Start-up controller for a power converter

Patent metadata

Field	Value
Publication number	US-2016149504-A1
Application number	US-201514945729-A
Country	US
Kind code	A1
Filing date	Nov 19, 2015
Priority date	Nov 20, 2014
Publication date	May 26, 2016
Grant date	—

Abstract

Power converters, e.g., AC/DC and DC/DC, typically have unique circuitry for a proper graceful start-up and to develop correct operating voltage biases. Typically this unique circuitry is incorporated in a primary-side controller. This primary-side controller could also be the primary means of control of the power converter once started. However, a secondary-side controller is typically needed for more exact output voltage regulation, duplicating circuitry already present in the primary-side controller. Complication is typically added by linear communication between the two controllers across an isolation barrier. A simplified primary-side start-up controller is envisioned providing minimal circuitry to power up a converter until a secondary-side controller activates and takes control by sending discrete PWM commands across the isolation barrier instead of a linear signal. The start-up controller can provide voltage and current protection if the secondary-side controller fails. The secondary-side controller can be an analog and/or digital design for sophisticated converter control.

First claim

1 . A method for starting up a power converter, said method comprising the steps of:
applying a first DC voltage to a start-up controller; turning on and off a power switch with the start-up controller, wherein the first DC voltage and the power switch are coupled to a primary winding of a transformer, whereby an AC voltage is produced on a secondary winding of the transformer; rectifying the AC voltage from the secondary winding of the transformer with a second rectifier to provide a second DC voltage for powering a secondary-side controller and a load; and transferring control of the power switch from the start-up controller to the secondary-side controller when the second DC voltage is at a desired voltage value.

2 . The method according to claim 1 , wherein the start-up controller is initially powered directly from the first DC voltage and then from a tertiary winding of the transformer.

3 . The method according to claim 1 , wherein the step of turning on and off the power switch with the start-up controller comprises the steps of:
turning on the power switch until a maximum current through the primary winding of the transformer is reached; and thereafter turning off the power switch for a fixed time period.

4 . The method according to claim 3 , wherein the fixed time period is determined by a capacitance value of a capacitor coupled to the start-up controller.

5 . The method according to claim 1 , further comprising the step of decoupling the load from the second DC voltage until requested to couple the load to the second DC voltage.

6 . The method according to claim 1 , wherein the load is coupled to the second DC voltage after the secondary-side controller starts controlling the power switch.

7 . The method according to claim 1 , further comprising the step of preventing an overvoltage of the second DC voltage by coupling a voltage shunt thereacross.

8 . The method according to claim 7 , wherein the voltage shunt is a Zener diode having a breakdown voltage higher than a desired value for the second DC voltage.

9 . The method according to claim 1 , wherein the step of transferring control of the power switch from the start-up controller to the secondary-side controller comprises the steps of:
sending PWM signals from the secondary-side controller to the start-up controller when the second DC voltage is at the desired voltage value; detecting the PWM signals from the secondary-side controller with the start-up controller; and turning on and off the power switch with the detected PWM signals from the secondary-side controller.

10 . The method according to claim 9 , wherein the second DC voltage is regulated by the secondary-side controller after the start-up controller detects the PWM signals from the secondary-side controller.

11 . The method according to claim 1 , wherein the step of controlling the power switch further comprises the steps of:
turning on and off the power switch at a low frequency with the start-up controller to conserve power; and turning on and off the power switch at a higher frequency with the secondary-side controller.

12 . The method according to claim 9 , wherein the step of sending PWM signals from the secondary-side controller to the start-up controller further comprises the step of sending PWM signals through a voltage isolation circuit.

13 . The method according to claim 12 , wherein the voltage isolation circuit is an optical-coupler.

14 . The method according to claim 12 , wherein the voltage isolation circuit is a pulse transformer.

15 . The method according to claim 1 , wherein the power converter comprises a flyback power converter.

16 . The method according to claim 1 , wherein the power converter comprises a forward power converter.

17 . The method according to claim 1 , wherein the start-up controller protects a power switch driver from under and over voltages.

18 . The method according to claim 1 , further comprises the step of limiting a maximum allowable transformer primary winding current with the start-up controller.

19 . The method according to claim 15 , further comprising the step of preventing the flyback power converter from going into too deep a continuous conduction mode with a current-sense comparator, whereby the flyback power converter is protected from an over-current fault.

20 . The method according to claim 15 , further comprising the steps of:
providing bias voltage to the start-up controller from a primary-side tertiary winding of the transformer, wherein the bias voltage is coupled to the second DC voltage and provides voltage feedback thereof; detecting an overvoltage condition from the bias voltage when the secondary side controller fails to properly operate; and locking out the start-up controller when the overvoltage condition is detected.

21 . The method according to claim 16 , further comprising the step of providing a linear regulator between an output of a primary-side tertiary winding of the transformer and a bias input of the start-up controller.

22 . The method according to claim 16 , further comprising the step of clamping a secondary side reset winding of the transformer to provide a transformer reset.

23 . The method according to claim 22 , further comprising the step of providing initial bias for the secondary-side controller from an active clamp circuit until a bias from a tertiary winding of an output filter inductor is established.

24 . The method according to claim 1 , further comprising the step of applying AC power to a first rectifier for providing the first DC voltage.

25 . A power converter, comprising:
a start-up controller coupled to a first DC voltage; a transformer having primary and secondary windings, wherein the transformer primary winding is coupled to the first DC voltage; a current measurement circuit for measuring current through the primary winding of the transformer and providing the measured primary winding current to the start-up controller; a power switch coupled to the transformer primary, and coupled to and controlled by the start-up controller; a secondary-side rectifier coupled to the transformer secondary winding for providing a second DC voltage; a secondary-side controller coupled to the start-up controller and the secondary-side rectifier; wherein when the start-up controller receives the first DC voltage it start to control the power switch on and off whereby a current flows through the transformer primary, an AC voltage develops across the transformer secondary winding, a DC voltage from the secondary side rectifier powers up the secondary-side controller, and the secondary-side controller takes over control of the power switch from the start-up controller when the second DC voltage reaches a desired voltage level.

26 . The power converter according to claim 25 , wherein the power converter comprises a flyback power converter.

27 . The power converter according to claim 25 , wherein the power converter comprises a forward power converter.

28 . The power converter according to claim 25 , further comprising a switching post regulator between the secondary side rectifier and a load, wherein the switching post regulator is controlled by the secondary-side controller.

29 . The power converter according to claim 25 , wherein the power switch is a power metal

Assignees

• Microchip Tech Inc

Inventors

• Quigley Thomas

Classifications

• H02M1/36 Primary

  Means for starting or stopping converters · CPC title

• H02M3/33523 Primary

  with galvanic isolation between input and output of both the power stage and the feedback loop · CPC title

• H02M3/33546 Primary

  with automatic control of the output voltage or current (H02M3/33561 takes precedence) · CPC title