Transformer flux estimation and limiting in isolated DC-DC voltage converters

Patent metadata

Field	Value
Publication number	US-9812971-B1
Application number	US-201615235872-A
Country	US
Kind code	B1
Filing date	Aug 12, 2016
Priority date	Aug 12, 2016
Publication date	Nov 7, 2017
Grant date	Nov 7, 2017

Abstract

A switching voltage converter using an isolated topology includes a transformer for coupling power from an input source to an output load. The transformer must be protected to prevent saturation of its core due to excessive magnetic flux density as the transformer transfers power from its primary side to its secondary side. The magnetic flux is estimated using a voltage measured on the primary or secondary side of the transformer, wherein the secondary-side voltage may be a rectified voltage. If the estimated magnetic flux is detected as approaching a saturation level of the transformer core, any power being input to the transformer is curtailed. This may be accomplished by modifying pulse-width modulated (PWM) waveforms controlling power switches that control the input power transferred to the transformer. Using these techniques, transformer saturation may be avoided without requiring a significantly oversized transformer within the voltage converter.

First claim

What is claimed is:

1. A voltage converter, comprising:
a power stage coupled to an input power source, the power stage comprising one or more power switches;
a transformer comprising a primary winding coupled to the power stage, and a secondary winding;
a conditioning circuit coupling the secondary winding to an output node that is operable to supply power to a load of the voltage converter; and
a controller operable to:
generate pulse-width modulated (PWM) waveforms for controlling the one or more power switches, wherein each of the PWM waveforms is comprised of a series of PWM pulses;
estimate magnetic flux within the transformer based upon at least one of a voltage across the primary winding and a voltage within the conditioning circuit;
detect an overflux condition in which a magnitude of an estimated magnetic flux has exceeded a flux magnitude limit for the transformer; and
responsive to detecting the overflux condition, truncate a pulse within the series of PWM pulses of the PWM waveforms before the PWM waveforms are provided to the one or more power switches.

2. The voltage converter of claim 1 , wherein the controller is operable to estimate the magnetic flux based upon the voltage across the primary winding.

3. The voltage converter of claim 2 , wherein the estimation of the magnetic flux is performed by:
generating or inputting a clock signal having a clock frequency and a corresponding period; and,
for each period of the clock signal:
measuring a primary voltage across a first and a second terminal of the primary winding, wherein the primary voltage is a signed quantity; and
adding the primary voltage to a volt-second value that represents the estimated magnetic flux.

4. The voltage converter of claim 2 , wherein the estimation of the magnetic flux is performed by:
generating or inputting a clock signal having a clock frequency and a corresponding period;
classifying the voltage across the primary winding as being within a first polarity pulse interval, a second polarity pulse interval, or an inactive interval, wherein the first and second polarities are opposite to each other; and
for each period of the clock signal:
increasing a flux count by a flux amount responsive to the voltage across the primary winding being classified within the first polarity pulse interval, and
decreasing the flux count by the flux amount responsive to the voltage across the primary winding being classified within the second polarity pulse interval,
wherein the flux count represents the estimated magnetic flux.

5. The voltage converter of claim 4 , wherein the flux amount is determined by measuring the voltage across the primary winding at a rate that is less than that of the clock frequency.

6. The voltage converter of claim 1 , wherein the controller is operable to estimate the magnetic flux based upon the voltage within the conditioning circuit, the voltage within the conditioning circuit corresponding to a secondary voltage across a first and a second terminal of the secondary winding.

7. The voltage converter of claim 6 , wherein the secondary voltage is a signed value and the estimation of the magnetic flux is performed by:
generating or inputting a clock signal having a clock frequency and a corresponding period; and,
for each period of the clock signal:
measuring the secondary voltage; and
adding the secondary voltage to a volt-second value that represents the estimated magnetic flux.

8. The voltage converter of claim 6 , wherein the estimation of the magnetic flux is performed by:
generating or inputting a clock signal having a clock frequency and a corresponding period;
classifying the secondary voltage as being within a first polarity pulse interval, a second polarity pulse interval, or an inactive interval, wherein the first and second polarities are opposite to each other; and
for each period of the clock signal:
increasing a flux count by a flux amount responsive to detecting that the secondary voltage is within the first polarity pulse interval, and
decreasing the flux count by the flux amount responsive to detecting that the secondary voltage is within the second polarity pulse interval,
wherein the flux count represents the estimated magnetic flux.

9. The voltage converter of claim 8 , wherein the flux amount is determined by measuring the secondary voltage at a rate that is less than the clock frequency.

10. The voltage converter of claim 1 , wherein:
the controller is operable to estimate the magnetic flux based upon the voltage within the conditioning circuit;
the conditioning circuit further includes a rectified voltage node; and
the voltage within the conditioning circuit that the magnetic flux estimation is based upon is a rectified voltage at the rectified voltage node.

11. The voltage converter of claim 10 , wherein the controller is further operable to estimate the magnetic flux by:
generating or inputting a clock signal having a clock frequency and a corresponding period; and,
for each period of the clock signal:
measuring the rectified voltage;
increasing a volt-second value by a measured rectified voltage during intervals when the voltage across the primary winding corresponds to a first polarity, and
decreasing the volt-second value by the measured rectified voltage during intervals when the voltage across the primary winding corresponds to a second polarity,
wherein the first and second polarities are opposite to each other and the volt-second value represents the estimated magnetic flux.

12. The voltage converter of claim 10 , wherein the controller is further operable to estimate the magnetic flux by:
generating or inputting a clock signal having a clock frequency and a corresponding period;
comparing the rectified voltage with a voltage threshold, such that the rectified voltage is classified as being within a rectified voltage pulse interval whenever the rectified voltage exceeds the voltage threshold, and is otherwise classified as being within an inactive interval; and
for each period of the clock signal:
increasing a flux count during each rectified voltage pulse interval corresponding to a first polarity of the voltage across the primary winding, and
decreasing the count during each rectified voltage pulse interval corresponding to a second polarity of the voltage across the primary winding,
wherein the first and second polarities are opposite to each other and wherein the flux count is representative of the estimated magnetic flux.

13. The voltage converter of claim 10 , wherein:
the rectified voltage node is coupled to a first terminal of the secondary winding, and
the controller is further operable to estimate the magnetic flux based further upon a voltage corresponding to a second voltage node, the second node being coupled to a second terminal of the secondary winding.

14. The voltage converter of claim 1 , wherein the flux magnitude limit is based upon a number of turns in the transformer, a cross-sectional area of the transformer, and a flux saturation limit of the transformer.

15. The voltage converter of claim 1 , wherein the flux magnitude limit is given by:
B LIMIT =N 2 A e B sat −Δ,

wherein N 2 is a number of turns of the secondary winding,
A e is a cross-sectional area of the transformer,
B sat corresponds to a saturation limit of the transformer in volt-seconds, and
Δ is a non-negative margin.

16. The voltage converter of claim 1 , wherein the controller is further operable to balance the magnetic flux within the transformer by:
capturing a positi

Assignees

• Infineon Technologies Austria Ag

Inventors

• Tschirhart Darryl
• Babazadeh Amir

Classifications

• H02M1/40

  Means for preventing magnetic saturation · 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

• H02M3/33523 Primary

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

• H02M3/156

  with automatic control of output voltage or current, e.g. switching regulators · CPC title

• H02M1/32

  Means for protecting converters other than automatic disconnection · CPC title