Control circuit for controlling output current at the primary side of a power converter

• US 6,977,824 B1
• Filed: 08/09/2004
• Issued: 12/20/2005
• Est. Priority Date: 08/09/2004
• Status: Expired due to Term

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: System General Corporation (ON Semiconductor Corporation)
• Inventors: Yang, Ta-yung, Hung, Guo-Kiang
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US10/914,802
• Time in Patent Office: 498 Days
• Field of Search: 363/21.12, 363/21.16, 363/21.17, 363/56.01
• US Class Current: 363/21.16
• 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 control circuit controlling output current at the primary side of a power converter is provided. A waveform detector generates a current-waveform signal. A discharge-time detector detects a discharge-time of a secondary side switching current. An oscillator generates an oscillation signal for determining the switching frequency of the switching signal. An integrator generates an integrated signal by integrating an average current signal with the discharge-time. The average current signal is generated in response to the current-waveform signal. The time constant of the integrator is correlated with the switching period of the switching signal, therefore the integrated signal is proportional to the output current. An error amplifier amplifies the integrated signal and provides a loop gain for output current control. A comparator controls the pulse width of the switching signal in reference to the output of the error amplifier. Therefore, the output current of the power converter can be regulated.

Claims

1. A control circuit for controlling an output current at a primary side of a power converter, comprising:

a waveform detector, generating a current-waveform signal by sampling a primary side switching current of a transformer through a current-sense device;

a discharge-time detector, coupled to said transformer for detecting a discharging time of a secondary side switching current;

an integrator, for generating an integrated signal in response to said discharging time and said current-waveform signal; and

a switching control circuit, including a reference voltage, wherein said switching control circuit generates a switching signal in response to said integrated signal and said reference voltage;

wherein said switching signal switches said transformer and regulating the output current of said power converter in accordance with said reference voltage.

2. The control circuit as recited in claim 1, further comprising an oscillator, for generating an oscillation signal for determining a switching frequency of said switching signal.

3. The control circuit as recited in claim 1, wherein a time constant of said integrator is correlated with a switching period of said switching signal.

4. The control circuit as recited in claim 1, wherein said waveform detector comprises:

a first comparator, having a positive input coupled to a current-sense terminal, wherein said current-sense terminal includes a primary side switching current signal, wherein a value of said primary side switching current signal is proportional to a value of said primary side switching current;

a first capacitor, for holding a peak value of said primary side switching current signal, wherein a negative input of said first comparator is coupled to said first capacitor;

a first constant current source, for charging said first capacitor;

a first switch, coupled between said first constant current source and said first capacitor, wherein said first switch is turned on/off by an output of said first comparator;

a first transistor, coupled in parallel with said first capacitor for discharging said first capacitor;

a second capacitor, for holding an initial value of said primary side switching current signal;

a second switch, coupled between said current-sense terminal and said second capacitor, wherein said second switch is turned on/off by a store signal, wherein said store signal is a pulse signal generated in response to the rising edge of a delayed switching signal, wherein said delayed switching signal is generated in response to the rising edge of said switching signal after a delay time;

a second transistor, coupled in parallel with said second capacitor for discharging said second capacitor;

a third capacitor;

a third switch, for periodically sampling a voltage across said first capacitor to said third capacitor;

a fourth capacitor; and

a fourth switch, for periodically sampling a voltage across said second capacitor to said fourth capacitor, wherein said current-waveform signal is obtained across said third capacitor and said fourth capacitor, wherein said third capacitor generates a first current-waveform signal and said fourth capacitor generates a second current-waveform signal.

5. The control circuit as recited in claim 1, wherein said integrator comprises:

a first V-to-I converter, having a first timing operational amplifier, a first timing resistor and a plurality of first timing transistors, wherein said first V-to-I converter generates a first programmable charge current in response to said second current-waveform signal;

a second V-to-I converter, having a second timing operational amplifier, a second timing resistor and a plurality of second timing transistors, wherein said second V-to-I converter generates a second programmable charge current in response to said first current-waveform signal and said second current-waveform signal, wherein said second timing resistor is correlated to said first timing resistor;

a timing capacitor;

a fifth switch, wherein a first terminal of said fifth switch is supplied with an average current signal and a second terminal of said fifth switch is coupled to said timing capacitor, wherein said average current signal is generated by summing said first programmable charge current and said second programmable charge current;

a third transistor, coupled in parallel with said timing capacitor to discharge said timing capacitor;

an output capacitor; and

a sixth switch, for periodically sampling a voltage across said timing capacitor to said output capacitor, wherein said integrated signal is obtained across said output capacitor.

