Glaringly obvious errors

[MrHeckles]

Hi guys.

Could someone please have a quick look at the circuit below and let me know if there are any glaring obvious errors that would result in smoke and/or flames when a battery is connected to it.

Many thanks

[audioguru]

The circuit doesn't have anything to turn off the Mosfets.
The transistors are wired strangely.

[indulis]

Looks like a PWM (around 59KHz??) motor speed controller, and you're trying to multiplex the half bridge driver?

How big is the motor , and what's it going to do... ???

[MrHeckles]

Yes it is a PWM motor controller. Its a test to see if I can get a variable forward speed, and a set reverse speed with the switch activated.

The transistors are supposed to be switches that will allow the appropriate mosfets to be used in either forward or reverse. I thought that grounding the bases of the transistors in certain circumstances (ie switch on or off) could disable the PWM pulse to various mosfets but allow others.

Since I am kind of just making it all up as I go along, its very possible that i've missed  some point entirely as to how something works.

audioguru - can you explain a little more about being nothing to turn the mosfets off. Are the transistors allowing them to stay on even if the pulse from the half-bridge driver is low? Also what do you mean by the transistors wired stangely?

[audioguru]

audioguru - can you explain a little more about being nothing to turn the mosfets off.
Are the transistors allowing them to stay on even if the pulse from the half-bridge driver is low?

The gate of a power Mosfet has a faily high capacitance. One of your transistors pulls a gate high to turn on the Mosfet then the gate stays charged when the driver transistor turns off, then the Mosfet stays on.

Also what do you mean by the transistors wired strangely?

Power Mosfets usually use complimentary emitter-follower transistors to charge and discharge a Mosfet's gate capacitance quickly. An NPN to turn it on and a PNP to turn it off.
Your circuit has transistors all over the place.

[MrHeckles]

My idea behind this mess was to have a motor driver that would have variable forward speed, and when the throttle pot was pulled all the way back, trip a microswitch to activate a preset reverse speed.

I thought that I could use the transistors as switches to enable or disable the pulse train from the half-bridge driver to the gate of the appropriate highside and lowside mosfets.

Could you perhaps point me in a simpler direction for doing this? I'm a bit lost now as to how to do it.

[indulis]

Varying the duty cycle will change the speed, and a fixed duty cycle will cause the speed to remain constant, but that will ONLY work if you have a constant load on the motor. If the load can change, then you'll need feedback (like a tachometer) if you want to maintain the constant speed. Why not just use 2 of the half bridge drivers. Then you don't have to worry about how the MOSFET's get turned off.

[MrHeckles]

Okay, how about this then.

I figure with this setup, when the PWM signal is switched to the other half-bridge driver, the one not selected will hold the lowside mosfet on (which is needed for the reverse). The selected driver will switch the highside and lowside mosfets on as needed.



Comments?