Start-up controller for a power converter
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.
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, wherein the start-up controller is configured to operate in a first and in a second mode, wherein in the first mode during start-up, the start-up controller operates as an open loop current regulator turning on and off a power switch, 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, wherein current in the primary winding of the transformer is allowed to rise to a maximum current level monitored by the start-up controller, wherein a fixed OFF time period of the power switch is set by an external capacitor;
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
wherein control of the power switch is transferred from the start-up controller to the secondary-side controller when the second DC voltage is at a desired voltage value by operating the start-up controller in the second mode, in which the start-up controller receives PWM signals from secondary side and a switch is controlled to feed said PWM signals to the power switch.
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 , further comprising the step of decoupling the load from the second DC voltage until requested to couple the load to the second DC voltage.
4. 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.
5. The method according to claim 4 , wherein the voltage shunt is a Zener diode having a breakdown voltage higher than a desired value for the second DC voltage.
6. The method according a 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 said PWM signals from the secondary-side controller to the start-up controller when the second DC voltage is at the desired voltage value;
detect said PWM signals from the secondary-side controller with the start-up controller; and
controlling the power switch to turn on and off with the detected PWM signals from the secondary-side controller.
7. The method according to claim 6 , wherein the second DC voltage is regulated by the secondary-side controller after the start-up controller detects said PWM signals from the secondary-side controller.
8. The method according to claim 6 , wherein the step of sending said PWM signals from the secondary-side controller to the start-up controller further comprises the step of sending said PWM signals through a voltage isolation circuit.
9. The method according to claim 8 , wherein the voltage isolation circuit is an optical-coupler.
10. The method according to claim 8 , wherein the voltage isolation circuit is a pulse transformer.
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 1 , wherein the power converter comprises a flyback power converter.
13. The method according to claim 12 , further comprising the step of protecting the flyback power converter from an over-current fault using a current-sense comparator.
14. The method according to claim 12 , 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.
15. The method according to claim 1 , wherein the power converter comprises a forward power converter.
16. The method according to claim 15 , 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.
17. The method according to claim 15 , further comprising the step of clamping a secondary side reset winding of the transformer to provide a transformer reset.
18. The method according to claim 17 , 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.
19. The method according to claim 1 , wherein the start-up controller protects a power switch driver from under and over voltages.
20. The method according to claim 1 , further comprises the step of limiting a maximum allowable transformer primary winding current with the start-up controller.
21. The method according to claim 1 , further comprising the step of applying AC power to a first rectifier for providing the first DC voltage.
22. A power converter, comprising:
a start-up controller coupled to a first DC voltage and configured to operate in a first and a second operating mode;
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 winding and the start-up controller, and is 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
the start-up controller is configured to operate in the first operating mode to receives the first DC voltage and to control the power switch on and off whereby a current flows through the transformer primary winding which is allowed to rise to a maximum current level monitored by the start-up controller such that an AC voltage develops across the transformer secondary winding, wherein a fixed off time circuit is adapted to keep the power switch off for a certain time period after the start-up controller has turned off the power switch, wherein the certain time period is set by an external capacitor, and
the secondary-side controller is configured to receive the second DC voltage from the secondary side rectifier and is further configured to control the start-up controller to switch to the second operating mode, and
wherein the secondary-side controller is configured to takes over control of the power switch from the start-up controller when the second DC voltage reaches a desired voltage level wherein the start-up controller is switched to operate in the second mode, in which the start-up controller receives PWM signals from secondary side and a switch is controlled to feed said PWM signals to the power switch.
23. The power converter according to claim 22 , wherein the power converter comprises a flyback power converter.
24. The power converter according to claim 22 , wherein the power converter comprises a forward power converter.
25. The power converter according to claim 22 , further comprising a switching post regulator coupled between the secondary side rectifier and a load, wherein the switching post regulator is controlled by the secondary-side controller.
26. The power converter according to claim 22 , wherein the power switch is a power metal oxide semiconductor field effect transistor (MOSFET).
27. The power converter according to claim 22 , wherein the secondary-side controller controls the start-up controller, and is coupled to the start-up controller through an isolation circuit.
28. The power converter according to claim 27 , wherein the isolation circuit is an optocoupler.
29. The power converter according to claim 27 , wherein the isolation circuit is a pulse transformer.
30. The power converter according to claim 22 , further comprising an AC-to-DC rectifier and filter adapted for coupling to an AC power source and for providing the first DC voltage.