6. The control circuit as recited in claim 2, wherein said oscillator comprises:

a third V-to-I converter, having an osc operational amplifier, an osc resistor, an osc transistor, wherein said third V-to-I converter generates a reference current;

a first osc current mirror, having a first osc transistor, a second osc transistor and a third osc transistor, wherein said third osc transistor generates an osc charge current;

a second osc current mirror, having a fourth osc transistor and a fifth osc transistor, wherein said fifth osc transistor generates an osc discharge current;

an osc capacitor;

a first osc switch, coupled between a drain of said third osc transistor and said osc capacitor;

a second osc switch, coupled between said osc capacitor and a drain of said fifth osc transistor;

an osc comparator, having a positive input coupled to said osc capacitor, wherein said osc comparator generates a oscillation signal, wherein said oscillation signal determines said switching frequency, wherein said oscillation signal turns on/off said third switch and said fourth switch;

a third osc switch, having a first terminal supplied with a high-threshold voltage and a second terminal coupled to a negative input of said osc comparator;

a fourth osc switch, having a first terminal supplied with a low-threshold voltage and a second terminal coupled to said negative input of said osc comparator;

an osc inverter, having an input coupled to an output of said osc comparator for producing an inverse oscillation signal, wherein said oscillation signal turns on/off said second osc switch and said fourth osc switch, wherein said inverse oscillation signal turns on/off said first osc switch and said third osc switch;

a first inverter, a second inverter, a third inverter and a fourth inverter coupled in series, wherein an input of said first inverter is supplied with said oscillation signal; and

an AND gate, for generating a clear signal, wherein a first input of said AND gate is coupled to an output of said fourth inverter, wherein a second input of said AND gate is coupled to an output of said first inverter, wherein said clear signal turns on/off said first transistor, said second transistor and said third transistor.

7. The control circuit as recited in claim 1, wherein said discharge-time detector comprises:

a delay circuit, having a first zcd inverter, a first zcd transistor, a first zcd constant current source, a first zcd capacitor and a first zcd AND gate, wherein an input of said delay circuit is supplied with said switching signal, wherein said delay circuit provides a propagation delay for a falling edge of said switching signal, wherein the timing of said propagation delay is determined by a current of said first zcd constant current source and a capacitance of said first zcd capacitor;

an one-shot signal generator, having a second zcd inverter, a third zcd inverter, a second zcd transistor, a second zcd constant current source, a second zcd capacitor and a second zcd AND gate, wherein said one-shot signal generator generates a voltage sample signal, wherein an input of said one-shot signal generator is coupled to an output of said delay circuit, wherein a pulse width of said voltage sample signal is determined by a current of said second zcd constant current source and a capacitance of said second zcd capacitor;

a zcd operational amplifier, performing as a buffer amplifier, wherein a positive input of said zcd operational amplifier is coupled to a voltage-detection terminal, wherein said voltage-detection terminal is coupled to said auxiliary winding of said transformer via a zcd resistor;

a sampling capacitor;

a sampling switch, coupled between an output of said buffer amplifier and said sampling capacitor, wherein said voltage sample signal turns on/off said sampling switch;

a zcd comparator, wherein a positive input of said zcd comparator is coupled to said sampling capacitor;

a reference voltage threshold, coupled between a negative input of said zcd comparator and said output of said buffer amplifier;

a fourth zcd inverter, having an input supplied with said switching signal;

a fifth zcd inverter, having an input supplied with said voltage sample signal;

a third zcd AND gate, having a first input coupled to an output of said zcd comparator;

a fourth zcd AND gate, for generating a discharge-time signal, wherein a first input of said fourth zcd AND gate is coupled to an output of said fourth zcd inverter;

a first SR flip-flop, having a set-input and a reset-input, wherein an output of said first SR flip-flop is coupled to a second input of said fourth zcd AND gate, wherein said set-input of said first SR flip-flop is coupled to said output of said fourth zcd inverter, wherein said reset-input of said first SR flip-flop is coupled to an output of said third zcd AND gate, wherein said discharge-time signal turns on/off of said fifth switch, wherein the pulse width of said discharge-time signal is correlated to said discharge-time of said secondary side switching current; and

a second SR flip-flop, having a set-input and a reset-input, wherein said set-input of said second SR flip-flop is coupled to an output of said fifth zcd inverter, wherein said reset-input of said second SR flip-flop is supplied with said switching signal, wherein an output of said second SR flip-flop is coupled to a second input of said third zcd AND gate.

