integrating the propeller with a power mos h bridge.

Zap-o

I am designing a circuit board and I am stumped!

Using the propeller and several other digital devices that operate on 3.3Volts, its is one of my restriction to this circuit. My problem is driving an h-bridge using power mosfets with a 5volt 10amp power supply and the propellers 3.3v. You can imagine that the gate of a power mosfet begins to operate fully at ~12Volts with respect to the source. So I implemented a charge pump circuit that takes a PWM i/o pin from the propeller at 3.3V and pushes out 13.5V. This works for turning on and off the h-bridge but leaves me no way to control the low side of the bridge. Any ideas on what I can do? Keep in mind that buying a IC to do this task is almost out of the question. It seems every time I design a product these days and have to order parts for that product, I can no longer get the part because of the long lead times. In some situations over a year long lead time is implemented (Texas Instruments is famous for this). That said I want to try to make this part using diodes, resistors and mosfets / transistors.

Phil Pilgrim (PhiPi)

Your job would be much easier if you use a MOSFET with a lower turn-on voltage, such as the IRF3708. With those, the lower legs can be driven directly from the Prop, and you won't need such a high multiplier to drive the top ones.

-Phil

Zap-o

I feel stupid! thanks for the eye opener. I seem to get tunnel vision and things get complicated. KISS (keep it simple stupid) tattoo is needed.

Phil Pilgrim (PhiPi)

You can also eliminate any need for a voltage multiplier if you use pMOSFET transistors in the top sections. Since your motor voltage is only 5.5V, you can drive the upper transistors via 74HCT logic, powered from the motor supply. Just be sure to provide adequate filtering for the portion that powers the logic.

Also, be sure to guard against "shoot-through." This occurs when the upper and lower transistors on one side of the H-bridge are turned on at the same time. It's always best to prevent this with hardware, rather than relying on your program to do it.

-Phil