Back-to-back MOSFET switch - Page 1
Forum section: electronics / general

author: roman.isaikin
author_rank: Contributor
timestamp: May 21, 2020, 04:20:59 pm
content: Hello!Similar questions have already been asked, but I still can't wrap my head around this back-to-back mosfet arrangement. Below are relevant schematics of the high side switches for the battery management system. The idea was to create separate charge and discharge paths, and to be able to completely shut them off, so that you won't discharge through charge port and won't charge through discharge port, when this is undesired.Additionally there is predischarge path for capacitive loads.As you might probably guess, there is a problem with turning the fets off... But what is interesting - only with the charge path. Discharge and predischarge paths seems to be working flawlessly, when turned off, I measure 0V from +V_DSG to GND, 0V at Vgs, when turned on voltage drop across fets are very small as expected. But with the charge path, Vgs is around 4V, so fets are slightly open. And even if I short R34/R35 nothing changes.I've assembled small test board only with the charge mosfets, and 10K gate resistor to be shure. If gate is not connected to anything, fets are completely off. I've charged a big capacitor to 10V and connected it between +V_BAT and gates to act as a floating power supply, fets turned on, but even when capacitor discharged I've observed exactly the same behaviour as before.There is a SPICE model of this mosfet, but I've had a hard time trying to simulate this, division by 0, singularity matrices and so on..Would be great to understand what is going on and how to fix this. Thanks in advance!

author: f4eru
author_rank: Super Contributor
timestamp: May 22, 2020, 10:50:32 am
content: The two driver outputs of the floating driver have the same source connection, means you have to tie all those 4 "S" together in your configuration.If you look at hte DS of your device, the configuration of the transistors is different, you have to respect that else your charge pump is not referenced correctly

author: roman.isaikin
author_rank: Contributor
timestamp: May 22, 2020, 06:28:51 pm
content: Please elaborate a bitDoes it mean, that everything is completely wrong, or that I should flip the mosfets into common drain configuration? Is it expected that discharge path seems to be working properly, even though it is exactly the same configuration or it is just luck?By the way, charge path picture didn't attached for some reason, here it is.

author: f4eru
author_rank: Super Contributor
timestamp: May 22, 2020, 07:42:09 pm
content: The charge pump drivers have each a specific reference (input, output, etc)The source of the mos needs to be connected to that reference.On your schematic, Q6 and Q7 dont have the source connected to anything defined. How do you expect the driver to set their gate voltage ? In this state, it will just float around undefined.

author: roman.isaikin
author_rank: Contributor
timestamp: May 23, 2020, 10:12:50 am
content: Yeah, that is exactly what I can't fully understand..Correct me if I'm wrong, but there seems to be at least 2 types of high side drivers - the ones that tie gate to source to turn the fet off, and the ones that tie gates to ground. For example LTC4365 -https://www.analog.com/media/en/technical-documentation/data-sheets/LTC4365.pdf, clearly can drive back-to-back configuration. Am I missing something here?

author: ocset
author_rank: Super Contributor
timestamp: June 02, 2020, 02:18:58 pm
content: Those charge pump high side driver ICs for fet switchs are very poor and often dont allow you to have a resistor connected from gate to source of say 10k, which is needed to protect fets from eg esd damage, and to damp the gate a little from ringing. (they cant supply the current through the 10k)So if i were you...just use an isolated dcdc, and put a fet driver in the high side, and use that to turn the back-to-backs on...you can refer the switch on signal to the high side with an opto or digital isolator, or even a bootstrap high side driver ic.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 03, 2020, 12:15:58 am
content: As indicated you can just use an isolated dc/dc converter, even without any additional gate driver if there is no need. The small Newport Components modules (or whoever they are now, and the myriad clones) were excellent for providing simple isolated gate control - each gate needs a relatively low-resistance gate-source resistor and possibly capacitor to manage gate voltage transients from drain dV/dt.Such a back-to-back controllable device, when used with a parallel diode, was good for allowing a battery to instantaneously discharge when needed, but otherwise have a controllable charge switch (which can then allow intermittent charging to prolong service life for VRLA batteries that were used for battery backup applications).