Control method and device employing primary side regulation in a quasi-resonant AC/DC flyback converter

Patent metadata

Field	Value
Publication number	US-9455636-B2
Application number	US-201414572627-A
Country	US
Kind code	B2
Filing date	Dec 16, 2014
Priority date	Dec 16, 2014
Publication date	Sep 27, 2016
Grant date	Sep 27, 2016

Abstract

The present disclosure is directed to a primary-controlled high power factor quasi resonant converter. The converter converts an AC power line input to a DC output to power a load, generally a string of LEDs, and may be compatible with phase-cut dimmers. The power input is fed into a transformer being controlled by a power switch. The power switch is driven by a controller having a shaping circuit. The shaping circuit uses a current generator, switched resistor and capacitor to produce a reference voltage signal. The controller drives the power switch based on the voltage reference signal, resulting in a sinusoidal input current in a primary winding of the transformer, resulting in high power factor and low total harmonic distortion for the converter.

First claim

The invention claimed is:

1. A device for controlling a power transistor of a power stage, comprising:
a divider having a first input, a second input and an output, the divider being configured to produce a voltage reference signal;
a first current generator configured to produce an output current;
a shaper circuit configured to output to the first input of the divider a first signal based on the output current of the first current generator;
a bias circuit configured to output a second signal to the second input of the divider based on the first signal; and
a driver circuit having a first input, configured to receive the voltage reference signal, and an output configured to drive the power transistor.

2. The device of claim 1 , the shaper circuit including:
a capacitor coupled between the first current generator and a supply voltage terminal; and
a switch coupled between the first current generator and the supply voltage terminal, the switch being configured to couple the first current generator to the supply voltage terminal when the power transistor is on.

3. The device of claim 2 , the shaper circuit including:
a voltage input terminal configured to receive an input voltage from the power stage; and
a multiplier having a first input configured to receive a first voltage based on the output current of the first current generator, a second input coupled to the voltage input terminal and an output coupled to the first input of the divider.

4. The device of claim 1 , comprising a voltage input terminal configured to receive an input voltage from the power stage, wherein the output current is proportional to the input voltage at the voltage input terminal.

5. The device of claim 1 , further comprising:
a phase angle detection circuit configured to compare a input voltage to a threshold voltage and to output a mask signal when the dimmer voltage is less than the threshold voltage; and
a dimming current generator configured to increase the second signal based on the mask signal.

6. The device of claim 5 , wherein the dimming current generator is configured to output a dimming current that is proportional to a voltage of the second signal.

7. The device of claim 1 , the bias circuit having:
a second current generator configured to produce a current proportional to the first signal;
a second capacitor coupled between the second input of the divider and a supply voltage terminal;
a first switch configured to connect the second current generator to the second capacitor when the first switch is on.

8. The device of claim 7 , the bias circuit having a second switch configured to couple the second current generator to the supply voltage terminal when the first switch is off.

9. The device of claim 1 , the bias circuit having:
an amplifier configured to amplify the first signal and produce an amplified signal;
a resistor coupled between the amplifier and the second input of the divider;
a first switch coupled between the amplifier and the resistor and configured to couple the amplifier to second input of the divider through the resistor when the first switch is on; and
a second switch coupled between the resistor and a supply voltage terminal, and configured to couple the second input of the divider to the supply voltage terminal when the first switch is off.

10. The device of claim 1 , wherein the shaper circuit includes:
a voltage feedforward circuit, having:
an input node configured to receive an input voltage from the power stage;
a peak detector configured to detect a peak of the input voltage;
a quadratic divider configured to receive an output of the peak detector and to output a feedforward signal based on the detected peak of the input voltage; and
a multiplier configured to multiply the feedforward signal by a signal based on the output current of the current generator and provide the first signal to the first input of the divider.

11. A device, comprising:
a divider having a first input, second input and an output;
a shaper circuit configured to provide a first signal to the first input of the divider, the shaper circuit having:
a first current generator having an output at which the first current generator is configured to produce an output current;
a first capacitor coupled between the output of the first current generator and a voltage reference terminal;
a first resistor coupled to the first current generator;
a first switch configured to couple the first resistor in parallel with the first capacitor when the first switch is on; and
an intermediate node configured to provide the first signal to the first input of the divider based on a voltage of the capacitor; and

a bias circuit configured to provide a second signal to the second input of the divider, the bias circuit having:
a current source configured to produce a current proportional to the first signal; and
a second switch configured to couple the output of the second current generator to the second input of the divider when the second switch is on.

12. The device of claim 10 , the bias circuit having:
a second resistor coupled to the second input of the divider; and
a third switch configured to couple current source to the voltage reference terminal when the second switch is off.

13. The device of claim 10 , the shaper circuit having:
an input voltage terminal configured to receive an input voltage; and
a multiplier having a first input coupled to the first capacitor, a second input coupled to the input voltage terminal, and an output coupled to the first input of the divider.

14. The device of claim 10 , wherein the bias circuit includes:
a phase angle detection circuit configured to compare an input voltage to a threshold voltage and to output a mask signal when the dimmer voltage is less than the threshold voltage; and
a dimming current generator configured to increase the second signal based on the mask signal.

15. The device of claim 13 , wherein the dimming current generator is configured to output a dimming current that is proportional to a voltage of the second signal.

16. A system, comprising:
a rectifier configured to output a rectified voltage;
a transformer having a primary winding coupled to an output of the rectifier;
a power transistor coupled to the primary winding of the transformer and configured to control the transformer;
a driver circuit configured to control the power transistor and configured to receive a reference signal;
a control circuit having:
a divider having a first input, a second input and an output, the divider being configured to produce the reference signal at the output of the divider;
a first current generator configured to produce an output current;
a shaper circuit configured to output to the first input of the divider a first signal based on the output current of the first current generator; and
a bias circuit configured to output a second signal to the second input of the divider based on the first signal.

17. The system of claim 16 , wherein the transformer includes a secondary winding, the system further comprising at least one light emitting diode coupled to the secondary winding of the transformer.

18. The system of claim 16 , comprising a voltage divider configured to provide a reduced voltage based on the rectified voltage, wherein the shaper circuit includes:
a multiplier having a first input configured to receive a first voltage based on the output current of the first current generator, a second input coupled to the voltage divider, and an output coupled to the first

Assignees

• St Microelectronics Srl

Inventors

• Gritti Giovanni
• Adragna Claudio

Classifications

• H05B45/10

  Controlling the intensity of the light · CPC title

• H02M1/4241

  using a resonant converter · CPC title

• H02M1/08

  Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters · CPC title

• H02M1/12

  Arrangements for reducing harmonics from AC input or output · CPC title

• H02M3/33507 Primary

  with automatic control of the output voltage or current, e.g. flyback converters (H02M3/33561, H02M3/33569 take precedence) · CPC title