Orson Cart
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V cool, the caps on the bus also helped reduce the turn off spike (to nearly zero) after the turn off, leading to the cooler operating temps...!!!!!
Yes, its all looking very good, the reset time for this level of power is about 4.5uS looking at the scope screens, it is proportional to on time and input voltage, looks like you can get more power if you wish, you may need more electrolytics on the main DC bus as there is quite a rise in HVDC after turn off (from 160V to 210V on the screen) - you may not mind this - so it isn't necessarily some thing that needs addressing, the turn off spike on the igbt could be addressed with 680pF (1kv ceramic cap) and 220 ohm (2-3W) as a guess for starters, across each igbt (RC in series) , this may reduce the spike a bit without incurring too much in the way of extra losses - also good for RFI / EMC...
Look forward to hearing how it all goes...!
Always good to experiment with snubbers, the D-C part gives you the ring and overshoot at turn off, try 10 ohms in series with the diode (or bigger) this may help, or just use an RC snubber (no diode) say 2.2nF and 220 ohm (3W) for starters - you need to vary the values to see what works in your circuit...
The transformer will have originally been designed around a maximum input voltage at the 80 Khz switching frequency.
You can run it at 50 Khz, but the maximum input voltage you can safely apply, needs to be limited to 5/8 of what was the originally specified maximum.
That may or may not be an issue for you.
Temperature rise is a very good indication of losses.
Another way is to measure dc input and output power, and calculate efficiency. That is a much faster way as you don't have to wait for everything to temperature stabilize.
Try it at 50, 60, 70 Khz and see how it goes.
That should give you a pretty good feel for what is going on, and where the main losses are.
As Orson says, its all jolly good fun, and very instructive.
You can tell what switching frequency is best by the temp of the transformer, 50kHz seems to be going OK, so maybe 60kHz max, as you have a reduced input voltage, i.e. lower than the welder, looking at your scope shots, as you go higher in freq the igbt losses go up as do the snubber losses and the Tx wire losses, the core losses may stay the same.
Its all about temp rise - it may be good for you to raise the freq and see what gets hot - this is the best way to learn and teaches you a lot - good luck..!
Most of your losses must be conduction losses rather than switching losses, so that is where it might need a little more work.
Still its not too bad considering the high output current which is always problematic.
What is getting hot should be a pretty good guide to where the watts are escaping.
The measured forward voltage drops will be a strong clue.
Compare what you are seeing to the device specifications, and see if it is reasonable.
Compare some different available devices, high voltage mosfets as well as IGBTs, and how they might perform in your circuit.
When paralleling multiple devices, the combined conduction losses of mosfets and IGBTs behave very differently.
You are running two devices in parallel. Perhaps three or four in parallel might significantly reduce conduction losses, or it may make little overall improvement.
But you can work it all out from published data, conduction voltage drop versus current for each device.
I am working on a 1.5Kw boost converter right now.
One IGBT beats one high voltage mosfet easily.
Two IGBTs in parallel are about the same as two mosfets in parallel.
Adding more, perhaps four mosfets in parallel works out being miles ahead in conduction losses compared to four IGBTs in parallel.
It does not always work out quite like that, depending on relative voltage and current involved, and the particular devices, but there may be significant gains to be had by exploring a few hypothetical alternatives.
That looks pretty normal, do you have snubbers on your o/p diodes? I take is the scope shot is from a current sensor?
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If that is your o/p voltage scope shot the extra bits on top are due to the ESR & ESL in your big o/p electro, more film caps needed to smooth this out 10uF say, or 5 x good quality electro's in parallel
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