H-Bridge

These end up getting tricky to build discretely, but high power versions get costly.

I think DRV8701 allows you to mount your own mosfets, which may allow me to increase the power capacity. look for "Full-Bridge Gate Driver" or "full-bridge pre-driver"

• http://www.modularcircuits.com/blog/articles/h-bridge-secrets/
• https://www.youtube.com/watch?v=xbJ0dT-VZIQ

Discrete Driver

All

Need protection diodes for inductive loads, so just always include them. A smoothing cap is also a good idea.

BJT

Uses PNP high side and NPN low side.

The microcontroller can simply provide the driver logic, though this will need 4 pins and require a little code. Something like an ATTiny can be used as a dedicated driver.

Driver Logic

• Input: F (forward), On, V, G
• Output: FH, FL, RH, RL (forward/reverse high/low)

Needs deadtime, enforced with shcmitt triggers.

• B: transistor buffer
• N: transistor not
• S: schmitt trigger
• Von = B(V, On)
• X = B(Von, F)
• Y = N(Von, F)
• F = S(V,X)
• R = S(V,Y)
• X' = N(Von, F)
• Y' = N(Von, R)
• F' = S(V,X')
• R' = S(V,Y')

The whole thing takes 25 resistors and 13 transistors.

For low power, this should be able to directly drive the HB, but I need to look at it to make sure there's not an Emitter/Base path through the circuit. For higher power circuits add a buffer before.

Voltage Range

If a narrow voltage range is used, circuits can be made quite simple. If a wide voltage range is needed, the circuit gets more complex to compensate.

Opto-isolators need current regulation. For small voltage ranges, resistors can be used. For wide ranges, two transistors and two resistors can form a current regulator.

Mosfets

Using a combination of P-channel and N-channel mosfets produces a fairly simple circuit. But the P fets will be more expensive, have a higher resistance and a lower max voltage.

To use all N-channel, a high side driver is needed. If the circuit can be pulsed, bootstrapping can drive the high side. But the caps will drain slowly so it can't be left on. To have a high side that can be left on, a charge pump is needed. For both, voltage regulation is needed to make sure the gate never exceeds its rated voltage.

VFD

3 half bridges can be used to create a VFD for a 3 phase induction motor.

Microcontroller Driver

Out

• A high
• A low
• B high
• B low
• PWM signal for high side driver

Probably accept input over something like SPI. This would allow it some flexibility.

• Off
• Hold (A/B)
• Direction/Speed
• High side voltage feedback

These could also be provided as steps with times. So a whole program could be sent to the controller then a "START" signal could trigger it. This would allow multiple controllers to run in unison.

The high side voltage would be a small flyback driver. The PWM provides a driver for a BJT. The input would be driven with a opto-coupler and would go high when the voltage is above the threshold.

The actual bridge controller would be designed to use BJTs to drive the MOSFETS.