MOSFET switching circuit transition issue help - Page 1
Forum section: electronics / beginners

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: April 02, 2025, 05:08:03 pm
content: I am having an issue where there happens to be a voltage dip down to 2.2V and due to the POR of the microcontroller which is set at 2.2V it would've dropped lower.What Happens :3V3 drops down to 2.2V or lower causing instability in system.When it happens :During transition between VBCAP and 3V3 and vice versaWhat I am seeking:Eliminate the voltage dips that happens in transitions of voltage rails through the Mosfet and Diode switching circuitWhat I have tried to do to eliminate but failed :Put a voltage divider to a voltage supervisor set at 4.5V which then controls an N-Channel that is tied to a P-Channel mosfet which disables/enables the VIN of the +3V3 LDOResult:1 voltage dip at transition from VBCAP to +3V3 and another from +3V3 to VBCAP.What I try to avoid :use BJT or Diode at the path of VBCAP to avoid any losses of battery life while keeping cost of components at minimum if possible.-Target Information :Curent load at 3V3 varies between 50mA to 150mABelow is Analog capture session of a 10 second on 10 second off cycle where the transition happens between either VBCAP or +3V3 taking over and providing Source to the target microcontroller.

author: xvr
author_rank: Frequent Contributor
timestamp: April 04, 2025, 11:18:08 am
content: QuoteThis sould work if supervisor connected oninputor 3.3V LDO.There is no treshold on 3.3V sence in your current circuit - it will switch on VBCAP when 3.3V already dropes too much.

author: Peabody
author_rank: Super Contributor
timestamp: April 04, 2025, 04:19:11 pm
content: You have the mosfet oriented in the opposite direction of a typical circuit of this type. You have it oriented as a switch, but in this circuit the body diode should be part of a diode-OR circuit where the source with the higher voltage supplies the load. So the body diode and the shottky should both be pointed toward the load. That means the source pin of the mosfet should be connected to the load. Here's an app note that shows this "load sharing" circuit.http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdfWith this orientation, the output should never go below the battery voltage minus the body diode drop whether the mosfet turns on properly or not.However, if you make that change, the 3.6V battery may be a bit too high voltage, so that it will supply the load until it discharges a bit even if the 3.3V is also present. If 3.3V is present, you want 3.3V minus the shottky voltage drop to be greater than the battery voltage minus the body diode voltage drop. So you may need to insert another shottky into the battery line just to drop its voltage a bit.Alternatively, if the 3.3V is a regulated voltage that comes from an upstream 5V supply through an LDO, it may make sense to move the LDO to the downstream side of your circuit. So you would feed 5V through the shottky, and the battery through the mosfet, both feeding into the 3.3V LDO. That would also let you supply your circuit at 3.3V instead of 2.9V.One other thing that may be causing part of your reset issue is the reverse current that flows back through a shottky diode when it switches from forward bias to reverse bias. That current increases exponentially with temperature. So when the shottky has been carrying the load for a while, and has warmed up, then suddenly the 3.3V supply goes away and the mosfet turns on, reverse current may flow back through the shottky for an instant, until it cools down, and that may be enough to raise the gate voltage enough to turn off the mosfet. So in effect the battery supply turns itself off for an instant because the shottky is leaking reverse current. The fix for that is typically to use a lower value pulldown resistor on the gate - like 10K or so. Zak Kemble has an excellent post on this problem, including how to calculate the value of the pulldown resistor:https://blog.zakkemble.net/a-lithium-battery-charger-with-load-sharing/