# Why PWM?



## xtian999 (Apr 26, 2009)

I am famous for missing the obvious, so will someone please tell me why the controller mosfets need PWM? It seems that if their output is proportional to the gate voltage, then a simple 5 volt source with a variable resistor before the the gate would regulate the mosfets just fine. No switching on and off, just an analog amplifier. Besides losing a few watts of efficiency through heat, what other disadvantages are there?


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## Anaerin (Feb 4, 2009)

xtian999 said:


> I am famous for missing the obvious, so will someone please tell me why the controller mosfets need PWM? It seems that if their output is proportional to the gate voltage, then a simple 5 volt source with a variable resistor before the the gate would regulate the mosfets just fine. No switching on and off, just an analog amplifier. Besides losing a few watts of efficiency through heat, what other disadvantages are there?


MOSFETS, and especially IGBTs work their best when either on or off. The "few watts" you would lose on a small (400W) amplifier become not so small when you're dealing with EV-type power levels (several kilowatts throughput makes for several hundred watts lost in heat). Without a LOT of cooling (phase-change cooling, at the very least), running MOSFETs and IGBTs at 50% capacity would burn them out in a matter of moments. You'd have the option of 50% load for 2 seconds, or a 50% PWM duty cycle for hours, even days.


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## xtian999 (Apr 26, 2009)

OK, Thank you. So it is a matter of heat.


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## Qer (May 7, 2008)

xtian999 said:


> OK, Thank you. So it is a matter of heat.


Not only. Let's say you have a pack of 100 Volt, want 50 Volt over the motor and that that results in 100 Ampere (yes, I've picked very easy numbers, I know ). Now, if you run that in linear mode it means that you will get 50*100=5kW power from the motor, but you will also get 5kW over the silicon that produces nothing but heat, draining a total of 10kW out of the battery pack. Not only will this result in a LOT of power to cool off from the silicon before it burns up, it also means that half of the energy you pull out of the pack will go to waste as heat and thus cut your range in half.

If you instead chop up the current with a PWM signal that runs at 50% duty cycle you will get an average Voltage over the motor of 50 Volt (same as above) and when the transistors are switched on the current through them will be 100 Ampere, but since the duty cycle is 50% you will only drain the battery with 50 Ampere in average. Now, since the motor is a huge inductor it will strive to keep the current flowing so even when the battery is disconnected the built up energy in the motor will keep the current flowing in the same direction, but since it's the motor that generates the current it will shift polarity and flow through a diode, or several diodes, that is connected in parallel with the transistors (but the opposite direction). This means that the battery will have 100 Volt pack Voltage and the current out from the battery will be 50 Ampere but the motor will only have 50 Volt over itself while the current will be 100 Ampere. So you're draining 5kW from the pack and the motor will generate 5kW power, nothing goes to waste and your range suddenly got doubled!

In reality nothing can have 100% efficiency and you will typically lose 1-2 Volt over the controller which in this case will mean that about 100-200 Watt goes to waste, but it's still much better than 5kW as in the first example. It's also much easier to cool off from the transistors (since there's less heat to cool off) which means that you can handle all this power with fewer transistors as you would have been forced to use if you ran the controller as an ordinary amplifier.


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## aeroscott (Jan 5, 2008)

thanks I wondered about that .


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