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.
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)
One other thing that is crossing my mind is that you may have a spike in the waveform from your gating transformer. This will also reflect itself on the main transformer winding. I am not sure if you have a scope to monitor the pulses but this will tell you for sure.
It also can be the case that the IRFP460 are fakes!!
Some things you may try-
1) Rise the auxiliary voltage to 15-18v dc
2) Remove the emitter resistors from the totem pole transistors to enhance current in the GDT primary
3) Put a snubber across the main transformer primary or a snubber (across each switching Fet 1nF x 1Kv- 5.6R x 3W)
4) Ask a friend to take a look with the oscilloscope and monitor the low side wave form (Gate to source) The rise and fall times need to be sharp as possible, any slopes here will heat up your fets for sure.
5) Check the dead time and see it is adequate so that while one fet is switching off the other will not start switching on.
6) As for the gate resistors use between 4.7R to 10R. Do not go for a higher value than this on the large fets.
7) Put a parallel ultra fast diode with the body diode (drain source) MUR 440 or MUR460. across each fet.
Good luck regards Silvio