Hello,

I am startng work on a new project for a PowerPC 32 bit miicrocontroller (Freescale.) We have a normal PWM driver as part of the low level firmware for this micro, however the customer specified they want to use their own "Current controlled PWM driver."Droid Would anyone be able to explain what the difference is between a normal PWM driver and a current controlled?

Thanks in advance for your help!

 

The internal PWM generator is great for generating voltage source inverter drive--in this mode of operation, it is essentially an open loop control--the micro determines the output voltage and frequency and the PWM generator outputs the appropriate transistor drive signal(s). Possible limitations include fixed carrier frequency and/or synchronous operation of all phases which both drive up the carrier harmonic amplitude--this can be an issue.

With more modern inverter technology such as in three-phase motor drive vector control, voltage source does not cut it, because the exact phase relationship between the stator and rotor flux must be controlled--this is done via current source techniques that tend to drive the motor to whatever voltage required to obtain the desired stator current. In this system, the micro generates the stator current waveform and the inverter must deliver this current to the motor. It is not practical to do this via micro controlled voltage source PWM, so external constant current regulator drivers are employed for each phase. One good way of doing this is simply a current feedback comparator that simply decides if the phase current is above or below the reference current and controls the H-bridge accordingly--there is no carrier frequency in this case as it is totally asynchronous--or whatever happens. The result is remarkable in that there is no carrier whine, but simply white noise.

Hope that this is what you are looking for.