# Will replacing MOSFET with a MOSFET with lower rds(on) reduce the thermal characteristics?

I want to replace a MOSFET in a legacy product. The MOSFET is a P-channel Power MOSFET that is used for PWM switching purposes (VGS @ -12V, PWM is 20% @ 200us). The FET is switching in to a 200uH, 10ohm load (see image, 5 and 6 are a coil of 5ohms 200uH @ 10KHz). The FET has quite a large heatsink attached due to it getting hot. I'm wondering if the RDS(on) of 1.5 ohms may be the culprit as this is fairly high resistance in saturation mode? Would it be worth looking at alternatives with similar characteristics but with lower RDS(on) (in the milli-ohm range)? Or is the issue potentially more complicated / caused by the load? Any help would be great.

• The "thermal characteristics" are given by the MOSFET, if both MOSFETs have the same thermal resistance then the "thermal characteristics" are the same. What you mean is the power dissipation which will indeed decrease with a lower $R_{DS,on}$ assuming everything else stays roughly the same (if the low $R_{DS,on}$ switches much slower, things are not the same). A lower power dissipation means that less heat needs to be dissipated and that can result in lower temperatures. – Bimpelrekkie Feb 10 at 15:15
• I think you're assuming that a low RDSon does not come without a cost. But it does; increased gate capacitance. This makes each switching cycle possibly longer, which could cause more heat dissipation. You'll need to analyze this very thoroughly. – rdtsc Feb 10 at 15:47
• Looking at the thermal specifications, the proposed substitute device has a lower Rth(j-c) but a higher thermal derating coefficient. – vir Feb 10 at 16:55
• @rdtsc you're correct I was assuming that as this is a particularly old product - I was wrong to do so. It seems that the cost of increased gate capacitance tends to mean a slower turn on in the range of 10-100ns but this can be at a reduced RDSon of just a few 100 milliohms. As the switching frequency is fairly "slow" at 5KHz I'm wondering how much of a difference the slower switch on will make, I don't think it matter a lot. Time to experiment. – ChrisD91 Feb 12 at 8:23
• You could try to simulate the two using LTspice for a rough estimation. – rdtsc Feb 12 at 15:57