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Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
EDIT: soft switching is not an option, but I'm still curious as to the responses to my question.
Hello. I seem to have destroyed my IRF740's and all I have left are these much slower (but more durable) FETs. I don't know whether I can operate these at the frequencies I need. Should I soft switch them? What is the advantage of hard-switching over soft switching? Maybe I should just order new FETs...
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Not enough data ;) What are your "much slower" FETs exactly? What frequency are you going to run them at? What is the amount of the estimated forward and reverse power levels?
>What is the advantage of hard-switching over soft switching? There is no ;) - soft-switching provides way less switching loss.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
I tested it and they were switching at up to a few 100kHz. That's what my oscilloscope revealed anyway. Although when I tried connecting the primary, very little current flowed, though the oscilloscope showed a wonderful waveform. I also noticed that the voltage across my pseudo switch caps would drop... What is wrong with my half bridge? I will get more information in the morning... Just wondering if anyone has experience with overcoming this problem.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
I was having the current problem with IRF740s too. I believe the problem might be the capacitors, which are just 1 uF nonpolarized radioshack pieces of junk. What type of capacitor/capacitance is recommended for a frequency of about 300kHz, running from 170 (fullwave rectified mains). Current draw can be up to 10 amps. I ran the driver as low as a few kHz and still had the problem. When I test the voltage across the a half bridge cap or the voltage in series with the primary coil, it starts high and drops to zero. I don't think the protection diodes are part of the problem, but I am using MUR1560s in antiparallel to protect the internal diode. I am also using the same type of diode in "antiseries" with the built in mosfet diode in place of schottkys, until they arrive... Could that be the problem? I don't see how that would drain the capacitors, though...
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Slow MOSFETs
