MOSFETs IN SERIES

[arg733]

Nope it's 3 mosfets (irf 540) AIR coil = 80 ohm and i can't exceed 100v. The 1n4007's cathode is pointing to the + and the anode to the drain of the mosfet.

 

[Electrobrains]

Nope it's 3 mosfets (irf 540) AIR coil = 80 ohm and i can't exceed 100v. The 1n4007's cathode is pointing to the + and the anode to the drain of the mosfet.

Hmm...
And you say you can "turn on and off an 15w incandescent lamp at 350V" with basically the same circuit?
That's very mysterious.
1. Can you measure on that 1N4007 to see if it's really ok?
2. Do the MOSFETs get destroyed, or does the circuit just not switch on or off?
3. Maybe you need to increase the V1 voltage to securely switch on the higher transistors, if the current is high and the cascaded Rds(on) gets significant. Try with V1=16V

 

[arg733]

Hmm...
And you say you can "turn on and off an 15w incandescent lamp at 350V" with basically the same circuit?
That's very mysterious.
1. Can you measure on that 1N4007 to see if it's really ok?
2. Do the MOSFETs get destroyed, or does the circuit just not switch on or off?
3. Maybe you need to increase the V1 voltage to securely switch on the higher transistors, if the current is high and the cascaded Rds(on) gets significant. Try with V1=16V

I can turn on and off any incandescent lamp at 300v with 3 IRF540 (100V 33A) in series. The particular test that you are referring to was done with 6 2n7000 (60v ~200ma) in series and it worked fine up to the limit of 6*60=360v I used 15w lamp because of the amps problem.

1. The diode is alive if that't what you mean, anyway i changed it and i got the same result.

2. The MOSFETs get destroyed if i exceed the voltage rating of one mosfet (IRF540 = 100v) so i can't give more than 100v to an inductor (of course if i use another mosfet like IRF740 i could give more). So for some reason that circuit doesn't work with inductors.

3. Ok I will try this, in the meantime if you have more ideas plzzz share.
Thanks.

 

[Electrobrains]

This problem starts to become interesting... I think I soon have to put together that circuit and test it myself with an oscilloscope.

From point 2 above: That the MOSFETs get destroyed at approximately the limiting Vds (100V), could point to that they don't work well in the avalanche sequence and that one transistor (after the other) ends up carrying the full voltage.

A killer could be that the circuit is too slow to switch the higher current, thus the maximum power, (U*I)ds could be exceeded in the switch-on or -off slope.

1. You could try to switch a pure 80 ohm resistor instead of the coil and see if the result is the same.
2. You could try to reduce the gate resistors. Instead of 10k use 100 ohm. That will speed up the circuit very much.

Normally MOSFETs are very good at withstanding over voltage when a load is connected. That's because of the Avalanche Rating that is also built into IRF540. It means they simply start conducting when an over voltage appears. I have used MOSFETs switching very inductive loads, at high current, without any freewheel diode or snubber. The MOSFET catches those spikes wonderfully. It must only be assured that the max. power is not exceeded (P=clamped over voltage * current).
In fact, in the suggested circuit I don't think that ZD11-16 are necessary if it would work properly.

 

[Electrobrains]

p.s.
That the circuit works with 2N7000, could also point to a speed problem!
2N7000 has a much lower gate capacitance Cgs than IRF540 and therefore would tolerate a higher gate resistor.

 

[arg733]

p.s.
That the circuit works with 2N7000, could also point to a speed problem!
2N7000 has a much lower gate capacitance Cgs than IRF540 and therefore would tolerate a higher gate resistor.

NOOO it didn't work with the 2n7000 either it only worked with an incandescent lamp , never tried it with inductors because of the low amperage rating of those mosfets , thats why i used irf540. Sorry if my English is confusing .

 

[arg733]

I also tried this:

but it didn't work with inductors either (in fact it is worse as the last mosfet doesn't count, so if i use 4 mosfets it's actually 3 times the voltage of one and not 4 as the circuit that KrisBlueNZ drawed ).

 

[Electrobrains]

NOOO it didn't work with the 2n7000 either it only worked with an incandescent lamp , never tried it with inductors because of the low amperage rating of those mosfets , thats why i used irf540. Sorry if my English is confusing .

Try to reduce the Gate Resistors from 10k to 100 ohm.