Reverse polarity protection 4

[PhilipBiebach]

Well basicly what im looking for is a way on protecting a circut from a reverse battery condition.
I have considered several common solutions such as:

-Diode (.7 Voltage drop for Silicon Diodes is to high,germanium
voltage drop still not ideal)

-Fuse (only works once)

What i am looking for is a way to protect a microprocessor,H-brige,support circutry from reverse polerity.

The ideal solution would be to have something that will ither shut of power to the circut or even better to internally reverse the the polerity to still power the circut.
Also it cant have much of a voltage drop preferable no voltage drop

Anyone have any ideas,
thanks for your time

 

[waross]

I would use a bridge rectifier for polarity protection and then solve the voltage problem.
yours

 

[Comcokid]

With a H-bridge, your circuit probably uses a little current.

I once saw a design idea that used a MOSFET for polarity protection, but cannot now find where it was.

Instead, try a diode that activates a relay when the polarity is correct. That way you only have only the miniscule drop of the relay contacts.

 

[PhilipBiebach]

Thanks for the ideas, I like the idea behide the Mosfet polarity protect, only problems i see with it are the internal diode would shortcircut the whole thing but im sure theres a way to fix that

The Relay option sounds pretty good exept id like to keep this relativly small, but maybe il find a source of really small relays, il have a look around

Any one else have any idias, are there maybe some ICs specificly for this task?

 

[itsmoked]

You do the same thing using a N-FET with a low Ron. You use a diode circuit to turn the FET on. It would require a high side driver or other voltage boosting but it would be way smaller that any relay.

Easier yet is you use a P-FET pulled down to ground with about a 1M resistor. Hooked up backwards gives no current. Forward very little drop.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[IRstuff]

With a standard PMOS substrate tied to Vdd, there would still be conduction in the reverse direction, since the load would pull the p+ drain above the level of the substrate and the substrate diode would forward bias.

If you flipped the PMOS so that the source feeds the output, it might work as described, but the turn on will be soft.

You could try a Shottky diode:

http://store.americanmicrosemiconductor.com/1n5711.html

TTFN

 

[zeitghost]

This site shows how to use a mosfet as battery protection.

The parasitic diode is used to power the circuit until the mosfet is enhanced.

http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

 

[IRstuff]

As zeitghost's reference describes, the source-drain connections are reversed from normal operation for a p-channel device.

TTFN

 

[felixc]

What about a series polyswitch, and a diode in reverse between supply and ground after the polyswitch?

(polyswitches have voltage and Ipeak requirements, they are not ideal devices but they might suit your app.)

 

[cbarn24050]

You could try a BJT, it has only a low saturation voltage drop.

 

[PhilipBiebach]

Thanks alot everone, got some great ideas there
im leaning towards the MOSFET solution(great current capability, low voltage drop)

 

[cbarn24050]

You might find this usefull.

http://www.irf.com/technical-info/designtp/dt94-8.pdf

 

[zeitghost]

The other thing to remember is not to exceed the gate source breakdown voltage... otherwise... (

 

[benta]

ON Semiconductor has introduced an "active rectifier", which is basically a power MOSFET plus control circuitry. I think this should do exactly what you look for. Go to

http://www.onsemi.com

and search for NIS6111.
Another choice could be a Schottky diode (lower voltage drop than a normal rectifier).

 

[jimkirk]

You can use the MOSFET idea with four devices (2 P channel, 2 N channel) to make a full wave bridge that provides power regardless of the battery polarity with little voltage drop.

I'm not sure how to insert a picture here, so try this...

D S G
PFET 1: battery+ Load+ battery-
PFET 2: battery- Load- battery+
NFET 1: battery- Load- battery+
NFET 2: battery+ Load+ battery-

PFET 1 & NFET 1 are on at the same time when the battery is coonnected with polarity as indicated.

PFET 2 and NFET 2 are on when the battery is conencted opposite.

The load always sees the correct polarity.

Again, mind the maximum gate voltage. If the battery voltage is too low, the circuit will still provide full wave rectification through the substrate diodes.

You can also use this with an AC input, again, minding the voltage & current ratings.

Jim

 

[jimkirk]

Oops, typo!



D S G
PFET 1: battery+ Load+ battery-
PFET 2: battery- Load+ battery+
NFET 1: battery- Load- battery+
NFET 2: battery+ Load- battery-

Sorry about that,

Jim

 

[itsmoked]

Hi jimkirk,

Picture posting:
faq238-1161

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jimkirk]

Thanks itsmoked.

The above is a starting point. You may want to make additions like gate resistors depending on your application.
The source voltage can be either polarity, the load will always see positive on the top end.

Jim