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Voltage Clamp for Switched Mode Power Supply

IP.com Disclosure Number: IPCOM000020191D
Publication Date: 2003-Oct-31
Document File: 5 page(s) / 287K

Publishing Venue

The IP.com Prior Art Database

Abstract

ID613781

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Page 1 of 5

Voltage Clamp for Switched Mode Power Supply

This publication is related to voltage clamps for single ended flyback converters. Voltage clamps are used for three reasons:

1. Limit the peak voltage across the switch after turn off.

2. Improved EMC behaviour of the converter, because the HF-ringing generated after turn off is clamped.

3. Partial leakage energy recovery

An improved version of the conventional RCD (resistor capacitor diode) clamp is proposed. The diode used in the conventional clamp is replaced by a diode in series with a resistor. The diode has a slow reverse recovery time.

The diode starts conducting after the main switch is turned off and the voltage across the switch has reached the input voltage plus the reflected output voltage. After the diode starts conducting the voltage across the switch rises further until the maximum voltage is reached and the current through the diode has reached the maximum. The voltage across the switch and the current through the clamp continuous as a damped sinusoid. Where the damping is caused by the series resistor.

The reverse recovery time of the diode is large enough so that the diode stays in conduction until the current through the clamp is zero.

Power supply with PI-voltage clamp (prior art)

Circuit of the Voltage Clamp:

R2 560k

D1 D1N4006

C2 3n3

R1 1k

2

1

Measurement:

Ch1: Voltage across the switch

Ch2: Current through the clamp

[This page contains 4 pictures or other non-text objects]

Page 2 of 5

PM3394B

CH1 100 V= CH2 50mV= MTB 500ns- 2.02dv ch1+

ch1

ch2

T

1

2

[This page contains 1 picture or other non-text object]

Page 3 of 5

Power supply with 1N4006 voltage clamp

Circuit of the Voltage Clamp:

R2 220k

R1 330

C2 1n8

D1 D1N4006

Measurement:

Ch1: Voltage across the switch

Ch2: Current through the clamp

PM3394B

CH1 100 V= CH2 50mV= MTB 500ns- 2.02dv ch1+

ch1

ch2

T

1

2

Advantages 1N400X voltage clamp in comparison with the PI-clamp:

1. Very well EMC performance because of the damped ringing, comparable with or even better than the PI-voltage clamp described in the patent 5,986,905 (see graph last page)

2. Large leakage inductance of transformer is allowed because the HF-ringing is effectively damped. A large leakage inductance improves the EMC performance in the frequency range above 30MHz.

3. Reflected output voltage (n*Vout) can be measured on the clamp, an auxiliary winding to measure the output voltage can be left out. The voltage across the clamp capacitor is equal to the reflected output voltage.

4. Low cost voltage clamp (the active part is a simple diode, for example an 1N4006)

[This page contains 4 pictures or other non-text objects]

Page 4 of 5

Description of the 1N4006 voltage clamp

The description is carried out with the help of a simulation. The circuit diagram is shown beneath.

Simulated waveforms:...