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Old 20th November 2007, 02:44 PM   #21
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Quote:
Originally posted by Workhorse
Zero

Your Flyback cannot work with a 600V igbt with mains voltage at 240VAC, you need at least 1200V Igbt to do the job...
test clamping network for 600v igbt
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Old 20th November 2007, 06:43 PM   #22
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The voltage reflected to the primary side is 60V + 1V from the diodes * 64T / 20T = 195V. Thus good primary-side clamping is required for operation at 230V input and above.
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Old 20th November 2007, 10:47 PM   #23
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Primary rectified voltage 310V + 195V= 505V
Maybe a stretched safety margin?
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Old 21st November 2007, 01:05 AM   #24
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It's not a matter of margin. Unless the tranformer is really well made to exhibit extremely low leakage inductance, the collector voltage will rise up to avalanche upn turn off. The schematic shows no clamp, only an RCD snubber that is probably not going to provide good enough clamping effect.

Capacitors across the diodes help in reducing the turn-off spike but make turn-on behaviour worse.
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Old 23rd November 2007, 06:30 AM   #25
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Let's make a rough guess:

According the newer schematic the refelected voltage would be 284V. Means already without leakage inductance the IGBT would see values close to 600V.
Say 310+284V=594V.
In the snubber cap 594V will mean a energy of 388uJ.
Then let's consider 5uH leakage. This will have stored 250uJ if we consider an switching event at 10A.
These 250uJ will be added to the 388uJ. So you get 638uJ in the snubber cap, means it will go to 762V.
Not only your IGBT is underrated also the 400V snubber cap.

And I it might be worse, because I have doubts that you will be able to realize 5uH.... The transformer itself is not so easy to design and you will additionally see an additional upwards transformed portion from all parasitic inductances of your secondary circuit loop(s).

Last but not least take care in this game.
In most offline applications people are considering 400V as relevant voltage for the safety isolation, which would required a layout AND transformer desgin that ensures 8mm creepage (EN60065, Table 11).
In your case with the high reflected voltage and leakage peak... well, in such a design the requirements might be even higher...
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Old 23rd November 2007, 03:32 PM   #26
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hmmm, the second snubber with 47nF should be helpful to keep the peak less than my first calculation....

Nevertheless, especially with the new output voltages and turns ratio of your new schematic you will need more 600V for the switch and also for the snubber caps.
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Old 24th November 2007, 01:59 PM   #27
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Default transformer completed

for 1kw gapped E70/33/32 transformer

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Old 24th November 2007, 02:49 PM   #28
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I have experience with long air gaps. Yours is very large and requires litz wire to prevent the windings from overheating due to Eddy currents, even at a few A only.

Are you isolating primaries and secondaries properly?
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Old 24th November 2007, 04:14 PM   #29
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now not to be
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Old 24th November 2007, 05:32 PM   #30
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Keeping the windings away from the gap helps, this is achieved by using the outer portion of the winding space and leaving the inner portion unused. Using a gap in the three legs instead of a center-leg gap also helps a lot, but at the expense of stray magnetic fields that require some way of shielding.

You may consider several smaller transformers or the kind of iron powder toroid cores used for class D (low-perm low-loss RF iron powder and Arnold's MPP & Hi-Flux materials) as alternatives.
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