MOSFET Ideal Switch for Battery Isolation

[michalk]

Hobbyist here, and my first post.

I need to implement an ideal switch in a MOSFET. This is in a vehicle with multiple batteries, and I would like to manage these with the following functions:
1) Isolate a battery
2) Join the batteries

I made a PC board with ST Micro VN31, which is an IC SSR; basically the charge pump and other bits with a MOSFET in a single package. My board works exceptionally well, except that I'm not getting isolation. When I isolate a low voltage battery, the bus voltage becomes higher than the isolated battery, causing current to reverse flow through the intrinsic body diode.

I need to build another board with the FETs back-to-back, but I was wondering if I could simplify with something like this:
**broken link removed**
It's a two channel version of a solid state relay.

Can I connect Batt1 to out1, and bus to out2? I would leave Vcc floating.

So the idea is that when enabled conducting in a normal situation, curent would flow backwards through FET1, to VCC which is internally connected to FET2, then forward current through FET2 and then out to the bus.

When not enabled (not conducting), high voltage on the batt would try to drive current through FET1 through the body diode, but would hit FET2 that is not coducting.

The data sheets that I can find don't have any examples using it this way. Is there a problem with trying to do this?

 

[KlausST]

Hi,

there are solid state relays for both AC and DC.

Also photomos relays.

*******

Keep in mind, that if you connect a discharged and a charged battery together ther will be high current.
Try to match both voltages (maybe with a current limiting resistor, or a intelligent gate drive) before conducting low ohmic.

Klaus

 

[chuckey]

Why not just put a diode in series with the battery to bus feed to stop the reverse current flow?
Frank

 

[michalk]

Hi,

there are solid state relays for both AC and DC.

Also photomos relays.

*******

Keep in mind, that if you connect a discharged and a charged battery together ther will be high current.
Try to match both voltages (maybe with a current limiting resistor, or a intelligent gate drive) before conducting low ohmic.

Klaus

I've looked at some DC SSR's, namely the Crydom ones where I could find a schematic. These do not offer isolation, they allow reverse current when not enabled. I initially tried an AC relay, but due to its zero crossing sensing, it was unreliable when switching a load. I would love to buy off the shelf, do you have any references?

Sure, I could put a diode in series, but the voltage drop is a problem.

 

[KlausST]

Hi,

SSR without isolation....

I just tested the first hit with crydom: CMX series relays. They have isolation. I´m sure there are many others like this.

Klaus

 

[dick_freebird]

I would not be so quick to step away from the good ol'
relay (or starter contactor, if you need big current and
not continuous duty). Isolation is a freebie, replacement
is easy, thermal concerns are about nil and so on. You
can probably find a lot of info in this vein on RV sites as
these tend to have multiple batteries to manage.

Sure, you -could- do it solid-state. Maybe even at par
on cost. But what, besides a little coil current, would you
hope to gain?

 

[michalk]

I would not be so quick to step away from the good ol'
relay (or starter contactor, if you need big current and
not continuous duty). Isolation is a freebie, replacement
is easy, thermal concerns are about nil and so on. You
can probably find a lot of info in this vein on RV sites as
these tend to have multiple batteries to manage.

Sure, you -could- do it solid-state. Maybe even at par
on cost. But what, besides a little coil current, would you
hope to gain?

You are right on all counts. It's the coil current tyat has me worked up. I keep revisiting this in my mind, and perhaps I should just stick with relays.

- - - Updated - - -

Hi,

SSR without isolation....

I just tested the first hit with crydom: CMX series relays. They have isolation. I´m sure there are many others like this.

Klaus

Ah. I just pulled up the schematic for that. Yes, it's optically isolated. I was using isolation in a different context, which is to isolate the load from the battery. The MOSFET there shows the instrinsic diode, but nothing in that diagram prevents reverse current through the MOSFET.

 

[KlausST]

Hi,

i see.

look for AQZ102 or AQZ262.


Klaus

 

[michalk]

Hi,

i see.

look for AQZ102 or AQZ262.


Klaus

Oh wow. That's almost exactly what I'm looking for, except I need about 20 amps. Did you just go through each product and look at the schematics? I'm unable to specify a proper search that returns what I am looking for. I try to consider a class of SSR's and see if they have the back-to-back MOSFETs, and discard the whole class. Maybe I need to pull each datasheet.