What does the 2 diodes across a winding do ?

Thread Starter

DarthVolta

Joined Jan 27, 2015
111
What do, not does, how do I edit titles ??

I have seen a few isolated forward converters, of the type with 3 windings, where across the primary, there are 2 rectifier diodes, like this, pic of schematic below.

I know only the basics of magnetizing currents, and of Volt*seconds.
WRT to the neg. of the rectifier, between D8/9 I have measured over 328Vdc on a good DMM, when I was pulling 1.15A from Vstby. When the mosfet turns off, and the L1 current in L1 wants to continue, does a small reverse current run through D8 to demagnetize L1 ? And or does it get transferred into L2/3 ?

I was looking at the waveform's across both D8/D9, and IDK if they are conducting any amount or not.

My LTspice model of this has them only at pA.




Vaux does not have an output inductor, and is rated for 0.5A. Vstby does and is rated for 2A. Are they both the same type of converter? Or is Vaux without an OP inductor a Flyback winding ?? Both L2 and L3 are low when the primary is high. I thought a forward converter was conducting on the output, when the primary was. But IDK for sure what this is, or what A FC always does.
 

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Thread Starter

DarthVolta

Joined Jan 27, 2015
111
The FWR is 168V, the full circuit has the main PWM. The Vaux is the only 1 way too high compared to the real PSU


Vfwr To the D7/D8 was like 12V high, and -3.1V low, and when it went low, it spikes from 19V to -71V
 
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Ylli

Joined Nov 13, 2015
771
If they are standard diodes, they do nothing. Most likely they would be a pair of back to back Zeners or TVS diodes.
 

ronsimpson

Joined Oct 7, 2019
111
D8 is a Zener. D9 should be a high voltage fast switching diode. This clamps the ring from the transformer to a safe voltage. The diodes save the life of the MOSFET.
Often D8 will have a capacitor across it. Some times D8 will be a resistor and capacitor.
 
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