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Registered Member #2628
Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627
I am building a half bridge to drive a flyback ive got around,
the mosfets I have on hand are these guys:
I am a bit uncertain to how fast can they switch, judjing by the on/off time, I figured around 70-100KHz would be close to what they are rated for, does this sound fairly accurate? (the flyback will be driving at far lower, but I do wonder just how fast can this mosfet be pushed before the losses catch up)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I think youll be ok up to 100kHz maybe 150kHz, but I cant be sure. Without seeing your circuit, there is a difference between switching voltage at a frequency.... and driving current at a frequency. Your flyback ferrite character will limit you freq before your mosfet. At least i think, remember that stray capacitence and inductance will cause unwanted impedance, so you may be able to switch your moset at high freq, but your ferrite/coil may not do anything useful beyond 40-70kHz.
One thing that helped me, was looking at STmicro and Fairchild mosfet basic documents, which describe delay times and rise / fall times, and how these are related to max frequency of switching, the ferrite physics i learned the most from were the Unitrode/Texas Instruments websites.
Registered Member #2628
Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627
I just drew up a schematic of what Im trying to do, hopefully this will give you a better idea as to how its going to be driven, but If I understood what you said correctly, at a point, my core will start seeing losses, saturation, etc, which will affect the losses far more then the mosfets themselves.
the input to the inverter gates is from a square wave generator. The gates of the mosfets (bridge) are bieng driven at 24V
hope this helps, and if you spot any errors with the drawing, feel free to point them out. thank you.
Registered Member #2628
Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627
I am using ExpressSCH, it comes with ExpressPCB (which you can download for free at thier website) I pretty much use these two softwares all the time, for PCB design and schematic drawing.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
You likely need some resistance between the output of your inverting buffers and the bases of your BJT's in order to limit your base current to a reasonable value. You might also consider a high value resistor from gate to source on the low side, N channel FETs to help prevent a possible shoot through condition.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
cjk2 wrote ...
You likely need some resistance between the output of your inverting buffers and the bases of your BJT's in order to limit your base current to a reasonable value. You might also consider a high value resistor from gate to source on the low side, N channel FETs to help prevent a possible shoot through condition.
I second cjk2's comment. you may also find you need some dead time to prevent that shoot through.
As for the original question, I think this circuit will limit your upper freq before the mosfet or ferrite.
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Flyback bridge mosfets (frequency?)
