Here's simplified diagram of my circuit:

enter image description here

I am trying to build UPS-like thing with TP5100 but before connecting battery stage.

Since I need 12V output, I adopted a boost module MT3608. Also, I inserted low side MOSFET switch using IRLZ44N between TP5100 and MT3608.

The problem is, when the switch is turned off, -VOUT is 8.4V which is the charge voltage from TP5100.

I have probed several points in circuit and found that the drain of MOSFET and GND pad of the boost module is 8.4V.

I tried two test to figure out what is going on.

  1. Remove boost module and make direct connection instead of VIN-VOUT and GND line of the boost module and supply 8.4V instead of 12V: work as expected(the wired non-zero -VOUT disappeared and +VOUT is 8.4V and -VOUT is 0V)

  2. Separate and test the boost module: work as expected.

  3. Breadboard test for the combination of low-side N-Ch MOSFET switch and boost module: work as expected.

Now I am lost. What is wrong with my circuit?


1 Answer 1


There is nothing wrong with your circuit.

If you thing any load such as a resistor light bulb, when it has one leg disconnected, there will be no current through load and no voltage over the load.

Here one leg of your load is disconnected and 0V over your load.

So if one leg of the load is fixed to 8.4V, then the other disconnected leg must be 8.4V as well.

If you don't want that, don't switch the ground leg of your regulator. Usually switching the ground leg just creates more issues than it solves.

  • \$\begingroup\$ Understood. So, basically, do you recommend high-side switch? \$\endgroup\$
    – slyx
    Jul 23, 2023 at 20:19
  • \$\begingroup\$ @slyx When switching power to anything more complicated than an LED, a high side switch is usually preferred. You'll want to use a P-channel MOSFET to avoid drive complexity. \$\endgroup\$
    – Hearth
    Jul 24, 2023 at 15:11

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