Phase Control System For Switching Power Supply Pre-Regulator Allows Increased Power Transformer Leakage Inductance

• US 20080112198A1
• Filed: 11/15/2006
• Published: 05/15/2008
• Est. Priority Date: 11/15/2006
• Status: Active Grant

• Current Assignee: Agilent Technologies, Inc.
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Cheah, Chin Hong, Teoh, Lian Ping, Keng, Beng Wei
• Granted Patent: US 7,518,896 B2
• Time in Patent Office: Days
• US Class Current: 363/84
• CPC Class Codes: H02M 7/2176 comprising a passive stage to generate a rectified sinusoidal voltage and a controlled switching element in series between such stage and the output

Abstract

A switching pre-regulator for a bulk capacitor filter followed by a series pass regulator has a switching element controller that relies upon a large desirable leakage inductance in a main secondary winding of a power transformer acting as a swinging choke input to the bulk capacitive filter. This desirable leakage inductance limits inrush current and supplies some filtering. However, the effective value of the swinging choke is a function of load conditions, and introduces a varying phase shift that would potentially disturb the zero crossing detection used in properly activating the switching element. In addition to creating a reference ramp synchronized to the phase shifted, twice power line frequency, full wave rectified zero crossings of the principal current supplying secondary winding, the switching element controller receives a selectable one of an isolated indication of primary line voltage zero crossings or zero crossing indications for an un-rectified auxiliary secondary winding that has a separate leakage inductance that mimics the effect of the phase shift experienced with the main secondary winding. This gives the switching controller the ability to know true zero crossings. The controller for the switching element can now adjust the conduction of that switching element to turn off at a zero crossing, which reduces both acoustic noise from the power transformer and in-circuit electrical noise by eliminating a steep falling edge in the switched current pulse used to supply the bulk capacitive filter.

Claims

1. A method of regulating the output of a power supply having a switching pre-regulator operated from a rectified secondary winding voltage produced by a power transformer having an amount of leakage inductance sufficient to limit inrush current, the method comprising the steps of:

(a) detecting in the secondary winding voltage zero crossings occurring unequal amounts of time apart by an amount related to the amount of current being supplied by the secondary winding;

(b) adjusting an indication of the zero crossings detected in step (a) to have equal periods of time between successive indications of zero crossings while retaining the same frequency as the secondary winding voltage;

(c) detecting in a different winding of the power transformer voltage zero crossings occurring unequal amounts of time apart by an amount related to the amount of current being supplied by the secondary winding;

(d) adjusting the an indication of the zero crossings detected in step (c) to have equal periods of time between successive indications of zero crossings while retaining the same frequency as the voltage of the different winding;

(e) amplifying the voltage drop across a series regulation element whose input is supplied by a filter capacitor and whose regulated output voltage is applied to a load, the amplification having a transfer function that describes as a comparison voltage a location in phase when to begin charging the filter capacitor from a rectified half-cycle of a sinusoid voltage from the secondary winding to keep the voltage drop approximately constant;

(f) creating a reference ramp waveform of constant amplitude representing with each cycle thereof the duration in phase of a rectified half cycle of the secondary winding voltage, and having one such cycle for each adjusted indication of step (d);

(g) comparing the comparison voltage of step (e) with the reference ramp of step (f);

(h) connecting the rectified secondary voltage to the filter capacitor upon a selected comparison in step (g); and

(i) subsequent to step (h), disconnecting the rectified secondary voltage from the filter capacitor upon the occurrence of an adjusted zero crossing indication of step (b).

2. A method as in claim 1 further comprising the steps of filtering the comparison voltage to establish control loop dynamics prior to its use in step (g).

3. A method as in claim 1 wherein the different winding is a tertiary winding.

4. A method as in claim 1 wherein the different winding is a primary winding.