During the design of various motor control systems I’ve run into a few ICs that allow for a peculiar drive method. I’ve seen it referred to as phase drive and direction drive. I’ve never implemented it in a product that’s been produced, but lately I have decided to give it a shot. The traditional method I’ve used for driving a DC motor in a closed-loop application is, simply stated, to drive it forward or reverse. At a direction change the drive signal is typically small. This is particularly true if you implement step limits on the PWM signals change from update-to-update. Doing it this way reduces inductive voltage spikes from high speed reversals, saving your circuit from from death and destruction.
However, I’m looking at a design that may require a certain amount of stiffness at the point of reversal. I would normally use the mechanical design (gearing) to allow for some resistance to load changes at the motor shaft. But I’ve designed a test circuit that hopefully will be robust enough to use phase drive when mechanical resistance is not available. Click on the image above for a larger version of the circuit.
I’m using a newish part from Infineon, the BTN8960TA. The BTN8960TA is a half-bridge with built in shoot-through protection (high and low side switches in the half bridge cannot be enabled at the same time), adjustable slew rate, load current feedback, and lots of other stuff. These parts are rated for 40V max. operation, but are intended to be used in the range of 8-18V (pretty much automotive parts). They have a variety of max. current ratings, depending on how you are using them, but a ball-park figure is 30A. Obviously unless you’ve got a good thermal design you’ll get much less than that for normal operation. Without running the numbers I’m guessing that they’re good for about 5-10A on a small board. They should be overkill for what I’m using them for.
Phase drive operates the motor in a constant state of reversal. You can use a single PWM channel to drive the motor, but need to invert it for the 2nd leg of the H-bridge. In the schematic above I would connect the “INVERTED DRIVE” net to “DRIVE SIGNAL U2” net, and apply a PWM signal to the “DRIVE SIGNAL U1” net. If I were going to use a standard 2 channel PWM drive I would connect the second channel to the “DRIVE SIGNAL U2” net.
In phase drive mode you stop the motor by running forward half of the time and reverse the other half. To move the motor forward you begin increasing the duty cycle so the H-bridge is running forward more often. For example 51% forward at “DRIVE SIGNAL U1” would apply 49% reverse at “DRIVE SIGNAL U2”. That would be a 2% forward drive signal.
Here’s an image of the difference between the phase drive and normal PWM drive used to stop an H-bridge driven motor. You can imagine the pain applied to the H-bridge.
It’ll be interesting to see how much stiffness I can produce in the motor. If the circuit works I might add it to a future breakout module.