Preventing reverse flow for a DC-DC buck converter - Page 1
Forum section: electronics / projects

author: snoop33
author_rank: Contributor
timestamp: November 01, 2021, 01:24:06 pm
content: Hi all,After some thoughts on how to prevent reverse flow. My initial thinking is to add a second inverted driver to the DRVH pin and driving a PMOS.On some boost converters like TPS61022 it has some magical and PMOS type that hasn't any reverse flow (or they've connected two back to back but the RDS indicates they haven't).Any ideas?

author: exe
author_rank: Unknown
timestamp: November 01, 2021, 01:37:17 pm
content: I guess that depends on a particular scenario you have. A simple solution could be adding adiodepfet on high side. But that only works if buck's converter is zero. If it's not zero, then the fet will remain open. In this case there a plenty of ideal diodes/OR-ing controllers.Or, just use a converter that is designed to prevent discharge.Your solution seems an interesting idea. But are you sure that buck doesn't pull DRVH down when it's not active? Also, I'm afraid fet's body diode will conduct.

author: dtmouton
author_rank: Contributor
timestamp: November 01, 2021, 02:41:18 pm
content: Can you please explain in more detail why you are concerned about reverse flow. It can only happen if the output voltage of the buck converter is higher than its input voltage.

author: snoop33
author_rank: Contributor
timestamp: November 01, 2021, 04:36:59 pm
content: The output can be greater than the input which is the problem.I currently use an ideal diode (PFET) before it which works but I just wondered if there was another way. Texas Ins wouldn't explain how their TPS61022 is doing it and I've not seen a FET without a reverse diode characteristic.

author: exe
author_rank: Unknown
timestamp: November 01, 2021, 04:54:29 pm
content: Quote from: snoop33 on November 01, 2021, 04:36:59 pmMy best guess: looking at the schematic, it seems they bias bulk (or whatever it's called) to highest potential. Unfortunately, such mosfets in discrete form are not on sale anymore (like MIC94031/MIC94030).

author: dtmouton
author_rank: Contributor
timestamp: November 01, 2021, 05:51:11 pm
content: It sounds like you already have the right solution.An IGBT can block reverse voltages. However, I am not aware of any IGBT that is not packaged with a diode.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2021, 02:02:19 am
content: The solution in other contexts is to use a bipolar switch which has some reverse blocking capability.Another option, again if the voltage is low enough, is to use a bipolar transistor as a zero voltage drop diode with the base biased with current from a resistor. This works on boost converters to allow the output to fall below the input when the regulator is disabled, but it could be adapted to a buck converter for reverse isolation.If high voltage isolation is required, then replace the power MOSFET with a pair of back-to-back power MOSFETs.

author: snoop33
author_rank: Contributor
timestamp: November 02, 2021, 09:20:05 pm
content: I've not seen this bulk bias method before.I should have mentioned max voltage is 25v peak.At present I'm using the PMOS with 2xPNP method as an ideal diode but it's speed is limited by the pull-down resistor. It then feeds a bit of capacitance which then hits the NMOS for the buck.I think it'll be ok and will eventually test it in real-life.

author: exe
author_rank: Unknown
timestamp: November 03, 2021, 10:46:32 am
content: May be an OR-ing IC will get you a better speed, one of those:https://www.ti.com/power-management/power-switches/ideal-diodes-oring-controllers/products.htmlLike, LM74700-EP claimed to have less than 1us reaction time.