Transistor as battery switch question

[datagrid]

Hello,

I am trying to design a hot-swappable battery system which uses logic signals to control which of two Li-Ion battery packs are connected to the board I am trying to power. I have designed a prototype that works fairly well by putting a power mosfet in series with each battery on its way to the board. I also had to add a schottky diode to prevent one battery from charging the other through the internal body diodes of the mosfets. The problem I am having is that the .3V forward voltage drop across the schottky diode before it reaches the board is unacceptable. I am trying to find a solution in which I can switch all 8.4V of a fully charged battery to the board. One solution I thought of was to use two MOSFETs back to back which would not allow current flow through the body diodes. However, obviously current must flow through the MOSFET's from drain to source. If I change the orientation of the second inline MOSFET to prevent the reverse current flow, I would disrupt the normal drain to source current flow through the MOSFET's when they are on. I assume that current would flow through the internal body diode of the second MOSFET but that would introduce a voltage drop and would be no better than my original external diode design. However, if a conducting channel was formed that allowed current flow through the second MOSFET, current would choose the path of least resistance and I would have a minimal voltage difference between the supply battery and the board voltage. I have drawn a small picture representing what I am trying to accomplish and the link is below:

http://plaza.ufl.edu/spdylude/switch.JPG

(not longer a student, graduated in April)

I have not tested this alternate configuration but was hoping that someone here might be able to tell me if it will work or not?

I was also considering trying to find a POWER N-CHANNEL MOSFET that has an external body termial so that I could simply ground it. If that was the case, would current flow from source to drain? I know that the source and drain are dictated by the direction of current flow through the MOSFET but what confuses me is that the source and drain are said to be predetermined and labeled with most POWER N-CHANNEL packages.

 

[logbook]

Use the first circuit but with the MOSFETs wired with D-S backwards. The normal operating current deliberately flows through the body diode. Both battery packs can feed the load in this way.

When the desired MOSFET is switched on, the body diode current will be reduced and the volt drop will come down below a few hundred millivolts (depending on the size of the MOSFET used). The other battery circuit may not be fully turned off, but should be off enough for your purposes.

 

[datagrid]

Thank you logbook for the suggestion, however I have a few concerns with that option.

1) D-S backwards with the diode providing the normal current flow forces both batteries to discharge. I would prefer only one selectable battery to be discharging.

2) With D-S backwards there is no way to completely turn off the unit using logic. I would have to add a third transistor somewhere to stop all current flow.

3) If I understand you correctly, you are saying that if the circuit was wired with this D-S backwards orientation and a transistor was switched on that a conducting channel would reduce the voltage drop across the diode due to the presence of a less resistive path. If this is correct then you are implying that current will indeed flow through the transistor from S to D and if thats the case then wouldn't my second option work much better?

 

[IRstuff]

Source and drain ARE predetermined on 3-terminal MOSFETS, because the body is tied to the source. What you've drawn are n-channel MOSFETs, since the bypass diode is a manifestation of the body-source diode being shorted and represents the body-drain junction.

So, it's unclear what you are really using.

You could possibly find a 4-terminal p-channel MOSFET and tie the body to the highest voltage of the two batteries, which would guarantee that all junctions are reverse-biased, as they should be.

N-channels with separate body connections COULD solve the problem, BUT, you'd need a gate drive on the order of 15V to ensure a good enough drive.

Alternately, you could consider using relays.

TTFN

 

[datagrid]

IRstuff

Thank you for the response. You have reinforced that if I want to use N-channel mosfets that I need seperate body terminals. Thanks for your time.

 

[sdmays]

Is there some reason you can't use a BJT? Zetex has some great medium power PNP's (ZTX951) with Vec saturation voltages as low as, if not lower than, the Ids*Rdson of your MOSFET. The PNP will not allow current to flow into the collector.

Side note: N-Channel MOSFETs and NPNs (possibly excluding the NPN of a typical opto-coupler) are appropriate for sinking current. To source current like your drawing shows requires a high side switching device, such as a P-Channel MOSFET or a PNP. To use a low side switching device such as an N-Channel MOSFET or an NPN, you will need a source of either voltage or current referenced to the high side of the given battery for which your transistor is responsible for switching. For example, to take advantage of the low Rdson of a power MOSFET, you will have to have the Gate a MINIMUM of 5V (for logic level power MOSFETS) above the Source, etc...

 

[Comcokid]

Check out the Maxim MAX1538 and MAX1773 devices at

http://www.maxim-ic.com

. Even if you choose not to use these chips, they do show a MOSFET arrangement that will allow you to build a circuit avoiding the schottky diode drop. Maxim may have some other hot-swapping or battery/wall adapter switch devices that will perform the operation you are looking for.

 

[itsmoked]

Give a thought to relays run by 5pin PICs or some other minimal micro. Then you can stage the batteries monitor them, cycle them, control charging, shut them down b4 damage, show status, etc., etc.

 

[sreid]

Perhaps a latching relay. Low contact resistance and only brief power is required to switch the relay from one state to the other. Below, one of many from Aromat

http://www2.catalognavigator.com/ar...rigin=viewitems&by=prod&assetid=spec&filter=0