Need help with Low voltage protection circuit

[eccerr0r]

Re: Need help with Low voltage protection circuit

Once again, the question is - to accomplish what?

You cannot put enough of them to satisfy the fact copper clad and vias simply do not conduct and radiate heat as well as a bona fide anodized aluminum heatsink...

[flinx]

Re: Need help with Low voltage protection circuit

and I take back what I said about the assumptions. As I didn't know the characteristics of a IRF5305 I was assuming it was similar to the 75N06 which I am somewhat familiar with that is also quite high in current capability. The 75N06 will be marginal on the board with no heatsink at 12A.

The IRF5305 will roast at 12A without a heatsink. Your current maximum is more like 6A.

(Note that the 75N06 is n-channel which is not appropriate for this application, however, same principles can be applied.)

I figured out how to add thermal vias, and can add them in
square 6, 8, and 10mm sizes.

How much would I need?

[flinx]

Re: Need help with Low voltage protection circuit

oh fiddlesticks this is what happens when I don't play with discrete P-channel devices and assume it works like PNP. You're right, there's a problem there, really need to have simulated it and got my current directions wrong. I'm still not going to simulate since there's no warranty expressed or implied :-P

Now I was hesitant on doing the lower left no-BJT version in the first place, though it may well just work. Do not use a "logic level" MOSFET here, IRF9540 and your IRF5305 should work fine. If you you have to use a logic level or the no-BJT version doesn't work, try the upper left version with three BJTs.

I must thank you profusely for taking the time to redesign this.

[eccerr0r]

Re: Need help with Low voltage protection circuit

and I take back what I said about the assumptions. As I didn't know the characteristics of a IRF5305 I was assuming it was similar to the 75N06 which I am somewhat familiar with that is also quite high in current capability. The 75N06 will be marginal on the board with no heatsink at 12A.

The IRF5305 will roast at 12A without a heatsink. Your current maximum is more like 6A.

(Note that the 75N06 is n-channel which is not appropriate for this application, however, same principles can be applied.)

[flinx]

Re: Need help with Low voltage protection circuit

before any one says it, I changed the component labels to make them less confusing.

[flinx]

Re: Need help with Low voltage protection circuit

Warning: I did a quick reextract of the new PCB and noticed you didn't remove the diode, and in fact put it in the worst possible position that I feared. Please REMOVE the diode completely, or if you must use one, use a schottky diode which is still very bad. If you really want the diode there, you _must_ use either the LM6142 or use the three BJT version, else you risk running into weird leaking behavior.

ok, it was only there because it was on the original board no other reason.

now removed

[eccerr0r]

Re: Need help with Low voltage protection circuit

Warning: I did a quick reextract of the new PCB and noticed you didn't remove the diode, and in fact put it in the worst possible position that I feared. Please REMOVE the diode completely, or if you must use one, use a schottky diode which is still very bad. If you really want the diode there, you _must_ use either the LM6142 or use the three BJT version, else you risk running into weird leaking behavior.

Also, you will likely need a heatsink even at 12A, though this is at the borderline. If you have enough Cu foil and the aforementioned vias specifically for thermal conduction, you might be able to safely get away with no heatsink. At 20A the heatsink will be required.

[flinx]

Re: Need help with Low voltage protection circuit

You should put down Thermal Vias for the MOSFET.
Is the copper plane as heatsink for the MOSFET large enough for it to handle 20A or are you going to use heatsink to keep MOSFET junction at safe temperature?

Also consider VIA Stitching for high current.
https://www.microtype.io/high-power-...n-tips-tricks/

While i said it could handle 20 amps I would very likely not be running this much past 12.

[eccerr0r]

Re: Need help with Low voltage protection circuit

I look look at your first circuit:
When battery Voltage is good, the pot is set so the + INPUT (3) is >5.1Vref on the -INPUT (2) so Output (1) swings high (max out = VCC - 1.5V per spec) which turns on the 2N2222 and P-CH MOSFET since now the Gate is toward ground providing > 3V Vg-s.
When battery goes low to the point that - INPUT (3) is < 5.1V, the Output (1) will switch low (0.2V) which will cause the 2N2222 and MOSFET to be off.
Am I missing something?

