primary windings of the flyback power converter, wherein a drain of said switching transistor is connected to an input of the flyback power converter;

a current-sense resistor for generating a current-sense voltage, wherein said current-sense resistor is connected between a source of said switching transistor and said first terminal of said first primary winding;

a PWM controller for providing a PWM signal to drive a gate of said switching transistor, wherein said PWM controller has a VCC input for receiving power, a detection input for receiving a flyback voltage, a current-sense input for current detection, and a GND input connected to the first terminal of said first primary winding, wherein said current-sense input is connected to said source of said switching transistor;

a power-pin capacitor for starting up said PWM controller, wherein said power-pin capacitor is connected between said VCC input and said GND input of said PWM controller;

a start-up resistor for charging said power-pin capacitor, wherein said start-up resistor is connected from the input of the power converter to said VCC input of said PWM controller;

a power-pin diode connected from said second terminal of said first primary winding to said VCC input of said PWM controller;

a detection resistor for receiving a bias current to compensate for the voltage drop across said output rectifier, wherein said detection resistor is connected from said second terminal of said first primary winding to said detection input of said PWM controller;

a transit-voltage suppressor connected in parallel with said second primary winding; and

a snubber diode connected in series with said transit-voltage suppressor.

2. The flyback power converter as claimed in claim 1, wherein the output current of the power converter and the output voltage of the power converter are regulated in such a manner that they are substantially constant.

3. The flyback power converter as claimed in claim 1, wherein said flyback voltage is reflected from said secondary winding to said first primary winding and said second primary winding of said transformer substantially rapidly after the PWM signal is turned off.

4. The flyback power converter as claimed in claim 1, wherein during the on-time of the PWM signal, a primary current of said transformer is produced, wherein said primary current of said transformer generates a current-sense voltage across said current-sense resistor.

5. The flyback power converter as claimed in claim 1, wherein a bias current pulled from said detection input of said PWM controller is modulated in proportion to the output load of the power converter.

6. The flyback power converter as claimed in claim 1, wherein the PWM signal is modulated in response to said flyback voltage sampled from said detection input of said PWM controller, wherein the modulation is performed in a manner that keeps the DC output voltage substantially constant.

7. The flyback power converter as claimed in claim 1, wherein the peak primary current of said transformer is limited in response to the voltage sampled from said detection input of said PWM controller, wherein the peak primary current of said transformer is limited in a manner that keeps the output current substantially constant.

8. The flyback power converter as claimed in claim 1, wherein said PWM controller further comprises:

a voltage detection circuit for sampling said flyback voltage from said detection input of said PWM controller, wherein said voltage detection circuit produces a hold voltage and a feedback voltage, wherein said hold voltage is the voltage sampled from said detection input of said PWM controller;

a V-limit generator for producing a limit voltage in response to said hold voltage, wherein said limit voltage is used for limiting the peak primary current of said transformer;

a PWM circuit for generating said PWM signal, wherein said PWM signal is a function of said feedback voltage, said limit voltage, and said current-sense voltage; and

a pulse generator for generating a sampling pulse in response to said PWM signal, wherein said sampling pulse is used for sampling said flyback voltage.

9. The flyback power converter as claimed in claim 8, wherein said voltage detection circuit comprises:

a sample switch having an input terminal connected to said detection input of said PWM controller, wherein said sample switch is turned on and turned off by said sampling pulse;

a SH capacitor for producing said hold voltage, wherein said SH capacitor is connected to an output terminal of said sample switch;

a first SH operational amplifier having a positive terminal coupled to said SH capacitor;

a second SH operational amplifier having a positive terminal coupled to a SH reference voltage;

a SH transistor having a gate connected to an output terminal of said second SH operational amplifier, wherein said SH transistor has a source connected to a negative terminal of said second SH operational amplifier;

a SH resistor connected between said source of said SH transistor and an output of said first SH operational amplifier;

a SH current mirror having a SH input mirror transistor, a first SH output transistor, and a second SH output transistor, wherein said SH input mirror transistor is connected to a drain of said SH transistor;

a reference resistor for generating said feedback voltage, wherein said reference resistor is coupled to a drain of said second SH output transistor; and

a bias current sink for generating said bias current, wherein said bias current sink has a bias input mirror transistor and a bias output mirror transistor, wherein a drain of said first SH output transistor is connected to a drain of said bias input mirror transistor, wherein a drain of said bias output mirror transistor is connected to the detection input of said PWM controller, wherein said drain of said bias output mirror transistor pulls said bias current from the detection input, wherein said bias current is generated in response to said feedback voltage.

10. The flyback power converter as claimed in claim 8, wherein said V-limit generator comprises:

a means for generating a first limit current, wherein said means for generating said first limit current comprises a VL operational amplifier, a VL transistor, and a VL input resistor;

a means for generating a second limit current, wherein said means for generating a second limit current comprises a first VL mirror transistor and a second VL mirror transistor, wherein said means for generating said first limit current is connected to a drain of said first VL mirror transistor;

a first VL current source for restricting the maximum value of said second-limit current, wherein said first VL current source is connected to a source of said first VL mirror transistor and a source of said second VL mirror transistor;

a second VL current source; and

a VL output resistor for generating said limit voltage, wherein said VL output resistor is connected to said second VL current source and the drain of said second VL mirror transistor, wherein said limit voltage is produced in proportion to the magnitude of said hold voltage.

