Almost everybody knows about the simple MOSFET/ZENER-Diode/Resistor circuit that prevents that power will flow into your
circuit when you connect the battery with reverse polarity.
That is well and good, I use it all the time on my boards, but it will not block reverse current like it can happen when you connect a power supply to a battery.

As it happened, I wanted to charge a large battery with my power supply and stupidly reversed the cables. The so-called protection diode across the outputs blew, but it did not prevent damage to the circuit.
Ever since I am looking for a better way to protect my (repaired) power supply from "everything", although it is being said, that there is always a clever idiot that beats idiot-proof designs".

An example would be the NCV68061 (datasheet attached, I don't own onsemi stocks).
The first thing I don't understand in the shown circuits are the battery and the "protected" battery. What kind of situation is that?
A solar-charged house battery and your electric vehicle? And why should both not be protected?

Figure1 in the datasheet (ideal diode) seems to be just an improvement of the mentioned MOSFET circuit in that it has a charge pump and will start at 3.2V.
It will also protect form back-flow, I guess and it WILL also protect from reverse polarity... or not?

So why is there a second suggestion for the reverse polarity option?

What I want is something that I connect to the output of my lab supply and then I can connect anything - a low or higher voltage battery, reverse or not - to that output without fear of damage. No smoke... magic or otherwise.

Do I have to chain an ideal diode plus a reverse polarity circuit?

 

what is the voltage? what is the current?
ideal diode only works with low voltage/current

simple polarity protection is to use fuse and diode which conducts when supply is reversed. i you do not want replacing fuses, consider poly fuses.

 

I don't think that is correct or matters for the question:

The chip can do 30+ volts and the current capacity is determined by the MOSFET.

But you can answer the question for a 30V power supply and a 7S lithium battery to be charged with 2 Amps.

 

The problem I see with a reverse connection to the supply output from a battery, is that current is drawn in the same direction as the supply normally delivers, so any type of diode (ideal or otherwise) does not work.
So you need a circuit that detects when the voltage goes below ground to then isolate the output.
And I assume you need a circuit that can operate when the supply is set to 0V, so that would require an external power source for any circuit.

 

One option would be a diode and a Normally OPEN relay, arranged so that the contact would close only if the correct polarity were present. Not vert efficient but rather effective.

 

One option would be a diode and a Normally OPEN relay, arranged so that the contact would close only if the correct polarity were present. Not vert efficient but rather effective.

Not sure that a mechanical relay response time of several ms minimum would be fast enough to protect against the sudden application of reverse voltage, such as from a battery.
A DC (MOSFET output) faster SSR would likely be better.

 

Not sure that a mechanical relay response time of several ms minimum would be fast enough to protect against the sudden application of reverse voltage, such as from a battery.
A DC (MOSFET output) SSR would be better.

Read the first line again!! The NORMALLY OPEN contacts would not close unless the polarity was correct. S with reversed polarity response time to release will not matter BECAUSE the relay will not engage. The diode would be reverse biased and so the relay would not operate. And being a NORMALLY OPEN set of contacts, no current would flow.

 

Read the first line again!!

I did!!! (notice the extra explanation point).
But suppose the supply voltage is set to provide some positive output so the NORMALLY OPEN contact is CLOSED and a reverse battery connection is then made to the PS output.
Guess what!
A mechanical relay is unlikely to then OPEN fast-enough to protect the power supply.

 

The diode and relay would be on the external side so that no matter what is inside the portion being protected, the relay will not operate if the polarity is wrong.. Evidently my description was inadequate. It is power from the external device that operates the relay IFF the polarity is correct. (IFF=If and ONLY IF)

 

It is power from the external device that operates the relay IFF the polarity is correct. (IFF=If and ONLY IF)

Okay.
But that protection only works for protection with a battery load.
That won't work for loads that require power, and I think (perhaps incorrectly) the TS wants the power supply protection to work for any type of load, since it appears to be a general purpose supply, not just a charger.

 

Here's the LTspice sim of a circuit using a DC output SSR that has a MOSFET to block any reverse voltage to the supply:
LM339 comparator turns the SSR off when the voltage at the power supply output drops below about 100mV (red trace), preventing it from going negative from a reverse connected battery (yellow trace).
The power supply will work normally for any other type of load also.

The circuit does require a separate supply voltage (such as the shown 5V wallwort).
A different supply voltage can be used with the adjustment of R3's value to maintain a Vref voltage of about 100mV.

 

My reverse polarity protection scheme was intended to protect the power supply against ANY reversed polarity voltage. It was not intended for any other application EXCEPT BATTERY charging. BUT ALSO, it will not supply power to totally dead batteries. A similar scheme was used in the larger battery chargers by at least one major supplier of service equipment. Their charger also had a reversed polarity warning light.

 

Hi guys,

I appreciate your deep discussion, but we have strayed from the original question:

An example would be the NCV68061 (datasheet attached)
The first thing I don't understand in the shown circuits are the battery and the "protected" battery. What kind of situation is that?
A solar-charged house battery and your electric vehicle? And why should both not be protected?
Do I need a chained "ideal diode' - "reverse voltage protection" circuit.

BTW: I have not managed to get the NCV68061 to work as I think it should. For instance turning the charge pump on at 4 V or so.
The gate output is always following the source.

 

My post is intended ONLY to protect a power supply used to charge a battery, from damage if the battery to be charged is accidentally connected backward. It does not help with anything else.

 

we have strayed from the original question:

No we haven't.
It is all a direct response to your question.

Do I need a chained "ideal diode' - "reverse voltage protection" circuit.

As I noted, an "ideal diode" will not protect against a reverse battery voltage since the reverse battery current is in the same direction as normal charging current.

I don't think the NCV68061 will either.

 

So you need a circuit that detects when the voltage goes below ground to then isolate the output.

Isn't that what ideal diode mode is supposed to do?

 

Isn't that what ideal diode mode is supposed to do?

Of course a diode (ideal or otherwise) will block reverse current.
But as I've noted several times here, the current doesn't reverse direction from a reverse connected battery being charged.
Why does that seem difficult to understand?

 

Here's a simplified version of my circuit using just a transistor and SSR, which has a slightly higher cutoff voltage than the one with the comparator:
It cuts off the output when the Vs supply voltage goes below 0.7V.

The 5V supply can be a USB wallwort charger of which most of us have several spares.

Edit: As with the other circuit, this one allows normal normal operation of the supply for other types of loads, while protecting against the reverse connection of a battery being charged.

 

The circuit that I suggested long ago provides total protection from the application of reversed polarity. BUT it does not provide ant protection to a reversed connection LOAD. So it is a trade-off, like many engineering decisions are. It will not protect a totally dead battery at all..

 

BUT it does not provide ant protection to a reversed connection LOAD. So it is a trade-off, like many engineering decisions are. It will not protect a totally dead battery at all..

Okay.
But I don't think the TS is concerned about those two scenarios.