[clintonedward]

Hi Guys,

Sorry I have a noob question: I have just finished the driver for 5 phase stepper motor. To limit the current, I use an op-amp to compare the V-sense with the reference voltage, if the v-sense is higher than the reference voltage, the op-amp goes low => turn off the signal => the current drops. When the current drops below the threshold, the op-amp goes high, turn on the signal again.

However, I see that in 2 phase stepper motor driver, they use the pwm chopper drive. I don't understand why they don't use the simple constant current solution above but they have to use the pwm chopper?

Really appreciate your advice!
Edward

[doorknob]

What does the switching waveform look like in your circuit? Are the power transistors being turned solidly on or off, or are they spending a lot of time in the in-between linear region? I would suppose that it would depend on factors such as the slew rate of your op amp, the inductance of your stepper windings, the characteristics of your transistors, the regulation of your power supply, etc.

I'd expect that with PWM/chopper drive you would probably achieve much higher efficiency in the driver because the power transistors are either (mostly) on or off, rather than in an in-between region where they would dissipate a lot of power as heat. I would also suspect that there are advantages to having a known, fixed switching frequency rather than a possibly dynamically-changing frequency with your approach (for selection of component values, filtering, etc.).

But that's just a guess on my part, since I have never actually designed a chopper drive.

[clintonedward]

Hey, thanks man

[clintonedward]

Hmm, I haven't got time to test the theory yet but I have some other questions, may you please advise?

1. Why in most constant current circuit, they use power transistor rather than mosfet? Is it because of historical reason (because those circuits appeared before mosfet)? Or any other reason?

2. I use mosfet instead of transistor, so it should be either on or off within 1us, not in the middle, right? So the advantage of chopper drive is not clear here?

Thanks!

[doorknob]

This is just a guess, but "historical reasons" sounds like as good a reason as any. More modern designs are probably going to be based on mosfets.

As for how quickly your mosfet switches on or off, I would expect that it would depend on the drive circuitry more than anything else (for example, as mentioned before. op-amp slew rate). Also, IIRC circuit/gate capacitance in general can affect how quickly a mosfet switches.

But again, I'm no expert in this area. So perhaps someone who is an expert will chime in with their ideas.

[clintonedward]

Hehe, thanks man

[Al_The_Man]

International Rectifier on IGBT or Mosfet.
Al.