Hi, I am trying to make a variable frequency supply unit using 555 timer, TLP250 driver, a mosfet P80NF55, and a flyback transformer 1V/250V. I need to drive a capacitive load (Plasma reactor) at 10 kV, 0.1 A, if the frequency is 3kHz. Circuit diagram is attached herewith. Before connecting the flyback transformer, I am getting variable frequency pulses across MOSFET (between drain and source). But when I connect the MOSFET (drain) through the primary of flyback transformer to the DC supply, I think MOSFET is burning. I have seen it not working later when I test it alone again removing flyback transformer. I can understand that if I connect the load also, then a huge current will flow in the primary that can destroy the MOSFET. But here, I didn't even connect any load. Please help me. Is there any mistake in my circuit.

 

Hi, I am trying to make a variable frequency supply unit using 555 timer, TLP250 driver, a mosfet P80NF55, and a flyback transformer 1V/250V. I need to drive a capacitive load (Plasma reactor) at 10 kV, 0.1 A, if the frequency is 3kHz. Circuit diagram is attached herewith. Before connecting the flyback transformer, I am getting variable frequency pulses across MOSFET (between drain and source). But when I connect the MOSFET (drain) through the primary of flyback transformer to the DC supply, I think MOSFET is burning. I have seen it not working later when I test it alone again removing flyback transformer. I can understand that if I connect the load also, then a huge current will flow in the primary that can destroy the MOSFET. But here, I didn't even connect any load. Please help me. Is there any mistake in my circuit.

 

For starters, you should have some bypass capacitors on your power supplies. I

f you have a scope, tell us what you see on the gate of the MOSFET when the load is connected and not connected.

You can also do this with a voltmeter but the interpretation will be less certain. If measuring with a voltmeter, please tell us what the power supply voltage reads with and without load.

 

But when I connect the MOSFET (drain) through the primary of flyback transformer to the DC supply, I think MOSFET is burning.

Not surprising. You have nothing to protect the MOSFET from back-emf spikes.

 

Try using an IRF510 /540 Mosfet..... And Back emf diode across the coil...

 

Hi Dave,
According to the TS's first post he requires 10KV output at 100 mA (1KW) so even if the converter was 100% efficient he would need 67 amps from the 15 volts supply so he would need a mosfet with a higher current rating than an IRF540. Also as the transformer ratio is 1:250 and he needs 10KV output there would have to be at least 40 volt pulses across the primary. He would have to use a zener diode to clamp the back emf rather than a normal diode.

Les.

 

Hi, I am trying to make a variable frequency supply unit using 555 timer, TLP250 driver, a mosfet P80NF55, and a flyback transformer 1V/250V. I need to drive a capacitive load (Plasma reactor) at 10 kV, 0.1 A, if the frequency is 3kHz. Circuit diagram is attached herewith. Before connecting the flyback transformer, I am getting variable frequency pulses across MOSFET (between drain and source). But when I connect the MOSFET (drain) through the primary of flyback transformer to the DC supply, I think MOSFET is burning. I have seen it not working later when I test it alone again removing flyback transformer. I can understand that if I connect the load also, then a huge current will flow in the primary that can destroy the MOSFET. But here, I didn't even connect any load. Please help me. Is there any mistake in my circuit.
View attachment 124117

 

For starters, you should have some bypass capacitors on your power supplies. I

f you have a scope, tell us what you see on the gate of the MOSFET when the load is connected and not connected.

You can also do this with a voltmeter but the interpretation will be less certain. If measuring with a voltmeter, please tell us what the power supply voltage reads with and without load.

Thank you sir for your response. I was giving 15VDC supply from a DC test set, and I connected 0.44 uf capacitors across pins 5 and 8 of TLP250, which was not shown in figure.
PFA for the waveform I am getting across mosfet's drain and source before connecting transformer.
At the instant I switch on the supply, some spikes and gone. Later when I remove TF, only DC voltage is passing through the MOSFET, no pulses.

 

@LesJones ,
@Dodgydave ,
@Alec_t .
Thank you very much for your responses. I will add a Zener diode to clamp the back emf.
thank you all once again.

 

He would have to use a zener diode to clamp the back emf rather than a normal diode.

Can you please suggest me, what could be the rating of Zener diode and how it can be connected in this circuit.

 

Try using an IRF510 /540 Mosfet..... And Back emf diode across the coil...

That MOSFET has a built in zener diode, but its current rating is too low for my application. So please suggest me the rating of Zener diode for this. Should I still have to use a diode across coil, even after using zener across MOSFET? If yes, what could be the rating of that diode.
Thank you in advance.

 

You're better putting one across the fet and one across the transformer, use Ultrafast diodes like Vishay or other manufacturers, search for 5000V reverse voltage..you may have to put two in series..


http://www.futureelectronics.com/en/Search.aspx?dsNav=Ny:True,Ro:0,Nea:True,N:666-4294918011


Mouser only do 1200V...

http://www.mouser.co.uk/ProductDeta...urbxH0/352xx0BCcNOaYZum6dsFLUPsoX1R22ZiEs0g==

 

That MOSFET has a built in zener diode

The built-in body diode will not protect the MOSFET from back-emf spikes when the drain is connected to an inductive load.

 

You're better putting one across the fet and one across the transformer, use Ultrafast diodes like Vishay or other manufacturers, search for 5000V reverse voltage..you may have to put two in series..


http://www.futureelectronics.com/en/Search.aspx?dsNav=Ny:True,Ro:0,Nea:True,N:666-4294918011


Mouser only do 1200V...

http://www.mouser.co.uk/ProductDetail/Littelfuse/DUR75120W/?qs=/ha2pyFaduiurbxH0/352xx0BCcNOaYZum6dsFLUPsoX1R22ZiEs0g==

Thank you for your suggestion.
I found one diode https://www.vishay.com/docs/93173/vs-sd263cs50lseries.pdf
but it seems like availability of these diodes in India is quite less.
Can I use IGBT instead of MOSFET as they usually available in high voltage ratings.

 

If I connect my capacitive load (0.3nF) also, will this back emf reduces?

 

Go for an IGBt if you can make it work just as good in India.

 

Go for an IGBt if you can make it work just as good in India.

I am very sorry, I am new to this area. So with very less knowledge, I asked that question. I just came to know that the reverse voltage problem will still be there. I'd once again say sorry as I hurt you with my question.
I am searching for GP02-40 now.
Thank you.

 

I think your mosfet is being destroyed by the circuit. The positive pulses from the 555 are turning the mosfet on. When the mosfet is turned off the reverse polarity in the transformer primary is exceeding the ratings of the mosfet. A diode in parallel with the primary, the cathode connected to the + 15VDC supply and the diode anode to drain of the mosfet will most likely prevent damage to the mosfet. The diode voltage rating should be near or greater than 100V, the current rating should be near the drain current of the power supply

 

I think your mosfet is being destroyed by the circuit. The positive pulses from the 555 are turning the mosfet on. When the mosfet is turned off the reverse polarity in the transformer primary is exceeding the ratings of the mosfet. A diode in parallel with the primary, the cathode connected to the + 15VDC supply and the diode anode to drain of the mosfet will most likely prevent damage to the mosfet. The diode voltage rating should be near or greater than 100V, the current rating should be near the drain current of the power supply

Hi, Thank you for the response. If I connect my capacitive load also at the end, will this back emf effect (due to inductive nature of TF) reduces?

 

No........