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TP4056 useless protection circuit? (discuss.tchncs.de)
submitted 10 months ago* (last edited 10 months ago) by Rolive@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de
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I got these TP4056 modules from an AliExpress vendor and fail to understand how the protection circuit works or if it's just typical Ali shovelware. It could be my limited understanding of electronics.

The protection circuit appears to be just for show. To the right there's a DW01S chip that prevents over charging and discharging in combination with the 8205 dual channel MOSFET.

It looks like the drain of this MOSFET isn't connected anywhere. I've tried following the traces using a multimeter and no other pin shows continuity with the drain. Source1 is connected to Battery - and Source2 is connected to Terminal -.

I suppose the Drain starts participating in the circuit when one mosfet activates.

What was the idea behind this? That the 8205 acts as an AND gate by having them both in series?

I'm trying to make an 18650 testing circuit that uses these modules to charge and discharge a battery and wanted to use the protection circuit mosfet as a trigger for discharging.

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[-] cmnybo@discuss.tchncs.de 3 points 10 months ago

The drains in the 8205 MOSFET are connected together internally. In a DW01 circuit, they are not connected to anything else.
One MOSFET stops the battery from charging and the other stops it from discharging.

[-] Rolive@discuss.tchncs.de 1 points 10 months ago

Yes that I found as well but have/had trouble understanding why it would be built like this. Also why a MOSFET would be designed internally like this. If you want more power capability you'd get a bigger MOSFET rather than two tiny ones in parallel right?

This page helped me understand the setup. I'll post it here just for informational purpose. It took me a while to find this. https://electronics.stackexchange.com/questions/203463/dual-mosfet-8205a-lithium-battery-protection-circuit

It's related to the internal body diode of the N channel mosfet, so two of them are in series but reversed. When one MOSFET is activated, current may flow easily in one direction but be reduced by the body diode of the other. When both are activated, current may flow easily in either direction.

It seems they don't really prevent discharging or charging separately due to the body diodes but they can cut off the battery alltogether.

[-] cmnybo@discuss.tchncs.de 5 points 10 months ago

The MOSFETs are in series, but in opposite polarities. Two MOSFETs are needed to block current in both directions.

The DW01 uses the voltage drop across the MOSFETS to measure the current. The overcurrent threshold voltage is fixed at 150mV. Using a larger MOSFET that has a lower RDS(on) will increase the current limit.

this post was submitted on 01 Jan 2024
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