MOSFETs used in driving BLDC motors

Thread Starter

electronicsLearner77

Joined May 26, 2012
125
Really very critical question for me. What characteristics of MOSFETs effects the performance of a BLDC motor. The performance of BLDC motor i mean to say the speed and torque of the motor. And characteristics of the mosfet i mean to say like switching speed or which characteristic affects the most and what should i look into mosfet selection? Any mosfet will do the job or some company make will do better? Please advise.
 

MrChips

Joined Oct 2, 2009
21,400
One usually looks at the following parameters:

N or P-Channel
Supply Voltage VDS
Drain Current ID
Power Dissipation PTOT
On Resistance RDS
Input Capacitance Ciss
Switching Times td, tr, tf
Package

STP75NF75 is a typical N-Channel 75V 80A MOSFET in a TO-220 package.
 

Thread Starter

electronicsLearner77

Joined May 26, 2012
125
So the above characteristics control the speed of the motor if the switching algorithm is proper. Am i correct ? Why i am asking is even if my pwm signals from the controller are proper but if the mosfets add delay the voltage may not be properly applied to the motor terminals. Is it true?
 
So the above characteristics control the speed of the motor if the switching algorithm is proper. Am i correct ? Why i am asking is even if my pwm signals from the controller are proper but if the mosfets add delay the voltage may not be properly applied to the motor terminals. Is it true?
There will always be some time offset. First, the MOSFET gate looks like a big capacitor and the gate driver needs to charge that up in order for the MOSFET to conduct. The time it takes depends on the gate resistor value and the type of gate driver used. Then, each MOSFET has a specified ton value and corresponding toff value. One must also be careful not to choose too fast of a MOSFET or ringing could occur. In demanding applications, picking a drive MOSFET is not straightforward, there are a lot of nuances to be considered.
 

avayan

Joined Oct 30, 2015
38
I don't think the FET's characteristics will affect the motor performance “that” much. Some, of course, but not a whole lot, if properly selected. What it will affect is the system and whether it will work or not before catching flames. What do I mean by this?

The MOSFET VDS will tell you what is the maximum voltage it will tolerate. For example, if your battery voltage is 20V and the FET is rated at 40V, then you better be sure the motor's BEMF never exceeds the 40V mark as once that happens, you may need to kiss the FET goodbye. What can cause the motor voltage to increase the voltage so much? There are many mechanisms, but usually it is transients such as changing speed, dumping lots of currents into the battery, problems with parasitics (i.e. long wires, little electrolytic capacitance, etc.). But, if your application voltage never exceeds the 40V, then no worries!

Then comes current. This is a super tough area because you will see a current rating, but this is often associated with ambient temperature and the system's thermal impedance. Just because a FET is rated at 100A does not mean you can do 100A for saecula saeculorum. If your thermal impedance is really, really good (i.e. lots of heat sinking, air flow, etc.), then you may be able to sink/source 100A for minutes or hours. But if the thermal impedance is poor (and it is often the case), then you will start to gain temperature rise and eventually the FETs will over heat. Once again, this is the moment in which the FET smokes out. Do note the current requirement is driven by the application torque, not the FET or even the motor, although the motor will kind of have a little bit of a say. The load has the last word, though.

100% tied to current is the RDSON. This is probably the most important aspects of FET selection as ideally you would want a 0 Ohm RDSON to make the power loss 0. Yet of course, such a dreamy thing remains to become real… What RDSON will do, however, is tell you how much energy is lost on the FET. And whatever power is lost on the FET, is a power you can’t juice into the motor.

There are two components. I^2*R is how much power will be dissipated in the FET in the form of heat. And again, this power the motor will never see. Also, RDSON*I will tell you what is the voltage drop. Again, whatever voltage is dropped on the FET is a voltage the motor will not see. The result? This is a speed you won’t be able to reach. For example, if you have a 20V battery but you lose 1V in the FETs, wiring, and all that stuff, then your motor can’t obtain a speed equivalent to 20V, because the motor can’t see 20V. There are some algorithms which squeeze the voltage the motor sees, but these are way out there… Do note in most cases the RDSON is so low, this only starts to become noticeable at high current levels. This is the main motor performance impactor, though.

