Simple & Inexpensive Ideal-Diode MOSFET Circuits 2017-09-19

crutschow submitted a new article:

Ideal MOSFET Diode Circuit - Ideal diode circuit prevents reverse charging of batteries, etc.

When a battery is parallel with another battery or other source it is often required that that one source doesn't back charge the other. This can be done with standard diodes but that gives close to a half volt drop, even with Schottky diodes.
To minimize this forward drop you can configure a MOSFET as an ideal diode, which has a very low drop in the forward direction (equal to the current times the MOSFET's ON resistance) while blocking the current in he reverse direction.

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Hi Crutschow, I saw your article posted on the power electronic tips site. Thank you for the circuit diagram, it is just what I'm looking for to make a simple DC UPS for a computer, monitor, and router. I like that your circuit would provide immediate backup power with negligible delay and no reverse current into either the switch-mode supply or the boost converter . You said "Note that there is a reverse voltage limit of the circuit due to the 5V maximum reverse Vbe that can be applied to the BJTs." If my AC power goes down or the AC/DC power supply fails, there will be a reverse voltage of 19V on the top diode as the battery takes over. Will your suggestion of an additional D1 diode and R4 10k resistor be appropriate to protect transistor Q1a from a 19 Vbe reverse bias?

Thank you for your help

Eric K. said:

Will your suggestion of an additional D1 diode and R4 10k resistor be appropriate to protect transistor Q1a from a 19 Vbe reverse bias?

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Yes, the resistor and diode will prevent more than a 0.7V reverse bias on the transistor.
It will have a couple mA reverse current when the input is 0V, but hopefully that's not a problem.

Regarding your question on the PET website about the minimum voltage for changeover from off-to-on or on-to-off, the simulation shows less than 20mV so I would think it should be not more than 50mV in the actual circuit.

Crutschow thank you for your time on this, I'm looking forward to ordering some parts and making it.

Okay let me know the results, if you can.

Regarding the matched-pair DMMT3906W, I was looking for an equivalent part in DIP package to use in a breadboard, but I'm seeing "Obsolete". I guess the next best thing is to use a 33006 SMD-to-SIP "proto board adapter".

Hi Crutschow, thank you for sharing your article. I'd like to use your ideal diode circuit, but with a little bit different setup.
I use a 12V SLA battery, which I would like to protect with the ideal diode and the other power source is a 24V laptop PSU.
The voltage from battery or laptop PSUwill be further stabilized by a DC-DC converter, so for me it is okay to use such a different voltage sources.
I built your circuit on a breadboard with S8550 PNP transistors and a IRF9Z34NPBF P-ch MOSFET (I have these parts only currently at home).
I tested with a lab power supply without any load so far, and it works.
However if I set 24.5V instead of 24V the left hand side transistor (Q1a on your diagram) gets very hot quickly, although my lab power supply shows only about 40-50mA current consumption. The battery voltage is 13.1 V.
Do you have any idea what am I doing wrong?
What would you recommend to change in the circuit for such a setup what I have (12V battery, 24V laptop PSU)
Thank you for your help.
Best Regards,