I'm in need of a voltage isolation circuit for a system that includes a big battery connected to an external male connector for charging and for safety concern I'd like to isolate that connection whenever the charger in not connected and here is the sschematic I came up with:
Now the charger current is 5A and it does actually work as intended but the mosfet gets realy hot after just a few seconds and I'm concerned it has something to do with the design.
Any thought why or suggestions on how to improve it??
larryd:
Why not just connect the charger directly and leave it that way?
If the charger is designed properly, it should back off once the batteries are fully charged.
You didn't get it.
It'd more of a physical practical problem.
I'm afraid that because the charge port in exposed something can get between those pins and blow it up sins the battery is around 40V and can burst around 300A so that's a big safety concern.
I'm in need of a voltage isolation circuit for a system that includes a big battery connected to an external male connector for charging and for safety concern I'd like to isolate that connection whenever the charger in not connected and here is the sschematic I came up with:
If that 12V zener isn't between the gate and source something is very wrong and you'll blow the
gate oxide instantly with 40V flying around.
Use a p-channel and put the zener between its source and gate, source to the charger, drain to the battery.
Add a schottky in line with the drain to protect the charger from reverse driving is a very wise precaution.
You'll need to actually provide a switch to turn on the MOSFET too, connecting a resistor to ground from
the MOSFET gate - ie the resistor and zener form a fixed divider to give Vgs a nice -12V for on and 0V for
off.
MarkT:
If that 12V zener isn't between the gate and source something is very wrong and you'll blow the
gate oxide instantly with 40V flying around.
Use a p-channel and put the zener between its source and gate, source to the charger, drain to the battery.
Add a schottky in line with the drain to protect the charger from reverse driving is a very wise precaution.
You'll need to actually provide a switch to turn on the MOSFET too, connecting a resistor to ground from
the MOSFET gate - ie the resistor and zener form a fixed divider to give Vgs a nice -12V for on and 0V for
off.
The problem is that I want the actual charger to turn it on automatically when I connect it to the connector from somehow "sensing" the charger voltage but sins there are 2 voltage sources in this scenario this becomes as complicated as this and I believe that the final solution should also prevent reverse voltage of the charger incase a wrong charger gets connected.
As said previously you need a diode for this job. If forward voltage of a (Schottky) diode is too high for may use "ideal diode" - Google for it, you can either get a dedicated IC or build one from components.
Smajdalf:
As said previously you need a diode for this job. If forward voltage of a (Schottky) diode is too high for may use "ideal diode" - Google for it, you can either get a dedicated IC or build one from components.
This Ideal diode looks great and all but seems a little expesive and internally conplicated(reliability issues and around 2$ per IC per 1000 +the mosfet and other passives) I was hoping for a solution that just includes passives with no IC's
I was exploring the topic some time ago but in the end decided a Schottky voltage drop is not so large (but I was interested in uAs@2V, not As@40V).
You may try for example the circuits with comparator as shown here. He says they are not working well but I think he aims for "truly ideal diode". If you don't mind low voltage drop (about 50-100mV) and possibly trimming some resistors it should work with little modification with common parts. But Google shows plenty of different circuits worth trying.
Smajdalf:
I was exploring the topic some time ago but in the end decided a Schottky voltage drop is not so large (but I was interested in uAs@2V, not As@40V).
You may try for example the circuits with comparator as shown here. He says they are not working well but I think he aims for "truly ideal diode". If you don't mind low voltage drop (about 50-100mV) and possibly trimming some resistors it should work with little modification with common parts. But Google shows plenty of different circuits worth trying.
Yes the "Transistor pair" method seem good enough according to the schematic but I'm worried about putting the high voltage into it.
Is there anyhow I need to modify it to work with 50V (for a 12Vgs p channel mosfet)?
