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I'm trying to understand an IC for battery protection, code HY2111 (datasheet)

The internals are:

enter image description here

And usage example is:

enter image description here

My doubt is the part of the over-current protection. How this circuit is able to control the overcurrent charge/discharge of the battery?

By example, the circuit in discharging stage, when battery is used to power some load "Z", is:

enter image description here

HY2111 datasheet says:

If the voltage of CS pin exceeds the overcurrent detection voltage (Vdip) and the condition lasts beyond the overcurrent delay time (Tdip) discharging will be suspended by turning off the discharge control MOSFET (OD pin). This condition is called the discharge overcurrent status.

where Vdip is 0.2 V and Tdip is 10 ms.

What is relationship between Vcs and I that allows use Vcs to control a maximum I?

From circuit, Vcs = Vbat - I*Z - I'*R2 = Vmosfet1+Vmosfet2-I'*R2

I' (current across R2) is unknown. The internal diagram of YH2111 connects it to inputs of two op-amps in comparator configuration (that should have a near than cero input current) and to another circuit. Could be is correct assume I' small that can be ignored.

However, these equations doesn't provides the final relation between Vcs and I.

On the circuit I'm studding, the battery (18650 one) has a maximum for charge/discharge current of 0.5 A (normal) or 1.3 A (fast). The mosfets (8205A ) has a Rds(on) of around 0.025 mo. That means Vmosfet1+Vmosfet2 = 0.065v at maximum current.

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3 Answers 3

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It is actually very simple. During normal operation, the protection circuit uses the Rds(on) of the two mosfets in series as a current shunt to measure the charge and discharge currents. This means that the specific selection of those mosfets will affect the current thresholds.

The equation is VDIP = Idc * 2 * Rds(on)

Where VDIP is from the datasheet, Idc is the maximum discharge current, and Rds(on) is the on resistance of one MOSFET.

Please note that Rds(on) will vary from sample to sample of MOSFET. Likewise, VDIP has a tolerance. So in reality there will be an Idc min and an Idc max when you calculate Idc using range of Rds and VDIP.

Current through the 2k resistor is probably very small, but in any event, the IC designers would be able to compensate for it since it would be known to them and could be controlled if needed.

The datasheet could be a lot clearer on this point. But if you think about it, this is the only possible way it could work. There is no other mechanism for sensing current other than voltage drop across the two mosfets.

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  • \$\begingroup\$ Circuit I'm studing uses a pair of mosfet 8205A. These mosfets has a typical Rds(on) of 25 mo. That means a maximum discharge current of 0.2v/2*0.025mo=4 A and a maximum charge current of 2 A. Not far but not according to the battery18650 datasheet, that gives maximum of 1.5 A \$\endgroup\$ Commented Aug 16, 2021 at 18:49
  • \$\begingroup\$ That datasheet (36e3e7f3-2049-4adb-a2a7-79c654d92915.pdf) is for the cell only. Nothing to do with protection circuit. Whoever integrates the protection circuit and cell must make sure over-current charge and over-current discharge thresholds are acceptable. Do not forget tolerance for Rds(on) and VDIP, etc. \$\endgroup\$
    – user57037
    Commented Aug 16, 2021 at 19:18
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With the variable resistors, the actual value depends on the part voltage option. The actual resistance may be laser trimmed at the factory.

The current is sensed across the mosfets. The 2k resistor probably sets the gain depending on the actual RdSon of the mosfets.

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  • \$\begingroup\$ If you mean M1 and M2, gates of these mosfets are connected to OD/OC pins, that internally are the outputs of the "logic circuit". I do not see relation with the overcharge op-amps. Moreover, the 2ko is connected to PB- and to the op-amps "+" (something usually useless, taken into account op-amp input impedance) and to the "short circuit detector", of unknown implementation. Difficult to understand for me. Could you expand your answer ? \$\endgroup\$ Commented Aug 10, 2021 at 18:27
  • \$\begingroup\$ Don't take the block diagram too literally. Section 11.1 of the datasheet explains how the current is sensed. \$\endgroup\$
    – Kartman
    Commented Aug 10, 2021 at 18:32
  • \$\begingroup\$ The datasheet says "This IC monitors [,,,] the voltage difference between the CS pin and VSS pin to control charging and discharging". This voltage difference is the one between Vss=Battery- and CS, where CS is PB- less the voltage across the 2ko resistor. I'm sorry, still difficult to relate that with the current across battery. \$\endgroup\$ Commented Aug 10, 2021 at 18:51
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    \$\begingroup\$ the battery - flows through the two switched mosfets. These mosfets have a given 'on' resistance thus there will be a voltage drop proportional to the current between battery - and PB-. PB- is sampled via the 2k resistor to CS. \$\endgroup\$
    – Kartman
    Commented Aug 10, 2021 at 21:24
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From circuit, Vcs = Vbat - I*Z - I'*R2 = Vmosfet1+Vmosfet2-I'*R2

I agree to that, since VSS is the only available reference point, and M1 & M2 are on the current path.

I' (current across R2) is unknown. The internal diagram of YH2111 connects it to inputs of two op-amps in comparator configuration (that should have a near than cero input current) and to another circuit. Could be is correct assume I' small that can be ignored.

Yes, we can safely ignore I'. R2 should have some current, but it is negligible comparing to I.

  1. Over charging current I >= Vcs / (Rmosfet1+Rmosfet2) => 0.2V / (10m-ohms + 10m-ohms) = 10A
    where, 10m-ohm is a random pick, I' is out since it is negligible.

  2. I' max = max current on 2K-ohms = 4.2V / 2K-ohms = 2.1mA

  3. 10A >> 2.1mA --> Over charging current >> I' max

However, these equations doesn't provides the final relation between Vcs and I.

We can safely assume: I >= Vcs / (Rmosfet1+Rmosfet2)

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  • \$\begingroup\$ Circuit I'm studing uses a pair of mosfet 8205A. These mosfets has a typical Rds(on) of 25 mo. That means a maximum discharge current of 0.2v/2*0.025mo=4 A and a maximum charge current of 2 A. Not far but not according to the battery18650 datasheet, that gives maximum of 1.5 A \$\endgroup\$ Commented Aug 16, 2021 at 11:26
  • \$\begingroup\$ @pasabaporaqui Now, I see why you were asking almost obvious question. I noticed that, too. I would give a second thought to using Rds for various reasons. I have a very little experience in BMS, and remember using a shunt resistor. Check Ti's BMS devices, just as a reference. They are selling the devices on DigiKey. \$\endgroup\$
    – jay
    Commented Aug 16, 2021 at 13:05

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