Lower power battery voltage selection circuit - Page 1
Forum section: electronics / projects

author: angust_uk
author_rank: Regular Contributor
timestamp: April 07, 2025, 05:34:50 pm
content: I'm designing a very low power device with a small (200mAh) LiPo battery charged through by a USB interfce. In the past I've used a very simple circuit to automatically switch power to the load from battery power to USB when it is present, using a P-channel MOSFET. However having done a quick simulation I am seeing ~30uA drain from the battery with this arrangement. Is there a better method to achieve this and reduce this quiescent current further?

author: mikerj
author_rank: Super Contributor
timestamp: April 07, 2025, 08:35:09 pm
content: Schottky diodes are notorious for reverse leakage, you'll likely find this current will increase significant at high temperatures. The LM66200 "Ideal diode" may be a good fit for you, it has very low quiescent current.

author: Peabody
author_rank: Super Contributor
timestamp: April 07, 2025, 09:28:26 pm
content: You're seeing it in simulation? Are you seeing this battery drain only when USB is absent, or all the time?Well, I don't think your auto-switching circuit has anything to do with the current draw. If USB is absent, then the mosfet is turned on, and the battery will just supply the load current. I guess it's possible that the schottky is leaking, but it shouldn't be that much. But you can test that by replacing the schottky with a regular diode, or taking it out altogether, and see if the current goes away.Your regulator will draw some current even if there's no load. Also, what's connected to Vin?

author: Buriedcode
author_rank: Super Contributor
timestamp: April 07, 2025, 09:29:05 pm
content: Yes I have been bitten by this when using the PMEG2015** series of diodes - very low Vf but the reverse current drained the battery in my device in about 10 months. There are schottky diodes specifically for very low Vf, but I used a B0540W which was measured at <0.2uA @ 20C. Mostly because I just had it around, but good enough, and two orders of magnitude better than the smaller diode.

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2025, 09:42:14 pm
content: Quote from: mikerj on April 07, 2025, 08:35:09 pmI missed the schottky diode.I recall a similar circuit where I think I used p-channel MOSFETs for both inputs, so replace the schottky diode with a p-channel MOSFET?

author: mikerj
author_rank: Super Contributor
timestamp: April 08, 2025, 08:40:25 am
content: Quote from: Peabody on April 07, 2025, 09:28:26 pmThe OP's leakage current is right on the money for this diode at room temperature. A lot of people underestimate the magnitude of Schottky leakage, especially at higher temperatures. As a rule of thumb, the lower the Vf, the higher the reverse leakage is likely to be.Quote from: David Hess on April 07, 2025, 09:42:14 pmI think you'd also need another transistor to control the gate, unless there is a cunning scheme I'm unable to see (quite likely!).

author: angust_uk
author_rank: Regular Contributor
timestamp: April 08, 2025, 11:19:10 am
content: Thanks all for the very useful feedback. Looks like the reverse leakage is the culprit. The LM66200 device looks like a good fit - will give that a try.Appreciate the help!