Voltage rating for MOSFET in full bridge?

I think about Resonant LLC Converter with full MOSFET bridge on primary side (3kW on 230V).
My question is simple.

What minimum voltage must the transistors have?
Is 500V good choice or is better 600V?

Hi,

My question is simple.

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The simple answer: The rating schould be higher that the peak voltage you expect.

But there´s exactly the problem. The peak voltage depends on circuit, PCB layout, wiring... especially stray inductance.
On a well designed circuit you are safe with 20% margin.
On a bad designed circuit you are not safe even with 400% margin.

--> I can´t give you an absolute value.

Klaus

400% on full bridge?
How and where?

Hi,

carpenter said:

400% on full bridge?
How and where?

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I don't believe it was meant literally as a meaningful figure.

Hi,

I don't believe it was meant literally as a meaningful figure.

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True. It means that stray inductance may cause high voltage peaks of unknown value.

Klaus

As already mentioned it’s not a simple answer. First as Klauss said it depends on layout.

But second I’d point out that by “spending” watts in a snubber you can probably mitigate ringing. Then the question becomes: is 1W extra in snubbing worth it to go from 500V to 400V parts (for example).

Though finally reality may have good/bad news: above 200-300V it’s fairly slim pickings until you get to 600-650V. I currently have a similar full bridge with 250Vin and I have 650V parts because 650V parts are as good (or better in some cases) than the available 400-500V parts.

I would suggest double of the nominal voltage as a thumb rule. But i have seen many designs with 1.5 times also.

I will be more specific.
I would like to build an induction melting furnace 3.5kW.
Any as full-bridge LLC resonant inverter, see picture
Here Lcoil is largely variable depending on the content of the fused material in the crucible
I do not dare to estimate the properties of the transformer.
I do not know how to realize it yet

I built a fully insulated mosfet driver for full bridge with 9A peac current for each channel.
3,3V logic digital input , digital insulating barrier 3kV and 9A MOSFET driver on 12V.
For control I want to use fully digital control, for example STM32F334.

DC source is relized two big 400V capacitor charged directli from 230V over high current diode bridge..
Cp is 200A 1uF PP capacitor Lcooil is approx 10 turn 8mm Cu pipe 100mm diameter (wather cooled)

Now I'm solving the MOSFET transistors.
I prefer to use separate modules that I can screw directly onto the radiator without PCBs
it means SOT227 rather than TO-xxx
because of the prices something I can buy in China.
On Aliexpres in not problem buy 500V NMOSFET 50mOhm 80A on SOT227 for 9$/pcs IXYS.
In TO--xx case I find more modern MOSFET , for example STW70N60DM2 600V 66A 37mOhm TO247 3$/pcs but not ideal for mouting without PCB.

The essential point to avoid overvoltage in a bridge circuit is to have low inductance bypass capacitors near to each half bridge transistor pair. If the "big 400V capacitors" are suited for this purpose depends on the layout and the capacitor design. Best to have sandwiched bus plates directly screwed to the transistors and capacitors.

As capacitor I have **broken link removed**
and direct on high side MOSFET I thinking about **broken link removed** or **broken link removed** capacitors.