Hello.

I´m using a 555 oscillator with a MOSFET IRFP450 to run a small high frequency transformer. It runs at 12volts and drains 2 amps. 24watts and in the datasheet says max 180 watts. That max power is with or without heatsink? When I try it without heatsink, gets hot very quick and I don´t know if that is OK. I'll use for short periods or 10 or 15 secs, and I´d like to leave out the heatsink. What do you think?

Thank you

best regards

 

The drain to source on-state resistance depends on the gate to source voltage you are using. If you are using too low a voltage, the drain to source on resistance could be quite high. That would cause a lot of heat to be dissipated.

 

I have put a 54 ohm resistor between the gate and the pin #3 of the 555 to limit the current. But it soesn´t matter if I run at 9 or 12 volts and with or twithout resistors.

 

Why are you using a 500V MOSFET to switch a 12V transformer?
Power dissipated will be I^2R, where R=Rds(on) = 0.4Ω =1.6W.
A power transistor without a heatsink will dissipate 1W as a rule of thumb (perhaps a bit more for a TO247) - no wonder it is warm.
A better choice of MOSFET with Rds(on) <0.1Ω should sort it out.

What happens to the drain voltage when the MOSFET switches off? Do you have adequate protection circuitry? What are you doing to prevent a net DC voltage across the transformer?That could be a source of extra dissipation.

 

Hello.

I´m using a 555 oscillator with a MOSFET IRFP450 to run a small high frequency transformer. It runs at 12volts and drains 2 amps. 24watts and in the datasheet says max 180 watts. That max power is with or without heatsink? When I try it without heatsink, gets hot very quick and I don´t know if that is OK. I'll use for short periods or 10 or 15 secs, and I´d like to leave out the heatsink. What do you think?

Thank you

best regards

You are confusing the power in the load with the power in the MOSFET.

The power dissipated in the MOSFET is

Vds x Id

That is, the voltage across the MOSFET times the current through it. In a well designed circuit, this is far less than the power delivered to the load.

 

Somewhere in the FET's datasheet there should be small footnotes which tell you under what conditions the various parameters are obtained. Max power handling for the FET is usually given for a specified case temperature or junction temperature. To keep the case temperature down to, say, 25C may well require a fair-size heatsink.

 

How about a circuit diagram? You cannot run the transformer with current in a single direction, the current in the transformer must reverse on every cycle. This is done either with a full bridge, or a half bridge with capacitive coupling.

 

I was making this.



but as oscillator the circuit is this one.



I use a IRFP450 because I had one. If there is a better option please tell me but if you can tell me the model, better, I have not very much idea haha.

The transformer I´m using instead the flyback is



I made a mistake with the numbers. At 7 volts the circuit drains 4 amps

 

If you are running 7V to the 555, the output is more like 5V, not nearly enough to turn the MOSFET fully on. That is likely why it is getting hot. I doubt that that little coil is rated for 4A.

 

mmmm ok, and what should I do? using a resistor o capacitor to limit the current of the coil? And for that values (I forgot to say the best output is a 7,4 Khz) what MOSFET or other device should I use?

 

DO you also have the core for that coil?

 

It has a ferrite core inside

 

It has a ferrite core inside

Is it meant to be like that? I would have expected a rectangular core with the coil around one leg. With only a piece of ferrite down the middle it would be likely to radiate interference everywhere.

 

What kind of voltage/ current are you expecting to get out of it?

 

mmmm ok, and what should I do? using a resistor o capacitor to limit the current of the coil? And for that values (I forgot to say the best output is a 7,4 Khz) what MOSFET or other device should I use?

The MOSFET is fine as long as you drive the gate with 10V. If you want to use a lower voltage you need a logic level one like IRL44Z.

 

A much better choice for the MOS FET is IRFZ44. It has an on-resistancs of 9 milliohms with a minimum of 6V on the gate.

 

Keith, my IR datasheet for the IRFZ44 Mosfet shows a maximum on-resistance of 28 milli-ohms with a minimum of 10V on the gate. Its threshold voltage is 2V to 4V like all common Mosfets.

The IRLZ44 has a maximum on-resistance of 25 milli-ohms with a minimum of 5V on the gate or a maximum on-resistance of 35milli-ohms with a minimum of 4V on the gate. Its threshold voltage is 1V to 2V like all logic level Mosfets.

 

I tried a IRFZ44N and gets hot quicker than the IRFP450

I also have IRFP260N, IRFP250N, IRF840, IRL540N, IRFZ48N
TIP31C, TIP41C

Aniones of these is suitable? I mean, I can try the IRL44Z but I have to buy it and I´d preffer to use one of these.

 

Keith, my IR datasheet for the IRFZ44 Mosfet shows a maximum on-resistance of 28 milli-ohms with a minimum of 10V on the gate. Its threshold voltage is 2V to 4V like all common Mosfets.

My apologies! I was quoting from memory. The IR datasheet gives a rds-on as 17,5 milliohms.I should have checked. I designed several radio controlled DC motor speed controllers in the past, using those. I remember that I needed at least 6V vgs to switch them hard with 12 to 16V at up to 25A. They did not need a heat sink.
What does the output waveform look like? I think you may need a resistor from gate to ground to make sure the gate is discharging properly between positive pulses.

 

I think you may need a resistor from gate to ground to make sure it is discharging properly between positive pulses.

Not necessary when driven by a 555, which can both source and sink current.

I tried a IRFZ44N and gets hot quicker than the IRFP450

I also have IRFP260N, IRFP250N, IRF840, IRL540N, IRFZ48N
TIP31C, TIP41C

Aniones of these is suitable? I mean, I can try the IRL44Z but I have to buy it and I´d preffer to use one of these.

IRL540 is a logic level one, use it. But there are other problems with your circuit, so don’t expect good results.