how to parallel mosfet

i want to build induction furnace

i parallel 16 power mosfet(irfp460) 8 fet for high side and 8 fet for low side for half bridge

i use ir2104s and bd139 and bd140 totem pole for drive mosfet.

like this



According to irfp460 datasheet, Total Gate Charge=210 nc and td(on)=18 and tr=59

td(on)+tr=77ns then 210/77=2.7 amp. V=IR then R=6.6 and i use 6.8 ohm resistor for each gate and use 10 ohm for bd139 and bd140 bias.

i see the output of totem pole with oscop and its nice and clean square wave without any noise or ringing. like this



but when i connect output of totem pole to high side mosfet the output is ringing and have Too much noise with 12 volt switching power supply without any load for test.

like this




freq is 20khz

why this is happen?

how to reduce or Eliminate this ringing and noise?

thank you

Are you using common GND between high power circuit and controller circuit...???
if not use inductors to minimize the noise..

roboman said:

i want to build induction furnace

i parallel 16 power mosfet(irfp460) 8 fet for high side and 8 fet for low side for half bridge

i use ir2104s and bd139 and bd140 totem pole for drive mosfet.

like this

According to irfp460 datasheet, Total Gate Charge=210 nc and td(on)=18 and tr=59

td(on)+tr=77ns then 210/77=2.7 amp. V=IR then R=6.6 and i use 6.8 ohm resistor for each gate and use 10 ohm for bd139 and bd140 bias.

i see the output of totem pole with oscop and its nice and clean square wave without any noise or ringing. like this

but when i connect output of totem pole to high side mosfet the output is ringing and have Too much noise with 12 volt switching power supply without any load for test.

like this

freq is 20khz

why this is happen?

how to reduce or Eliminate this ringing and noise?

thank you

Click to expand...

I think the problem is in what you wrote: no load connected.
MOSFETs have a high input capacitance, so the drive source must be designed for it. DC power supply must be low-internal resistance, also DC blocking capacitors must be good for the high current. At 20 kHz, I would use parallel capacitors like 10 000 uF plus 10 uF plus 0.1 uF.

Try gradually increasing the frequency under a full load. Take care to use a good heat sink to. protect the MOSFETs.

I don't particularly see "ringing" in the gate waveform, just some overshoot. It's also not clear how you are driving the high side transistor. The gate voltage should be applied relative to source, how did you?

FvM said:

I don't particularly see "ringing" in the gate waveform, just some overshoot. It's also not clear how you are driving the high side transistor. The gate voltage should be applied relative to source, how did you?

Click to expand...

thank you for reply

but Unfortunately i currently no access to oscop image to show it.above image place for example

the output of 4046 generate 20 khz square wave and to ir2104 and next to totem pole with bd139 and bd140 and in over to gate resistor.lm2576 adj, 18 volt supply gate voltage.

according to this article ferrite bead series with each gate resistor can be effective.what your opinion?
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