h bridge question about mosfet needed

[maicael]

Hello everyone
Pla how does one calculate how many MOSFET will be needed for an full bridge inverter using N channel MOSFET.
If I have irfp260n having power at 300w then using four of it in the bridge will give how much power output assuming no losses. Basically how to calculate number of MOSFETs needed depending on power output.

 

[Audioguru]

We do not know your battery voltage.
We do not know if the inverter will be crude squarewave, fairly crude modified sinewave or pure sinewave.
We also do not know its PWM frequency.
You forgot to tell us how you will cool the Mosfets. You cannot assume no losses because it is the losses that make the Mosfets so darn HOT!

 

[maicael]

Hello
Sorry for that
Battery voltage is 24v,
Its a hopefully pure sine wave using a transformer to step up the voltage.
PWM frequency is about 25khz(ok?)
MOSFETs will be places with adequate heat sinks and a fan for cooling.
I know that losses would always occur.
But am just confused on how to determine amount of FETs needed.

 

[FvM]

Rather than maximal power dissipation numbers, transistors in switch mode converter will be selected based on maximum current and voltage, conduction and switching losses. To give a ballpark estimation, you can use the transistor up to rated continuous current, better at 50 to 70 % to achieve better efficiency.

 

[maicael]

Hello
Yes to achieve good efficiency we do not use the MOSFETs to its full capacity but maybe i should explain my confusion.
We use four N channel MOSFETs in h bridge xonnection, now if each MOSFET is rated at 300w and assuming no losses and we use them at the bridge what will be the power output at the transformer?
Will it be 300 * 4 = 1200W(assuming no losses)
Or will it be 600W since two diagonal MOSFETs are on at a time.?
That's my slight confusion with how to calculate how much power can be delivered.

 

[FvM]

As said, the H-bridge output power is not directly related to transistor power dissipation.

 

[KlausST]

Hi

As said, the H-bridge output power is not directly related to transistor power dissipation.

I´d emphase this:
Max. rated power disspation of a MOSFET is not at all related to output power of a H bridge.

The actual power dissipation of a MOSFET is mainly power dissipation caused by
* RMS current: P_d1 = I_RMS ^2 * R_DS_on
* and switching loss. Switching loss depend on: voltage, switched current, switching frequency, MOSFET parameters, gate drive current ...

Klaus

 

[Audioguru]

The irfp260n Mosfet is rated at 280W MAXIMUM when its case is cooled somehow down to 25 degrees C. But of course the Mosfet will be hotter than the heatsink and the heatsink will be hotter than the fan-blown air so maybe its REAL maximum rating is 100W or less.

If the Mosfets turn on completely and quickly then the efficiency will be high and they will heat with a total of about 100W if the inverter output power is 300W. Then only 2 Mosfets on a huge heatsink with a fan will be fine.
Where will you buy a transformer that works properly at 25kHz?

Why don't you and the other students (you said, "we") buy an inverter, take it apart and put it back together instead?