How can I choose a MOSFET based on Rds? I have understood all the parameters except this

Question

I have understood all the parameters except R_ds and conduction loss. Please explain them to me.

I am using a lithium-ion battery pack of 36 V and its maximum voltage is 42 V to drive a motor.

Motor specifications:
Minimum current: 500 mA
Maximum current: 15 A
I_pulse: 30 A

The MOSFET I am planning to use is the Vishay SQD50N05-11L

MOSFET specifications:

So based on the above specification, I have calculated the power dissipation:

Power dissipation: I² * R_ds = (15 A)² * 11 mohm = 2.47 watts

And the maximum power the transistor can dissipate without a heat sink:

(T_max - T_ambient) / R(jA) = (175 °C - 25 °C) / 60 °C/W = 2.5 watt

I am confused that how to check and relate the power dissipation with R_ds and select the MOSFET based on it. Is the MOSFET that I have chosen okay, based on the above specifications or should I go for a MOSFET with a low R_ds?

And please explain how to select the MOSFETs based on R_ds.

Answer 1

Your understanding of R_Dson and conduction loss is fine. Just know R_Dson varies with V_gs and increases with temperature, usually by 150% to 200% at its hottest, so you must account for this if you plan on running lots of current or hot weather. You can find the R_DSon vs. temperature graph in the datasheet.

But know there are switching losses if you are switching your transistor at high frequency (or have too long a transition time). Motor PWM is one of those cases where you have to pay attention to switching losses. It is more complex than conduction loss and is supposed to equal conduction loss in an optimal design.

You select one that can handle your current at your switching frequency where conduction loss = switching loss. Trying to select based on conduction loss without switching loss is like trying to pick a car based on top speed and nothing else: if you only care about top speed don't care about cost, fuel efficiency, handling, features, appearance, or anything else. You would just pick as fast as possible, because faster is better and you don't care about compromises made anywhere else, but that's not practical, because you would end up with a drag racer goes fast, but it can't even turn a corner.

Sizing near maximum using only conduction loss when operating at high frequency will burn out you MOSFET due to the heat from switching losses.

Answer 2

The MOSFET with the minimal R_dson is not always the most suitable. Those with a very low R_dson have a large Q_g gate charge. It means that they need a powerful gate driver if you want to do PWM. A higher gate charge makes turn on/off slower, thus higher switching losses. The good MOSFET is the one with the lowest product: R_dson * Q_g

The total loss of a MOSFET is the conduction loss + switching loss. If your application needs only to turn on/off a motor, then the MOSFET you choose will suit your demand.

Answer 3

Ideally, a transistor should have ZERO resistance when turned on, but we don't live in an ideal world, so we're stuck with just getting the lowest resistance we can.
I don't understand why would you even ask such a question, seeing that you understand that higher RdsON means more losses. OF COURSE you will try to get the lowest RdsON available.
There are very few special cases where a higher RdsON would be preferred.
Since you are able to calculate the power loss (the amount of heat generated) based on the RdsON, you are able to see if it will be too much for your circuit.
In the case above, the transistor is pretty good for its purpose, all you need is a small heatsink.
Sometimes, the limiting factor in choosing a MOSFET is also its price, its capacitance and/or speed. Higher capacitance will require a more powerful MOSFET driver and will increase losses at higher switching frequencies, so in that case we don't go for the lower RdsON if it means higher losses overall.