8. The control circuit as recited in claim 1, wherein a resistance of said first timing resistor is correlated to a resistance of said osc resistor and a capacitance of said timing capacitor is correlated to a capacitance of said osc capacitor.

9. A control circuit for controlling an output current at a primary-side of a power converter, comprising:

a waveform detector, for generating a current-waveform signal by sampling a primary side switching current signal, wherein said primary side switching current signal is generated from a primary side switching current of a transformer;

a discharge-time detector for detecting a discharging time of a secondary side switching current via an auxiliary winding of said transformer, wherein said secondary side switching current is generated from the secondary side of said transformer;

an oscillator, for generating an oscillation signal for determining a switching frequency of a switching signal, wherein said switching signal is used for switching said transformer and regulating the output of said power converter;

an integrator, for generating an integrated signal by integrating an average current signal with said discharging time, wherein said average current signal is generated in response to said current-waveform signal;

an error amplifier, having an operational amplifier and a reference voltage for amplifying said integrated signal and providing a loop gain for controlling the output current of said power converter; and

a comparator, for controlling a pulse width of said switching signal in reference to an output of said error amplifier, wherein the output current of said power converter is regulated in response to said reference voltage.

10. The control circuit as recited in claim 9, wherein a time constant of said integrator is correlated with a switching period of said switching signal.

11. The control circuit as recited in claim 9, wherein said waveform detector comprises:

a first comparator, having a positive input coupled to a current-sense terminal, wherein said current-sense terminal includes said primary side switching current signal, wherein a value of said primary side switching current signal is proportional to a value of said primary side switching current;

a first capacitor, for holding a peak value of said primary side switching current signal, wherein a negative input of said first comparator is coupled to said first capacitor;

a first constant current source, for charging said first capacitor;

a first switch, coupled between said first constant current source and said first capacitor, wherein said first switch is turned on/off by an output of said first comparator;

a first transistor, coupled in parallel with said first capacitor for discharging said first capacitor;

a second capacitor, for holding an initial value of said primary side switching current signal;

a second switch, coupled between said current-sense terminal and said second capacitor, wherein said second switch is turned on/off by a store signal, wherein said store signal is a pulse signal generated in response to the rising edge of a delayed switching signal, wherein said delayed switching signal is generated in response to the rising edge of said switching signal after a delay time;

a second transistor, coupled in parallel with said second capacitor for discharging said second capacitor;

a third capacitor;

a third switch, for periodically sampling a voltage across said first capacitor to said third capacitor;

a fourth capacitor; and

a fourth switch, for periodically sampling a voltage across said second capacitor to said fourth capacitor, wherein said current-waveform signal is obtained across said third capacitor and said fourth capacitor, wherein said third capacitor generates a first current-waveform signal and said fourth capacitor generates a second current-waveform signal.

12. The control circuit as recited in claim 9, wherein said integrator comprises:

a first V-to-I converter, having a first timing operational amplifier, a first timing resistor and a plurality of first timing transistors, wherein said first V-to-I converter generates a first programmable charge current in response to said second current-waveform signal;

a second V-to-I converter, having a second timing operational amplifier, a second timing resistor and a plurality of second timing transistors, wherein said second V-to-I converter generates a second programmable charge current in response to said first current-waveform signal and said second current-waveform signal, wherein said second timing resistor is correlated to said first timing resistor;

a timing capacitor;

a fifth switch, wherein a first terminal of said fifth switch is supplied with an average current signal and a second terminal of said fifth switch is coupled to said timing capacitor, wherein said average current signal is generated by summing said first programmable charge current and said second programmable charge current;

a third transistor, coupled in parallel with said timing capacitor to discharge said timing capacitor;

an output capacitor; and

a sixth switch, for periodically sampling a voltage from said timing capacitor to said output capacitor, wherein said integrated signal is obtained across said output capacitor.

