Deriving an Isolated 10 volts

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kinarfi

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I need to be able to disconnect my motor in an h bridge, I'm thinking an N FET in one leg with an independent vgs would do the trick.
Any one have any suggestions on how to achieve that from power supply for the bridge, I have an oscillator in the driving circuit and may be able to charge some caps to hold the voltage independent of the direction of the motor. Another idea is an isolation transformer.
Any one have any suggestions?
Kinarfi
 

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Why not disconnect the power from the h-bridge?

That circuit won't work anyway even if you do have an isolated 10V power supply, the MOSFET's internal diode will always allow current to flow in one direction.

The best way is to turn all the transistors off. If you're using an oscillator you drive the h-bridge, all you need to do is disconnect the power to it.
 
Here is what happened and why I need a disconnect in the motor lead, I had both P FETs fail shorted, and even though the circuit breaker had tripped, disconnecting power, the motor leads were shorted, and this makes backdriving the motor very hard. The motor is part of a power steering assist project, so being unable to backdrive the motor becomes a safety issue, I must have a disconnect.

Thanks for the pointer about the internal diode, guess I will have to use 2 N FETs, Source to Source and Gate to Gate and an isolated 10 supply.

Now, how to build an isolated 10 v supply.

Kinarfi
 
The circuit should be designed so it isn't possible for both the high and low side MOSETs to turn on simultaneously causing a short circuit.

If you use a circuit breaker with an auxiliary contact you can use it to disconnect the motor when it trips. You should NOT use a MOSFET for a safety critical device because they have a tendancy to fail short circuit.
 
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How much in love are you with the above circuit? What if instead of DC power rails you were to use AC and diodes instead of MOSFETs? Now positive pulses on the top rail will drive the motor in one direction and negative pulses will drive it the other way. If you are looking for a failsafe arrangement, that should a long way toward getting you there.
 
This unit is on a 4 x 4 recreational big boy toy (Joyner Trooper T2), AC would be interesting, but generating it might be a hassle, but the use of SCRs would be a good way to go. AS is, I'm pretty much stuck with battery voltage and the alternator which runs at 14.4 vdc. Since a turned on FET passes current in either direction, I figure this diagram might work, any ideas of how to get my "isolated" 10 +/- vdc to drive them. The other option is to use a 30 Amp relay.
Kinarfi
 

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hi,
You could consider using a couple opto-isolators to control the Gates of the FET's.
 
Did you read my previous post?



For the last time, it's safety critical so MOSFETs are no good, since they can easily fail short circuit.
 
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Oh well it's your funeral or you have to take responsibility for whoever is driving the car's funeral.
 
probably will end up using a relay, I have one on hand, rated for 30 amps, but coming up with the isolated 10 volts is an intriguing project all by it's self. Something like the charge pump and voltage double schemes is what I was thinking of trying to modify. Any suggestions?
 
Hero999 said:
Oh well it's your funeral or you have to take responsibility for whoever is driving the car's funeral.
Click to expand...

Easy there, Hero, it's not a critical system, worst case scenario for FET failure is a loss of power assistance, not a complete loss of steering, abilities, the failure of both P FETs just made it harder to turn and the speed is not that fast and it is definitely not a freeway type problem.
I do appreciate your concern, but I know of no other methods to accomplish my goals. Other safety features are there and when I get this perfected, FET failure should be at a minimum.
Kinarfi
 
Just took a quick look at your drawing, I need to derive the isolated 10vdc from the systems 12 to 14.4 vdc so that as the drain,and thus the source, goes positive for one direction and neg for the other direction, vgs is not effected.
 
That's exactly what it does.

Q1 and Q2 are isolated from the 12 Vto 15VDC power supply at DC via two 100pF capacitors.

The total capacitance between the MOSFETs and DC drive is 200pF which is no more than an a medium sized isolation transformer.

Providing it's built correctly, with Y1 rated capacitors, this circuit is well isolated enough for mains and is more than good enough for your application.
 
IF you used dedicated H bridge driver IC's to run each side of the H bridge it would cure a lot of problems. With driver IC's if the inputs are off the outputs are off and none of the switching devices in the H bridge are active so the load effectively can float without any short circuiting effects.
 
As Paul Newman put it just before they shot him in the movie, Cool Hand Luke, What we have here is a failure to communicate. What I think I need as an isolated 10 vdc that is derived from the 12 to 14 volt supply to supply power to a pare of source to source, gate to gate, series NFETs and this needs to be able to happen a hundred miles for any AC power or any external power. At this point, it may just be a case "just in case" because things seem to be working better at the moment after some changes I just made.
But now that I have thought about doing it, I would like to figure out how to do it, Maybe I need to wind another toroid transformer like the one use in a joule thief.
Kinarfi
 
All right, one last try beore I give up trying to help.

What's wrong with the circuit I posted?

Do you have any concerns or do not understand it?
 
Again, As Paul Newman put it just before they shot him in the movie, Cool Hand Luke, What we have here is a failure to communicate. My apologies Hero999, the problem seems to be mine, I kept seeing, at short glances, a schematic that progressed left to right and started with 120 volts AC, instead of ending, then saw the relay, so I flipped it all and added a few lines. I'll put it together and see what happens.
Thanks, and again, my apologies for not seeing.
Kinarfi
 

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