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I am building a device which is being powered by single cell Li ion battery (1100mAh), i am using MCP73831 Li ion battery charger and controller in my design to charge the battery.

I am using below Load sharing circuit in my design MCP73831 with Load sharing

I have got the schematics from this page : LINK

Note : R3 is 2K in my case.

I am having problem with B130LAW diode (D1), This diode is heating when the battery is kept on charge and it's extremely heating for the first few seconds when the battery is kept on charge from 0%

What could be the problem of heating of that diode.? OR anyone can provide me any other Load sharing circuit for my requirement

My load is around 200mA, i am also boosting battery voltage to 5V using LTC3426 chip, Keeping in mind MCP73831 is charging the battery at it's peak charge current 500mA.

Edited below :

I have just noticed that mcp73831, D1 and Q1 are all heating only for 5-8 seconds. Is this because of mcp73831 is in preconditioning state? It's only happening when the battery is totally down.

When I check current going to the battery from mcp73831 is about 750mA only for few seconds then it's coming to 500mA..

Testing Voltage across pin of Q1 and D1 (when the D1 is not heating):

I have disconnected battery to test voltage drops across diode and Mosfet, Vin to Diode is 5.06V and other end it is 4.88V.So voltage drop at diode is 0.2V, on the other hand voltage reading at mosfet pins are at Drain : 4.87V, Gate: 5.07V, Source : 4.33V

When connected to battery and charging voltages are as follows : Diode : Vin 5.00V and other end 4.74V voltage drop 0.26V MOSFET : Drain : 4.74V, Gate : 5.00, Source : 4.09

See voltages when all three are heating up, charging from 0% battery charge.

photo

Thank you

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  • \$\begingroup\$ Check the orientation of the FET. If it is reversed, and turned off, the body diode may conduct giving an unwanted battery charge path. \$\endgroup\$
    – user16324
    Commented Aug 28, 2017 at 11:49

2 Answers 2

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I am also using this load sharing circuit and it works for me. You say that the diode is heating while the battery is charged. This means you have 5V at the input and the Mosfet is closed. To help you we need some more information

  • voltage drop across Diode
  • voltage drop across the transistor

If you have 5V at the input you should have a voltage drop about 0.25V across the Diode and therefore about 4.7V at the load. If your Lipo is not fully charged, lets say 3.5V and you Transistor is not blocking, your Output will be pulled down to 3.5V. This means you will have a Voltage drop of 5V-3.5V across the diode. That might be the reason why your diode is heating. Please check that

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  • \$\begingroup\$ It's only heating when mcp73831 is in preconditioning state. Only for a few seconds like 5-8 seconds. Then it's slowly cooling down. Please see my edits \$\endgroup\$
    – Mudassir Hussain
    Commented Aug 28, 2017 at 12:46
  • \$\begingroup\$ still the same: check voltage drop across D and Q while D is heating \$\endgroup\$
    – S.G
    Commented Aug 28, 2017 at 18:01
  • \$\begingroup\$ Please see . my edit of my test results on voltage drop, I have to drain out battery to 0% then test again.. I will attach a load to battery and let it run over night. I will come up with the tests again when the D1 is heating. \$\endgroup\$
    – Mudassir Hussain
    Commented Aug 28, 2017 at 19:42
  • \$\begingroup\$ How did you measure the voltage drop? \$\endgroup\$
    – S.G
    Commented Aug 28, 2017 at 19:51
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    \$\begingroup\$ Ok than your wiring is wrong. Your Drain has to be connected to the battery and the Source to the load. Other Question: why do you have 4.68V at the gate? isn't it connected directly to 5V supply? \$\endgroup\$
    – S.G
    Commented Aug 29, 2017 at 5:44
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My guess is that your diode will have a voltage drop of 0.25V, which will be enough to polarize Q1 (since the gate is on the VIN) and thus charging the battery through D1 and Q1, which could be dangerous as you no longer have current limited circuit to charge the battery.

Try replace Q1 with a simple diode and check if the heat goes away.

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  • \$\begingroup\$ Shall I try with 1n4007 ? \$\endgroup\$
    – Mudassir Hussain
    Commented Aug 28, 2017 at 7:24
  • \$\begingroup\$ no, you need a low drop diode and this means a shottky diode is required. The 1n4007 is a silicon diode with a voltage drop of about 0.6V-0.7V \$\endgroup\$
    – S.G
    Commented Aug 28, 2017 at 10:49

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