Clamp voltage detection and over-voltage protection for power supply topologies

Patent metadata

Field	Value
Publication number	US-10148188-B2
Application number	US-201715676445-A
Country	US
Kind code	B2
Filing date	Aug 14, 2017
Priority date	Sep 6, 2016
Publication date	Dec 4, 2018
Grant date	Dec 4, 2018

Abstract

An active clamp flyback controller includes first and second input terminals, a clamp voltage detection circuit, and an overvoltage protection circuit. The first input terminal is adapted to be coupled to a terminal of a clamp capacitor. The second input terminal receives a feedback signal proportional to a voltage across an auxiliary winding of a flyback transformer. The clamp voltage detection circuit is coupled to the first and second input terminals, and detects a clamp voltage as a difference between a voltage at the first input terminal and an input voltage, the clamp voltage detection circuit calculating the input voltage using a signal from the second input terminal. The overvoltage protection circuit is coupled to the clamp voltage detection circuit for comparing the clamp voltage to a threshold and triggering a protection operation if the clamp voltage is greater than the threshold.

First claim

What is claimed is:

1. An active clamp flyback controller comprising:
a first input terminal adapted to be coupled to a terminal of a clamp capacitor;
a second input terminal for receiving a sense voltage proportional to a voltage across an auxiliary winding of a flyback transformer;
a clamp voltage detection circuit coupled to said first and second input terminals, for detecting a clamp voltage as a difference between a voltage at said first input terminal and an input voltage, said clamp voltage detection circuit calculating said input voltage using said sense voltage without measuring said input voltage directly; and
an overvoltage protection circuit coupled to said clamp voltage detection circuit for comparing said clamp voltage to a threshold and selectively triggering a protection operation if said clamp voltage is greater than said threshold.

2. The active clamp flyback controller of claim 1 , wherein said clamp voltage detection circuit comprises:
a first peak detector having an input selectively coupled to said first input terminal, a control input for receiving a startup control signal, and an output for providing a peak input voltage as a peak value at said first input terminal during an activation of said startup control signal;
a second peak detector having an input coupled to said second input terminal, a control input for receiving a sample signal, and an output for providing a peak input current as a value of a peak voltage at said second input terminal during an activation of said sample signal; and
a calculation block coupled to said first and second peak detectors and to said second input terminal, for calculating said input voltage by multiplying said peak input voltage by a ratio of a current at said second input terminal to said peak input current.

3. The active clamp flyback controller of claim 2 , further comprising:
a third terminal for conducting a power supply voltage to internal circuits of the active clamp flyback controller; and
a startup bypass circuit having an input coupled to said first input terminal, a first output coupled to said third terminal and to internal circuitry of the active clamp flyback controller, and a second output for providing said startup control signal.

4. The active clamp flyback controller of claim 3 , wherein said startup bypass circuit comprises:
a switch having a first terminal coupled to said first input terminal, a second terminal coupled to said third terminal, and a control terminal; and
a comparator having a first input coupled to said third terminal, a second input for receiving an undervoltage lockout threshold, and an output coupled to said control terminal of said switch.

5. The active clamp flyback controller of claim 1 , wherein said clamp voltage detection circuit comprises:
a switch having a first current electrode coupled to said first input terminal, a control terminal, and a second current electrode for providing said clamp voltage;
a resistor having a first terminal coupled to said second current electrode of said switch, and a second terminal coupled to ground; and
a transconductance amplifier having a first terminal for receiving said calculation of said input voltage, a second terminal coupled to a reference voltage terminal, and an output coupled to said first terminal of said resistor.

6. The active clamp flyback controller of claim 5 , wherein said overvoltage protection circuit comprises:
a comparator having a first input for receiving said calculation of said clamp voltage, a second input for receiving a threshold voltage, and an output for providing an overvoltage protection signal; and
a timing and control circuit having an input coupled to said output of said comparator, and an output coupled to said control terminal of said switch, wherein said timing and control circuit activates said switch to discharge a voltage on said first input terminal in response to an activation of said overvoltage protection signal.

7. The active clamp flyback controller of claim 6 , wherein said timing and control circuit activates said switch for a predetermined period of time to discharge said voltage on said first input terminal.

8. The active clamp flyback controller of claim 6 , wherein said timing and control circuit activates said switch until said clamp voltage falls below a low threshold.

9. The active clamp flyback controller of claim 1 , wherein said protection operation comprises:
discharging a voltage on said first input terminal.

10. An active clamp flyback controller for use in a power supply system having a flyback transformer and a clamp capacitor coupled in series with a switch between first and second ends of a primary winding of the flyback transformer, wherein the first end of the primary winding receives an input voltage, comprising: a clamp voltage detection circuit coupled to the clamp capacitor, for determining a clamp voltage across the clamp capacitor as a difference between a voltage at a first terminal of the clamp capacitor and the input voltage, said clamp voltage detection circuit calculating the input voltage using a sense voltage proportional to a voltage across an auxiliary winding of the flyback transformer without measuring the input voltage directly; and an overvoltage protection circuit for comparing said clamp voltage to a first threshold and triggering a protection operation if said clamp voltage is greater than said first threshold, wherein: when said overvoltage protection circuit is in a green mode, said overvoltage protection circuit selectively discharges said clamp capacitor if said clamp voltage is greater than said first threshold, and said clamp voltage detection circuit stops monitoring said clamp voltage while said active clamp flyback controller remains in said green mode, and when said overvoltage protection circuit is in not in said green mode, said overvoltage protection circuit periodically measures said clamp voltage and triggers said protection operation if said clamp voltage is greater than said first threshold.

11. The active clamp flyback controller of claim 10 , wherein in said green mode, said overvoltage protection circuit discharges said clamp capacitor until said clamp voltage is below a second threshold.

12. The active clamp flyback controller of claim 10 , wherein when said overvoltage protection circuit is not in said green mode, said overvoltage protection circuit discharges said clamp voltage until either said clamp voltage is below a second threshold lower than said first threshold, or a predetermined amount of time has elapsed.

13. The active clamp flyback controller of claim 12 , wherein after said predetermined amount of time has elapsed, said overvoltage protection circuit determines whether said clamp voltage is below said first threshold, and wherein:
if said clamp voltage is below said first threshold but not below said second threshold, then said overvoltage protection circuit continues to periodically monitor said clamp voltage; and
if said clamp voltage is not below said first threshold, then said overvoltage protection circuit performs an overvoltage protection operation.

14. The active clamp flyback controller of claim 10 , wherein said clamp voltage detection circuit determines said clamp voltage across said clamp capacitor by measuring a voltage on a first terminal of said clamp capacitor and calculating a voltage on a second terminal of said clamp capacitor without measuring said voltage on said second terminal of said clamp capacitor.

15. The active clamp flyback controller of claim 14 , wherein said clamp voltage detection circuit calculates said v

Assignees

• Fairchild Semiconductor

Inventors

• Koo Gwanbon
• RANA Dibyendu

Classifications

• H02M1/36

  Means for starting or stopping converters · CPC title

• H02M2001/0058

  Electricity · mapped topic

• H02M2001/0006

  Electricity · mapped topic

• H02M1/32 Primary

  Means for protecting converters other than automatic disconnection · CPC title

• H02M1/08

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