Author Topic: Stupid question about the GDT for a full bridge configuration  (Read 563 times)

Offline nick

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Stupid question about the GDT for a full bridge configuration
« on: November 22, 2019, 01:07:33 PM »
Hi, sorry but this is probably a stupid question. I've briefly reviewed all the excellent online sources (Kaizer, Loneocean, etc. etc.) and while this may have been explained somewhere when I read it ages ago, I can't find the answer now from a brief look.

Question: why do I need 4 secondary windings when moving from half bridge to full bridge?

2 of the 4 switches should be on simultaneously and in-phase - right? So why can't one secondary winding be used to control both of the switches? I.e. why would the GDT for a full bridge be different from a half bridge? What am I missing?

Thank you

Offline klugesmith

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Re: Stupid question about the GDT for a full bridge configuration
« Reply #1 on: November 23, 2019, 04:01:16 AM »
For a pair of FETs that ideally are On at the same time, and Off at the same time:

what are their absolute source and drain voltages in the On phase?  In the Off phase?













Offline DashApple

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Re: Stupid question about the GDT for a full bridge configuration
« Reply #2 on: November 23, 2019, 07:46:04 AM »
one reason for the 4 windings on a full bridge GDT is that even though the two mosfets/igbt are in phase they are not both at the same potential

One switch will be sitting near the supply voltage while the other will be at ground with respect to its source so you need windings for each to 

Offline nick

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Re: Stupid question about the GDT for a full bridge configuration
« Reply #3 on: November 24, 2019, 03:30:35 PM »
Thanks both! That makes complete sense and I'm embarrassed I didn't realize this myself.

Sorry to hijack my own thread (I can post this as a separate thread if needed):

So I built a full bridge (using these MOSFETs: https://www.datasheetq.com/datasheet-download/884900/1/Infineon/35N60CFD - I salved a bunch from a discarded 6kW SMPS - what a great source of awesome components! I'd like to avoid killing my nice IGBTs if possible so tried with MOSFETs first) and it blew up. Not right away (I triple checked polarities, etc) - the coil started making sparks but blew when I hit probably around 80v. I tripped a fuse in my house (not fun but I'm used to it by now :)).

At first I noticed my FULL BRIDGE RECTIFIER was shorted, and thought that was the end of it (bus capacitors are fine). However after replacing that, I tried the circuit again at relatively low voltage from a battery source to avoid tripping house fuses. The wires literally melted lol - clearly something else was wrong. I took Mads' advice to desolder and check everything following a failure, and noticed one of the MOSFETs' pins were shorted (remaining 3 were fine). Before blowing more switches, does anyone have advice as to what the issue could be?

My switches are protected with MUR1560s across D-S, and 10 ohm paralleled with a small signal diode in front of each gate. My next step would be to put 18v zeners back to back across G-S, but is there anything else you think could be the issue?

Thanks!

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Re: Stupid question about the GDT for a full bridge configuration
« Reply #3 on: November 24, 2019, 03:30:35 PM »

 


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