Battery charger reverse polarity protection.

[treez]

Hello,
I am doing a switch mode lead acid battery (12v) charger with UC3909 IC

Datasheet:-

http://focus.ti.com/lit/ds/symlink/uc3909.pdf

The circuit i am using is like page 7 on the datasheet.

I am trying to incorporate reverse polarity protection for when the battery is connected the wrong way round into the charger...

-i wish it to do the following when reverse polarity...

1. Shut off current flow from charger to battery.

2. Reverse polarity protection components should not provide a discharge path for the battery when the charger is switched off (when battery connected right way round)

3. LED lights when battery is in wrong way round.

.....i don't think there would ever be a short circuit current if wrong-way-round battery connection is done....this is because pin 10 of UC3909 would experience a low voltage (?) and so will go into trickle charge mode (just supplying a few mA max.).....however, i still wish to manage the situation and shut off current flow to the battery if its in wrong.

Here is my proposed reverse polarity protection circuit:-

..Do you believe this will work?....i am worried that the 30V input may somehow be felt at the battery negative terminal and keep mosfet 2N7000 ON...thus not allowing Q3 PFET to disconnect the battery?

 

[gabisuciu]

I think that:
1.you need a current sense for detecting fast a reverse current when battery is oppositely connected.
2.the shut-down of serie prot. mosfet to be activated by a current comparator, but also by a B-E`0.7V NPN transistor which will first limit the short current.

 

[treez]

thankyou for responding...

do you think the existing current sense resistor on page 7 of datasheet..

http://focus.ti.com/lit/ds/symlink/uc3909.pdf

..will suffice for the short circuit current sensing?

Also, in event of reverse battery connection, do you know what voltage will appear on output ?
(i think its funny with batteries...even when shorted you sometimes hear they dont have zero volt across them?)

 

[DHambley]

treez, Your Q3 location looks similar to what we've done for reverse protection on another project. While I don't have time to study the details of your UC3909, if you design in a provision which keeps Q3 gate-source low during reverse then it works.
DH

 

[Renovator1]

Well... you know, mosfets conduct equally well in *both* directions when turned on, and (power mosfets, anyway) still conduct very well in *one* direction when turned off because of the intrinsic anti-parallel diode. Bipolar transistors don't have this "feature" so you might want to consider using pnp bjt's instead of p-ch mosfets, depending on how high the switching frequency is (judging by the 1mH buck inductor, not that high) and/or the peak current.

There are many ways to skin this particular cat, but the simplest is to just put a Schottky rectifier in series with the charger output and sense the battery voltage after it...

 

[treez]

Renovator1:- thankyou for replies, you're right , switch freq = 50KHz.

Though about putting a Schottky in series with the charger output:-

...if the battery is connected in reverse, this Schottky will be forward biased and not cut off current flow through the reversed battery?

 

[Renovator1]

treez - D2 in the schematic already protects against the battery being connected to the charger backwards (use a Schottky for lower forward voltage drop). Leave in D1 and R3 for an "inserted wrong" indicator if you want but I'd get rid of the 2n7000, q3 and their associated components.

 

[treez]

Renovaror1: Thankyou for looking into this:

I am happy if D2 does protect against the battery being put in backwards..

...that's not actually why i put it there:-..

..D2 is there to prevent a "right-way" battery from discharging back into the charger and through the leakage conductance of the output capacitors when the charger is switched off with a "right-way" battery still connected.

I thought if the battery is "wrong-way" then D2 will be OFF (reverse biased) ?

 

[itsmoked]

Have you considered a relay? Set it up so it only closes when source power is present and only if the battery polarity is correct. This rids the design all sorts of semiconductors and cranks up the efficiency and down on the heat dissipation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Renovator1]

Aw, crap... I can't believe I missed that, treez... Is there a black star you can give someone for screwing the proverbial pooch, because I sure earned one on that...

Yes, D2 (and D3) will conduct heavily if a battery is inserted backwards into this charger. They'll pretty much fry if the battery has a high enough short-circuit current.

Rather than go through all these contortions to protect against a revered battery why not rely on the user to actually notice that the battery is connected backwards via the LED? Else, I gotta agree with itsmoked that a relay fed by a normally reverse-biased diode is the simplest/most reliable solution to this problem. Use the NC contacts for the "normal" configuration and have the relay activate only when the battery is inserted backwards.

 

[GonzaloEE]

Some ideas:

- Schottky in anti-parallel and NTC resettable fuse in series with the Battery jack. When the battery is connected backwards, the diode is forward-biased and turn the NTC fuse open.

- Optocoupler/optotriac with emitter in series with your alarm LED, and receiver between VCC and MOSFETgate (or BJT base). The circuit will work at any VCC level, getting rid of the biasing resistors, diodes, etc.

 

[treez]

thankyou for some great ideas,

though i am having a problem with the fact that a completely_discharged battery may be connected the wrong way round.

i am not sure what the electrical equivalent circuit is for a completely discharged lead acid battery.......

-and since its completely discharged, it would not be able to forward bias a diode or push current through a potential divider, and i am wondering how i am going to sense that its in wrong ?

 

[itsmoked]

This is why you would normally want to trickle charge for a while then check for voltage. Since a battery has no capacity below 12V, trickle charging at a low value, for a while, fairly quickly brings things up to the vicinity of 12V. Then you can commence with charging.

Battery chargers are liability devices. You should figure out how to prevent any charging of batteries that are below, say 8V, as you are otherwise setting yourself up for diving into abnormal situations. Least I would with a dumb charger.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[miiky]

hi treez, did you find a solution to your batt reverse cct. I have a similar problem with a 60A charger. Need soluton to be a 1 part one. IE FET or BJT. relay is no the best for reliability.

Cheers
miiky

 

[treez]

miiky,

sorry , i just used a fuse in the end for its cheapness as i couldn't find anything free of problems/expense.

the chip i used won't deliver more than a small trickle current if the voltage is less than 10.5V.