I think the error is in OP post #46:
I have the pot connected to In+, in-, and pin 3 of the lm358 set so that the transistor turns off at 17.26v and on at 16.3v.

Yeah I thought the initial version should work, but variations in the components used (specifically, leakage in the S9014/PN2222/... BJT is suspect) in the breadboard seems to make it fail. The later versions with active drive should compensate for non-ideal components.

[budm]

Re: Need help with Low voltage protection circuit

You should put down Thermal Vias for the MOSFET.
Is the copper plane as heatsink for the MOSFET large enough for it to handle 20A or are you going to use heatsink to keep MOSFET junction at safe temperature?

Also consider VIA Stitching for high current.
https://www.microtype.io/high-power-...n-tips-tricks/

[flinx]

Re: Need help with Low voltage protection circuit

circuit board

with 1oz traces it can handle about 8 amps, with 2oz it can handle 13, and with a 12 gauge bare copper wire soldered to the bottom traces for b- to out- and b+ to source it should be able to handle at least 20 amps.

The mosfet would be laid down, soldered to the board and the output wire taken directly from the tab.

board is 2.6"x1.4" I'm sure it could be made much smaller with SMD but that's not my bag.

also I would most likely not be using this with anything below 14 volts.

[eccerr0r]

Re: Need help with Low voltage protection circuit

should make it clear that this will not work at 5V. The minimum operating voltage is probably around 8V minimum in order to bias everything properly.

[budm]

Re: Need help with Low voltage protection circuit

I look look at your first circuit:
When battery Voltage is good, the pot is set so the + INPUT (3) is >5.1Vref on the -INPUT (2) so Output (1) swings high (max out = VCC - 1.5V per spec) which turns on the 2N2222 and P-CH MOSFET since now the Gate is toward ground providing > 3V Vg-s.
When battery goes low to the point that - INPUT (3) is < 5.1V, the Output (1) will switch low (0.2V) which will cause the 2N2222 and MOSFET to be off.
Am I missing something?

I think the error is in OP post #46:
I have the pot connected to In+, in-, and pin 3 of the lm358 set so that the transistor turns off at 17.26v and on at 16.3v.

[flinx]

Re: Need help with Low voltage protection circuit

Reduce that 100K resistor in line to 47K or 68K to increase hysteresis, and you need the LM358, the LM393 will not work as drop in.

47k gives 1.7 to 2.3 volt spread. excellent

[eccerr0r]

Re: Need help with Low voltage protection circuit

For LM358, when not tripped pin 1 should be around the same voltage as pin 8, minus 1.5 volts or so. This was my main concern for direct driving in the first place.
Pin 7 should be no more than 0.2V.

When tripped, swap the voltages around.

Dang if this really doesn't exist anywhere I should productize this. But this is a ridiculously simple circuit and those chinese mass producers will beat me to the punch. Grr.

[budm]

Re: Need help with Low voltage protection circuit

So you are using the lower left circuit utilizing both opamp?
What is the DCV at output pin 1 and pin 7 when it is not trip and when it trips?

[eccerr0r]

Re: Need help with Low voltage protection circuit

Reduce that 100K resistor in line to 47K or 68K to increase hysteresis, and you need the LM358, the LM393 will not work as drop in.

[flinx]

Re: Need help with Low voltage protection circuit

IT VERKS! IT'S VERKING!
hysteresis is about 1.09v second pot has barely any measurable effect. maybe .04 volts?
But it VERKS!

now if my power supply would ever get here from china I could set the voltages proper.

To easyeda!

Can the lm393 be substituted for the lm358? pinouts are the same.

[eccerr0r]

Re: Need help with Low voltage protection circuit

oh fiddlesticks this is what happens when I don't play with discrete P-channel devices and assume it works like PNP. You're right, there's a problem there, really need to have simulated it and got my current directions wrong. I'm still not going to simulate since there's no warranty expressed or implied :-P

Now I was hesitant on doing the lower left no-BJT version in the first place, though it may well just work. Do not use a "logic level" MOSFET here, IRF9540 and your IRF5305 should work fine. If you you have to use a logic level or the no-BJT version doesn't work, try the upper left version with three BJTs.

[flinx]

Re: Need help with Low voltage protection circuit

here is the corrected graph I hacked together in excel that illustrates what is going on.