Low voltage H bridge driver

H-bridges that use four n-channel MOSFETs, with a flying self- powered high-side gate-driver capacitor, usually need at least 10 volts or so to run the gate driver, but it's possible to get this from a different supply than the H-bridge MOSFETs, which allows you in principle to vary the MOSFET supply all the way down to zero. Usually one doesn't vary the MOSFET supply at all, but instead uses PWM modulation of the gate-drive signals to vary the effective output voltage.

Although it may be impractical to vary a conventional PWM signal all the way down to 0% (i.e., 5% of a 200kHz period is 0.25 us), it is practical to do this with a phase-shift modulator, such as the ucc3895. In the phase-shift scheme, both sides of the H-bridge have a 50% duty-cycle square-wave, but their relative phase sets the actual output. For example, if both square-waves go up and down together (exactly the same phase), no relative difference occurs and the actual output is 0%.

I made a 500-watt 300kHz driver using both schemes (phase-shift modulation and DC power-supply variation) to easily achieve a precision 2000:1 output range, and make a precision 5 volts to 10kV RF ramp for a mass spec. I placed the ucc3895 inside its own phase-shift feedback loop to ensure accuracy, made by simply adding an XOR gate and a resistor to the basic ucc3895 circuit.

Sorry, my initial post was a misleading: where I say I want to do either PWM or linear control, I meant I want to do both. The reason being in some situations I want to control the motor using a linear voltage to minimise EMC.

As you point out, some volts are required to run the gate driver, but this can be derived from the logic supply. All the integrated devices I've looked at so far appear to require the bridge supply to be greater than the logic supply.

regards, Andrew

In that case you're assuming 100% duty cycle? As I pointed out, I used both linear and phase-shift mode together. How? I used a linear square-root circuit to determine the linear-supply portion, and the rest came from the phase-shift-modulation chip.

The logic supply is generally 5 volts or lower, and you need 12V or more for the MOSFET-driver supply. But it's usually easy enough to have two supplies.