Replacing a Switch and Avoiding body diode conduction

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Replacing a Switch and Avoiding body diode conduction

Postby maryzhu » Tue May 30, 2017 11:29 pm

Hello all. Thanks for taking up your time to read my post.

I am trying to design a replacement for a 2:1 switch using FETs, which would be really easy if I didn't have to concern myself with the body diodes that are present on PMOS transistors.

My scenario is that I have two batteries in parallel to power a circuit, they are both fed into a dual schottky diode and then that output is used to power most of the circuit, this dual schottky is good as it prevents a major issue arising if one of the batteries were to short circuit - the other would drain through it and die quickly. However, unfortunately the voltage drop over the schottky is too high (even though it is a mere 0.25V) and would not be able to power a specific part of the circuit that I need, and it would stop functioning much earlier than if powered direct from the batteries.

I need to switch between the two batteries to power this part in order to keep their voltages relatively even. Originally I was using a TPS2112APW as the datasheet of multiplexer to do this, however I ran into issues when I realised that the analogue inputs would be higher than the supply voltage for the chip (the enable pin became redundant and the output would always be high [whatever was at NC / NO]). So I want to replace this with FETs as mentioned (PMOS or whatever will work / be cheapest) and need to avoid the possibility of one of the batteries shorting allowing the other to drain through the body diode that is present on PMOS transistors. The reason I am struggling is because any solution can't have a voltage drop over it as this would eliminate the point of the switch in the first place.

The FETs (switch) is / will be controlled by a microprocessor which only has an output voltage of +2V, while the battery voltages will be up to 3V at the beginning of life and then will be considered dead at about ~2.5V as some of the circuit won't work and they are crucial for the functionality of the circuit. The premise of this being that a 2V pulse from the microcontroller will switch off / on one of the batteries and then switch on / off the other one accordingly.

Just to reiterate, the question is to prevent the body diode of a PMOS conducting given events mentioned above without causing a voltage drop when they are functioning.

Cost is quite a big thing with it needing to be as cheap as possible, I have searched for alternative switches to replace the current one but all datasheets seem to say the same sort of thing regarding analogue inputs being higher than the voltage supply.

I would draw a circuit to go with this question, however, I do not feel it would actually provide anything as all it would be is two batteries each going into a PMOS and then have the two outputs connected to each other going to some load. If you want one I will oblige but as I say, it will only give a small visual aid rather than any technical input.

Can anyone help me ? I am very puzzled about this question. I do need your help.

thanks in advance

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