11. The flyback power converter as claimed in claim 8, wherein said PWM circuit comprises:

a feedback comparator having a positive input supplied with said feedback voltage, wherein said feedback comparator has a negative input connected to said current-sense input of said PWM controller;

a limit comparator having a positive input supplied with said limit voltage, wherein said limit comparator has a negative input connected to said current-sense input of said PWM controller;

a first PWM NAND gate having a first input connected to an output of said feedback comparator, wherein said first PWM NAND gave has a second input connected to an output of said limit comparator;

a second PWM NAND gate having a first input connected to an output of said first PWM NAND gate;

a blanking circuit for generating a blank signal to ensure a minimum on-time for said PWM signal, wherein said blanking circuit has an output terminal connected to a second input of said second PWM NAND gate;

an oscillator for generating a clock signal; and

a flip-flop for supplying said PWM signal to an input terminal of said blanking circuit, wherein said flip-flop is set by said clock signal, and is reset by an output of said second PWM NAND gate.

12. The flyback power converter as claimed in claim 11, wherein said blanking circuit comprises:

a means for producing a blanking time comprising a BLK transistor, a BLK current source, a BLK capacitor, and a BLK inverter;

a BLK input inverter for supplying said PWM signal to a gate of said BLK transistor to start said blanking time, said BLK input inverter having an input supplied by said PWM signal, wherein said input of said BLK input inverter is an input terminal of said blanking circuit, wherein said blanking time starts substantially soon following every rising-edge of said PWM signal; and

a BLK NAND gate for supplying said blank signal, wherein said BLK NAND gate having a first input supplied by said PWM signal and a second input connected to an output of said BLK inverter.

13. The flyback power converter as claimed in claim 8, wherein said pulse generator comprises:

a means for generating a minimum delay time having a first PG transistor, a first PG current source, a first PG capacitor, and a first PG inverter, wherein said first PG transistor has a gate supplied by said PWM signal;

a means for generating an off-time delay having a PG p-transistor, a PG n-transistor, a second PG current source, a third PG current source, and a second PG capacitor;

a means for generating a sample time having a second PG transistor, a fourth PG current source, a third PG capacitor, and a PG second inverter;

a third PG inverter having an input supplied by said PWM signal;

a PG NOR gate having a first input connected to an output of said first PG inverter and a second input connected to said second PG capacitor;

a PG NAND gate for supplying a signal to start said sample time, wherein said PG NAND gate has a first input connected to an output of said PG NOR gate, a second input connected to an output of said third PG inverter, and an output connected to a gate of said second PG transistor; and

a PG AND gate for supplying said sampling pulse, wherein said PG AND gate has a first input connected to said output of said PG NOR gate, a second input connected to said output of said third PG inverter, and a third input connected to an output of said second PG inverter, wherein the pulse width of said sampling pulse is equal to the sample time.

14. The flyback power converter as claimed in claim 13, wherein said second PG capacitor and said PG p-transistor produce a charge time while said PWM signal is turned-on.

15. The flyback power converter as claimed in claim 13, wherein said PG n-transistor, said second PG capacitor, and said third PG current source produce a discharge time while said PWM signal is turned-off, wherein said discharge time is substantially proportional to said charge time.

16. A flyback power converter comprising a transformer, an output rectifier connected from a secondary winding of said transformer to an output of the flyback power converter, a switching transistor for controlling the voltage across the primary windings of the flyback power converter, wherein a drain of said switching transistor is connected to an input of the flyback power converter, a current-sense resistor for generating a current-sense voltage, wherein said current-sense resistor is connected between a source of said switching transistor and the first terminal of said first primary winding, the flyback power converter comprising:

a PWM controller for providing a PWM signal to drive a gate of said switching transistor, wherein said PWM controller has a VCC input for receiving power, a detection input for receiving a flyback voltage, a current-sense input for current detection and a GND input connected to the first terminal of said first primary winding, and said current-sense input is connected to the source of said switching transistor, wherein said PWM signal is modulated in response to a flyback voltage sampled from the detection input of said PWM controller, wherein the modulation is performed in a manner that keeps the DC output voltage substantially constant, and a peak primary current of said transformer is limited in response to the voltage sampled from the detection input of said PWM controller, and wherein the peak primary current of said transformer is limited in a manner that keeps the output current substantially constant.

17. The flyback power converter according to claim 16, further comprising a power-pin capacitor for starting said PWM controller, wherein said power-pin capacitor is connected between the VCC input and the GND input of said PWM controller.

18. The flyback power converter according to claim 16, further comprising a start-up resistor for charging said power-pin capacitor, wherein said start-up resistor is connected from the input of the flyback power converter to the VCC input of said PWM controller.

19. The flyback power converter according to claim 16, further comprising a power-pin diode connected from the second terminal of said first primary winding to the VCC input of said PWM controller.

20. The flyback power converter according to claim 16, further comprising a detection resistor for receiving a bias current to compensate for the voltage drop across said output rectifier, wherein said detection resistor is connected from the second terminal of said first primary winding to said voltage detection input of said PWM controller.

21. The flyback power converter according to claim 16, further comprising a transit-voltage suppressor connected in parallel with said second primary winding.

22. The flyback power converter according to claim 16, further comprising a snubbed diode connected in series with said transit-voltage suppressor.