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Hello Stewin, well from what you are saying it seems that either you are not driving the big fets hard enough and due to that you may have a slow rise time. This will tend to switch the fets in a linear mode hence the heating. You are getting better results with smaller fets due to they do not need much to be switched hence they stay cool. Looking at the schematic I would rather put the snubber in the gating circuit across the GDT primary than at the output of the SG chip. The energy supplied from the chip I will expend it all to the totem pole bases. (47r-103 ceramic)hello all , i have an issue the smps i made heats up irfp460 or 21n50c fets but if i use irf740 the heat is not present at the fets. when i put the bulb in series with smps and load a class ab with the bulb still in series the irfp460/21n50c they do heat insanely with little load and must be mounted on the sink. but irf740 are ok when smps has little or no load.
i tried changing the gate resistors from 22ohms to 47ohms but the heating was worse so i settled on the 4.7ohms it reduced the heating a little bit but did not eliminate the heating.
but when i used irf740 the smps worked well also with a little load to the amplifier the irf740s did not overheat but warmed without a heatsink.
the smps was
- gate drive transformer is ee16 trifilliar 12turns
- gate resistors 4.7ohms
- i tried with two etd39 formers
former 1 etd39 - primary turns 23turns, secondary turns 7t - 0 - 7t aux 2turns
former 2 etd39 - primary turns 42turns, secondary turns 14t - 0 - 14t aux 4turns
both formers behaved the same regarding the heating of the irfp460/21n50c
- i used a1020 and c2655 as buffers for the gdt and 1ohm resistors .(i had used 10 ohms resistor but results were similar with 1ohm)
note the same circuit i tried using igbt an year ago but it over heated abnormally a change of gate resistor didn't help in anyway .
below are the pictures . kindly help out and any opinions are highly welcomed.
I don't think its that. 1A current is more than enough. My 1Kw smps aux supply can only source 600mA and I don't have any problems. I think your GDT may be consuming a lot of current due to low inductance. I made GDTs and the inductance is to be around 1.4 to 1.5mH. This was made on a green round core of around 20mm dia. This works very well and can drive the mosfets very well. The waveform was very faitfull to the output from the chip.hi Silvio , thanks for the reply , i think it is because i use 7812 regulator even before the gdt , and maybe the home wound gdt requires more current than the 7812 can provide . also not using bd139/140 as buffer transistors and using a1020 and c2655 as buffers for the gdt
how many turns did you use for your gdt? also can the heating and the failing of the fets be because i am not using full wave rectification at the dc +/-60 . i am just using one diode per rail! ??I don't think its that. 1A current is more than enough. My 1Kw smps aux supply can only source 600mA and I don't have any problems. I think your GDT may be consuming a lot of current due to low inductance. I made GDTs and the inductance is to be around 1.4 to 1.5mH. This was made on a green round core of around 20mm dia. This works very well and can drive the mosfets very well. The waveform was very faitfull to the output from the chip.
Only an oscilloscope can give you these answers.
All input filters in smps have high permeability cores so that there is no need to wind a lot of turns to get to the right inductance.