General ESC Theory Discussion

[BrianG]

I thought I'd start a thread on exactly how ESCs work in general. Some of this is to clear things up for me, and some to help others understand how they operate.

From what I understand;

• ESCs use PWM to send varying levels of power to the motor coils. Greater pulse width means more power.
  
  
• ESCs use FETs for a few reasons:
  
  
  1. FETs can easily be paralleled because they have a positive temperature coefficient (which means they conduct less as they heat up) preventing any one FETs in a parallel set going into thermal runaway - a sort of self-regulation. Regular bipolar transistors have a negative temperature coefficient, which means that if several are paralleled, one will conduct more than the others, which heats it up more, which makes it conduct more (etc) until it blows leaving you with one less device to handle the current.
     
     
  2. FETs have a lower "on" resistance than transistors, which means they drop less voltage, which means less heat generated. And paralleling FETs further reduces total "on" resistance for less heat.
     
     
  3. FETs drive inputs are high impedance, so you don't have to worry about current gain and any loading down the drive signal at high currents like you would with plain bipolar transistors. This makes FETs ideal to drive with logic signals.
  
  
• Heat generated by an ESC is caused by two things:
  
  
  1. The voltage drop caused by the on resistance (Rdson) value of the FET.
     
     
  2. Slew rate, which is the time it takes to go from 0v to max voltage. Faster slew rate means that it will not have a voltage drop for as long of a time (if that makes any sense).
  
  
• By looking at several different ESCs, they always use a multiple of 6 FETs (2 FETs times 3 phases). So, does that mean these use the classic H bridge configuration? If so, then an ESC like the MM with 36 FETs means that power is flowing through 12 FETs at once - battery power through 6 FETs and ground through 6 FETs. Correct?

One of the things I don't understand is why using the motor for braking is hard on controllers and batteries. For the induced voltage from the motor to even get to the battery, that would mean all the phases get turned on at once, right? So, why not simply short all phases to ground instead? The controller would still have to be able to shunt that power, but the motor would be taking the majority of the abuse.

So, feel free to comment or correct any of above statements, or add more information.