I've got a problem with a part of an automotive circuit. It seems like it should be easy, but I'm missing the boat on it.

Input voltage nominal 12 or 24 volts (13.5 to 26 volts). I need an output voltage that is 10 volts below the input voltage and floats with the input. The use of the circuit will be the "ground" for 2 mosfet drivers that will drive p-channel mosfets. So the circuit will have a small negative current.

I tried using a floating negative voltage regulator as per the attached image. But it didn't work as expected. See image:

Anyone have some bright ideas on how to achieve this? I think there is a simple solution, but it is evading me.

Thanks.

 

It looks to me a strange unregulated supply.
making a 10V supply connected to the + is no problem but you need a load at leased the current as defined by chip.
What do you really want to do with the fets?

 

Here is the simplest way:


all plotted vs the input voltage, V(bat). Note the power dissipation in the resistor. You might want to add a bypass capacitor across the Zener to "stiffen" the output for transients.

 

Show the MOSFET circuit..
It sounds like you are trying to do something odd that can be done in an easier fashion.

 

It looks to me a strange unregulated supply.
making a 10V supply connected to the + is no problem but you need a load at leased the current as defined by chip.
What do you really want to do with the fets?

Hmmm. I think that is the problem. I'll test today and report back. The FETs are driving a hydrogen cell (electrolysis). Current can go as high as 100 amps through 4 fets, although usually run around 25 amps.

 

Show the MOSFET circuit..
It sounds like you are trying to do something odd that can be done in an easier fashion.

Lol. What's the easier fashion? I'm all ears.

 

What part of, "Show the MOSFET circuit" did you not understand?

The usual results of the matter is that if you are ending up more like Rube Goldberg, you have missed something simple in the beginning.

 

What part of, "Show the MOSFET circuit" did you not understand?

The usual results of the matter is that if you are ending up more like Rube Goldberg, you have missed something simple in the beginning.

I agree. That's why I'm here for help with it.

PL3 and PL5 are the mosfet gates, and PL1 is the pic chip pin.

 

No. Adding load to the circuit didn't work. The output voltage should be ~10V below the input, but is actually .65V all the time. Changes to the adj resistors doesn't change that. Changes to the voltage source doesn't change it appreciably either.

 

I agree. That's why I'm here for help with it. PL3 and PL5 are the mosfet gates, and PL1 is the pic chip pin.

Much better!!! Now we have something solid to work with! I haven't taken any time to diagnose it yet, but at least a dozen competent people are more willing to try than they were a few hours ago.

Edit #1: The 337 is a negative voltage regulator. You have to feed it a negative voltage and it outputs a negative voltage. In this case, negative is used in the absolute sense, not the relative sense. That clears up the first problem (from post #1).

 

What is the part number for your mosfet driver (U1)?
I see in the written words that there are (2) mosfet drivers. I don't see the second driver in the schematic. Two copies of the circuit you posted? Mistake in wording?
Does, "Mosfet+" mean the 13.5V to 26V supply?
For, "PL1"...what is the power supply voltage to your PIC?
Please show how the mosfets are connected. From PL3 and PL5 being their gates, we need the circuits for the sources and drains.
Near the place marked, "SD1" is the upper diode connected to, "Mosfet+"?

So far, post #3 by MikeML seems appropriate.
ps, you have declared some odd resistor values. I think you might be trying to be more precise than necessary, like, a 10k resistor often fits where you labeled, "10.5k".

 

There are 4 fets and 2 drivers. 2 of the fets are on the other side of the board, along with it's driver. Since it's exactly the same, I'm just showing one side.

Yes, Mosfet+ means supply voltage. The source on the mosfets connects to the supply voltage. The drain connects to an external terminal where is connects to the load.

PIC runs on 5 volts.

Yes, I'm testing the post by MikeML now. So far, so good. It runs perfectly with no load, and really, no noise in the environment. Setting up the test under load now. Will report back. It may be solved...

 

..
Yes, I'm testing the post by MikeML now. So far, so good. It runs perfectly with no load, and really, no noise in the environment. Setting up the test under load now. Will report back. It may be solved...

The circuit I posted has limited current sinking ability at node V(out). If you need more current, there are many ways to buffer it...

 

Ok. I've been on the bench with this for most of the day. Here's the report. MikeML, your circuit works perfectly. It is handling the current needs of the mosfet driver with no problem. I'm actually running both fet drivers from one zener, but in my final design, I'll have a zener next to each fet driver, which will cut the sinking requirements in half. I used a 10uF chip cap across the zener, which I'm sure helped with the current spikes as the driver fired.

I had some really big spikes coming back off the load, which when run at 24 volts exceeded the mosfet specs. Was able to handle it with a diode from ground to drain. Now everything scopes out well within the parameters of the fets and driver. The gates are firing crisply, and the heat is easily being handled by the heat sinking on the fets.

This issue is resolved. Thanks for the help. I really like the simplicity Mike's solution.