13. The control circuit as recited in claim 9, wherein said oscillator comprises:

a third V-to-I converter, having an osc operational amplifier, an osc resistor, an osc transistor, wherein said third V-to-I converter generates a reference current;

a first osc current mirror, having a first osc transistor, a second osc transistor and a third osc transistor, wherein said third osc transistor generates an osc charge current;

a second osc current mirror, having a fourth osc transistor and a fifth osc transistor, wherein said fifth osc transistor generates an osc discharge current;

an osc capacitor;

a first osc switch, coupled between a drain of said third osc transistor and said osc capacitor;

a second osc switch, coupled between said osc capacitor and a drain of said fifth osc transistor;

an osc comparator, having a positive input coupled to said osc capacitor, wherein said osc comparator generates a oscillation signal, wherein said oscillation signal determines said switching frequency, wherein said oscillation signal turns on/off said third switch and said fourth switch;

a third osc switch, having a first terminal supplied with a high-threshold voltage and a second terminal coupled to a negative input of said osc comparator;

a fourth osc switch, having a first terminal supplied with a low-threshold voltage and a second terminal coupled to said negative input of said osc comparator;

an osc inverter, having an input coupled to an output of said osc comparator for producing an inverse oscillation signal, wherein said oscillation signal turns on/off said second osc switch and said fourth osc switch, wherein said inverse oscillation signal turns on/off said first osc switch and said third osc switch;

a first inverter, a second inverter, a third inverter and a fourth inverter coupled in series, wherein an input of said first inverter is supplied with said oscillation signal; and

an AND gate, for generating a clear signal, wherein a first input of said AND gate is coupled to an output of said fourth inverter, wherein a second input of said AND gate is coupled to an output of said first inverter, wherein said clear signal turns on/off said first transistor, said second transistor and said third transistor.

14. The control circuit as recited in claim 9, wherein said discharge-time detector comprises:

a delay circuit, having a first zcd inverter, a first zcd transistor, a first zcd constant current source, a first zcd capacitor and a first zcd AND gate, wherein an input of said delay circuit is supplied with said switching signal, wherein said delay circuit provides a propagation delay for a falling edge of said switching signal, wherein the timing of said propagation delay is determined by the current of said first zcd constant current source and a capacitance of said first zcd capacitor;

an one-shot signal generator, having a second zcd inverter, a third zcd inverter, a second zcd transistor, a second zcd constant current source, a second zcd capacitor and a second zcd AND gate, wherein said one-shot signal generator generates a voltage sample signal, wherein an input of said one-shot signal generator is coupled to an output of said delay circuit, wherein a pulse width of said voltage sample signal is determined by a current of said second zcd constant current source and a capacitance of said second zcd capacitor;

a zcd operational amplifier, performing as a buffer amplifier, wherein a positive input of said zcd operational amplifier is coupled to a voltage-detection terminal, wherein said voltage-detection terminal is coupled to said auxiliary winding of said transformer via a zcd resistor;

a sampling capacitor;

a sampling switch, coupled between an output of said buffer amplifier and said sampling capacitor, wherein said voltage sample signal turns on/off said sampling switch;

a zcd comparator, wherein a positive input of said zcd comparator is coupled to said sampling capacitor;

a reference voltage threshold, coupled between a negative input of said zcd comparator and said output of said buffer amplifier;

a fourth zcd inverter, having an input supplied with said switching signal;

a fifth zcd inverter, having an input supplied with said voltage sample signal;

a third zcd AND gate, having a first input coupled to an output of said zcd comparator;

a fourth zcd AND gate, for generating a discharge-time signal, wherein a first input of said fourth zcd AND gate is coupled to an output of said fourth zcd inverter;

a first SR flip-flop, having a set-input and a reset-input, wherein an output of said first SR flip-flop is coupled to a second input of said fourth zcd AND gate, wherein said set-input of said first SR flip-flop is coupled to said output of said fourth zcd inverter, wherein said reset-input of said first SR flip-flop is coupled to an output of said third zcd AND gate, wherein said discharge-time signal turns on/off of said fifth switch, wherein the pulse width of said discharge-time signal is correlated to said discharge-time of said secondary side switching current; and

a second SR flip-flop, having a set-input and a reset-input, wherein said set-input of said second SR flip-flop is coupled to an output of said fifth zcd inverter, wherein said reset-input of said second SR flip-flop is supplied with said switching signal, wherein an output of said second SR flip-flop is coupled to a second input of said third zcd AND gate.

15. The control circuit as recited in claim 9, wherein a resistance of said first timing resistor is correlated to a resistance of said osc resistor and a capacitance of said timing capacitor is correlated to a capacitance of said osc capacitor.