Amps vs Voltage In a MOSFET = Watts

Hi.

What is the maxium power we can get out of the IRF510. On the datasheet it states that the maxium voltage can be 100v and the maxium permissable amperage is 5.6 with RDs(On) 0.54 ohms.

Does this mean I can get 5.6 amps at 100v which would be 560 watts or would it be 5.6 amps at 1 volt and a correpondingly small fraction of amps at 100v.

The datasheet just does not indicate the power.

Thanks a lot for your help.

Hello,

The relation between simultaneous voltage and current versus the time duration is given in the so called Safe Operating Area (SOA, FBSOA). It shows curves in a U/I graph where time duration is the parameter. Time duration spans from DC in steps to ms or us.

The maximum current ant voltage may simultaneously only be applied during very short time (us range). Note that the graphs are for a starting temperature of 25 degrees. So when temperature at the beginning of a large power transient is higher, you have to reduce the peak power during certain time. That means less current, voltage or time duration.

The maximum power you can generate into a load depends on the circuit. When you use this mosfet as a fast switch, you don't have inrush current peaks and apply the correct drive, theoretically you can switch a 560W load. The mosfet itself has to dissipate MORE then 0.54*5.6^2 = 17W. The reason for that is that at higher junction temperature, Rdson increases, so you have to calculate with a higher value for Rdson. Rdson versus temperature is mostly given in a graph also.

I would reserve some margin and not push the component to the limits.

Power dissipation (for the device, not the load) is given under the assumption that the temperature of the case of the fet is 25 degrees. As this is normally not the case, you have to reduce this also, based on your heat sink.

It seems this matter is new to you, please don’t hesitate to come back in case of doubts.

Saludos y mucha suerte con su circuito.

Does it not give a total power rating as well?
I'm pretty sure I saw it being around 90 watts for the IRF530 in TO-220 package...

Does it not give a total power rating as well?

Click to expand...

Yes, but the question was about achievable power to the load, not the device dissipation. As WimRFP pointed out, the
Safe Operation Area must be considered in this case. Maximum static power dissipation is just one limit line in the SOA
diagram. The said static 90 W is however a mostly theoretical value, because it assumes an infinite heatsink that doesn't
exist in a real application.

Thanks again.

We have to calculate how many IRF 510 we'd need to operate a 500 watt motor via a pwm circuit. We see that when the Mosfet IRF 510 is pulsed it can be used to transport higher currents. In fact in the datasheet it states 48 amps!!!!!!!!!!!!!!!!!!!! That seems impossible, can it be?

We are starting to look at more useful circuits which seem to present many more difficulties than the circuits we've done up to now. Any help will be most appreciated.

Thanks very much.

Where are you seeing those 48 amps? I think an IRF510/530 have about 4-6 amps of constant drain current.

The 'pulsed' current may be higher, but that's at 10% duty cycle only.

You'er right! It states 20amps when pulsed.

Hello,

With regards to the DC motor, when using PWM, you can smoothly ramp-up the voltage to the desired level. This avoids the high inrush current what you get when you apply 100V directly to the motor.

When paralleling mosfets, it is best to give each mosfet its own gate resistor to have better current sharing during turn-on and turn-off. You also have to find a good balance between the number of mosfets, switching frequency and heat sink requirements. Many mosfets reduce conduction loss (I^2*(total Rdson), hence your heat sink requirements will be less. Many mosfets need a more powerfull driver to avoid significant switching loss. Very fast switching may result is more EMC problems.

I don't know what sort of load the motor is driving, but when there is a change on jamming or other circumstance that may result in high current, you may add a current measuring circuit. Based on the measured current, you can control the PWM.

It will results in higher component cost, but you may consider EMC filtering between the power stage and the motor to avoid additional costs because of inconvenient EMC test results. It also enables you to make the current waveform less dependent on motor type and wiring (in case of long cables between power stage and motor).