Finally comes all the mambo jambo with internal gate resistance, input capacitance, etc. All of this is important to the extent of what is called switching losses. RDSON represents conduction losses, and turning the FETs ON and OFF represents switching losses. Add these two losses up, and that is the power the motor will never see. If you can turn the FETs ON/OFF REALLY fast, then you have very little switching losses, albeit you increase your EMI. If you turn it ON/OFF really slow, then the switching losses increase. Also, how fast you turn the H Bridge leg ON and OFF defines your dead time, which also plays a role in efficiency. BLDC motors, however, have fantastic efficiencies! I think the record is something like 96% or something around there?

The propagation delay on the MOSFETs really doesn’t do a whole lot of impact, if any. First of all, they are super short (nanoseconds) and second, if everything is phased out equally, then to the motor nothing is phased out.

Sorry for the long post!
 

Thread Starter

electronicsLearner77

Joined May 26, 2012
125
I don't think the FET's characteristics will affect the motor performance “that” much. Some, of course, but not a whole lot, if properly selected. What it will affect is the system and whether it will work or not before catching flames. What do I mean by this?

The MOSFET VDS will tell you what is the maximum voltage it will tolerate. For example, if your battery voltage is 20V and the FET is rated at 40V, then you better be sure the motor's BEMF never exceeds the 40V mark as once that happens, you may need to kiss the FET goodbye. What can cause the motor voltage to increase the voltage so much? There are many mechanisms, but usually it is transients such as changing speed, dumping lots of currents into the battery, problems with parasitics (i.e. long wires, little electrolytic capacitance, etc.). But, if your application voltage never exceeds the 40V, then no worries!

Then comes current. This is a super tough area because you will see a current rating, but this is often associated with ambient temperature and the system's thermal impedance. Just because a FET is rated at 100A does not mean you can do 100A for saecula saeculorum. If your thermal impedance is really, really good (i.e. lots of heat sinking, air flow, etc.), then you may be able to sink/source 100A for minutes or hours. But if the thermal impedance is poor (and it is often the case), then you will start to gain temperature rise and eventually the FETs will over heat. Once again, this is the moment in which the FET smokes out. Do note the current requirement is driven by the application torque, not the FET or even the motor, although the motor will kind of have a little bit of a say. The load has the last word, though.

100% tied to current is the RDSON. This is probably the most important aspects of FET selection as ideally you would want a 0 Ohm RDSON to make the power loss 0. Yet of course, such a dreamy thing remains to become real… What RDSON will do, however, is tell you how much energy is lost on the FET. And whatever power is lost on the FET, is a power you can’t juice into the motor.

There are two components. I^2*R is how much power will be dissipated in the FET in the form of heat. And again, this power the motor will never see. Also, RDSON*I will tell you what is the voltage drop. Again, whatever voltage is dropped on the FET is a voltage the motor will not see. The result? This is a speed you won’t be able to reach. For example, if you have a 20V battery but you lose 1V in the FETs, wiring, and all that stuff, then your motor can’t obtain a speed equivalent to 20V, because the motor can’t see 20V. There are some algorithms which squeeze the voltage the motor sees, but these are way out there… Do note in most cases the RDSON is so low, this only starts to become noticeable at high current levels. This is the main motor performance impactor, though.

Finally comes all the mambo jambo with internal gate resistance, input capacitance, etc. All of this is important to the extent of what is called switching losses. RDSON represents conduction losses, and turning the FETs ON and OFF represents switching losses. Add these two losses up, and that is the power the motor will never see. If you can turn the FETs ON/OFF REALLY fast, then you have very little switching losses, albeit you increase your EMI. If you turn it ON/OFF really slow, then the switching losses increase. Also, how fast you turn the H Bridge leg ON and OFF defines your dead time, which also plays a role in efficiency. BLDC motors, however, have fantastic efficiencies! I think the record is something like 96% or something around there?

The propagation delay on the MOSFETs really doesn’t do a whole lot of impact, if any. First of all, they are super short (nanoseconds) and second, if everything is phased out equally, then to the motor nothing is phased out.

Sorry for the long post!
Thank you really for such a detailed mail.
 
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