Phase Drive Not So Cool


A few weeks ago I posted about a design where I would try a motor drive technique called “phase-drive”.  You can read that post at Phase Drive a DC Motor.

In summary, “phase-drive” runs a motor at its stall point.  To stop the motor you run it 50% forward and 50% reverse, switching back and forth over short periods of time.  To move the motor forward you would begin increasing the forward percentage and decreasing the reverse percentage of the drive signal.  For example, 51% forward, 49% reverse is the slowest forward speed you could have.   100% forward and 0% reverse would be the fastest.  You might run the pulse-width-modulation (PWM) at around 15KHz to keep the switching sounds less audible, and update the forward/reverse percentage every millisecond.

This is hard on the circuit as well as the motor.  Why do this?  We had a customer that was running an ungeared motor in a position control system and the load was overpowering the torque the motor could provide.  Since it was a position control design there was no electrical “push” resisting the motor’s load when the system was “in position” (PWM was 0 when PID output was 0).  I wanted to see if this method could provide some resistance to the load by driving the motor even when it was in position.  The eventual solution was to use a geared motor, but I added circuitry to test the phase drive technique on our design out of curiosity.

In the end phase-drive did provide additional resistance to the load.  And if that was the end of it, I might have used it.  But I never really got to the point of quantifying how much better the load resistance was using phase-drive.  The motor was extremely hot to the touch after just 10-15 minutes of hitting it with phase drive.  Within a half hour of testing I was back at the solder station reconfiguring the board to drive it in the standard fashion.  Phase drive was just not cool.

What was cool was the H-bridge circuit we were using.  A pair of Infineon BTN8960TA high current half-bridges with some external spike protection components seemed to work very well  (there’s a schematic of the h-bridge on the post linked above).  The picture above shows the H-bridge portion of the circuit, top and bottom.  This circuit regularly drew 6-7A without generating any fault conditions or getting warm.  The BTN8960TA are rated for much more current than that, something like 30A.  However, thermal issues always seem to be the key to providing large currents to the load and you can see that our footprint for the design is pretty small. I was happy with the efficiency we got from the parts.  We used a 10-bit PWM drive signal with a period of 7.8KHz.

In fact, I liked the BTN8960TA so much that I had extra parts ordered so I could put them on a breakout module.  I’m looking forward to experimenting with these parts and using them in new designs.

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