MOSFET heat question

[audioguru]

The on-resistance of the Mosfet increases when it gets hot. That causes it to get hotter. That causes the on-resistance to increase more. Thermal runaway.

A low resistance Mosfet switching very quickly gets cooler as it pulses PWM at a lower average current. It gets hotter if it switches slowly. It has a high gate-source and feedback capacitance so it is difficult to switch quickly.

 

[ronv]

Actually if you are only going to drive a light bulb you don't need the diode inductor or capacitor. It sounds like you have control of the PWM frequency so just drive it at higher than 60 Hz so your eye won't see it flicker and it won't cool much between cycles and it should work ok. What FET are you using now, and what are you driving it with?

 

[skibum100]

I think you dont understand that to get rated power dissipation when using any FET or transistor it MUST BE BOLTED TO A LARGE HEATSINK! A TO220 device in free air can only dissipate a couple of WATTS. To get rid of 50W of heat, it must be bolted to a large heatsink with an area of several HUNDRED square inches...

I understand what you are saying, but where on the data sheet is the free air number? I'm not trying to dissipate 50W, only 1/100 of that in conductive mode (0.5Watts conductive power dissipation). I would not expect to need a heatsink but what should I expect the temperature to be in the MOSFET without a heat sink. I'm asking this question to better understand my specific situation but also as a theoretical question so i can more thoroughly understand these devices and how to read the data sheets...

The on-resistance of the Mosfet increases when it gets hot. That causes it to get hotter. That causes the on-resistance to increase more. Thermal runaway.

A low resistance Mosfet switching very quickly gets cooler as it pulses PWM at a lower average current. It gets hotter if it switches slowly. It has a high gate-source and feedback capacitance so it is difficult to switch quickly.

This is opposite what I am seeing in my example. All other factors equal I changed the frequency, when doing this the heat in the MOSFET decreased with lower frequency. This is what I expected after I found the equation for switching heat dissipation,

PDSWITCHING = (CRSS × VIN2 × fSW × ILOAD)/IGATE

Also, I found a really good writeup on MOSFET heat calculations that validates my testing data.
**broken link removed**

But I also noticed the capacitor was getting hot to the touch, especially at the really low frequency (120Hz). Can somebody explain this and why the cap would get hot?

Actually if you are only going to drive a light bulb you don't need the diode inductor or capacitor. It sounds like you have control of the PWM frequency so just drive it at higher than 60 Hz so your eye won't see it flicker and it won't cool much between cycles and it should work ok. What FET are you using now, and what are you driving it with?

I'm using the P10T MOSFET, data sheet link as follows:
https://www.electro-tech-online.com/custompdfs/2012/09/DS100024AIXTA-TH-TP76P10T.pdf

I'm using an Arduino Mega controller

I consider your suggestion to remove the inductor, cap, and diode...but would this have negative effects on the bulb lifespan? I'd image this would be harder on the bulb filament without the smoothing element of the buck converter arrangment.

 

[alec_t]

But I also noticed the capacitor was getting hot to the touch, especially at the really low frequency (120Hz). Can somebody explain this and why the cap would get hot?

Capacitors have an equivalent series resistance (ESR). At low frequencies the ripple current through the cap is increased (the cap discharges and re-charges more) so the heat dissipated by the ESR increases.

 

[ChrisP58]

I understand what you are saying, but where on the data sheet is the free air number?

That is listed under "Junction to Ambient" thermal resistance.

 

[ronv]

I understand what you are saying, but where on the data sheet is the free air number?

Thermal resistance junction to ambient is what you are looking for, but I gotta tell you I don't see it on that data sheet. TO220 packages (like this one) are 62.5 C per watt.

I'm not trying to dissipate 50W, only 1/100 of that in conductive mode (0.5Watts conductive power dissipation). I would not expect to need a heatsink but what should I expect the temperature to be in the MOSFET without a heat sink.

Normally you would be correct, but in your case you must have like a 5 volt lamp if you are driving it from the high side with the micro? If thats the case your gate to source voltage is only 5 volts or so. The 25 Mohms is with 10 volts gate to source with your FET. -- See figure 3. If you want to drive it with 5 volts you need a logic level FET. So you really have 2 problems: The gate to source voltage and the micro can't drive much current so it can't discharge the gate capacitance very fast so the FET won't switch fast. You can calculate switching time using Qg. This is the time it takes (in ns) for the FET to switch with 1 amp of gate drive. So in this case it's 197 divided by say 20 ma from the micro it would take 10 usec. to switch it.

But I also noticed the capacitor was getting hot to the touch, especially at the really low frequency (120Hz). Can somebody explain this and why the cap would get hot?

You will need to look at the ripple current in the cap. At 120 HZ it is really high.

I consider your suggestion to remove the inductor, cap, and diode...but would this have negative effects on the bulb lifespan? I'd image this would be harder on the bulb filament without the smoothing element of the buck converter arrangment.

It might make a slight difference in life but not much if you keep the frequency up to the point where the bulb doesn't have time to totally cool between cycles.