N-channel MOSFET back to back do not turn off

Question

I'm an hobbyist without experience in electronics, but I like to understand how the things works. Just now I'm playing with a PWM solar charger circuit.

After some research I produced a schematic, but after the first breadboard test it doesn't work.

My goal is to make a working NMOS PWM solar charger (sure I can buy it for a few dollars, but I'm trying to learn how it works.)

This is my circuit:

• The gate driver is working fine, at GPV_DRV I have a perfect PWM in the range of 0-12V (so, enough to turn on the NMOS)
• MOSFET Q6 is working fine, I can switch on\off the load on the line, or dimmer a led light, etc.
• I can turn ON Q2 and Q8, the two back to back MOSFET with common drain (to protect the panel from reverse current and to connect\disconnect the solar panel from the battery)

The problem is, when I put GPV_DRV to GND, the panel don't disconnect from the battery, Q2 and Q8 don't turn OFF.

I read a lot to understand how MOSFETs and specifically NMOD works, but I can't figure out the problem. Can you help me to understand what is wrong with my schematic?

I tested this circuit with a 40W panel (erogating 2,4A at the time of test) with a VOC > 20V and a 12V lead acid battery with a voltage of 12,6V.

When I connect the panel and the battery, the voltage on BATT+ becomes 13,1V (with the battery absorbing 2,4A,) but if I connect GPV_DRV to GND nothing happens. The panel do not disconnect.

The MOSFETs used for the test: IRFB7545Pbf from Infineon.

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So, I found another post as reference: PWM solar charge controller MOSFET

It's the same configuration that I'm trying to replicate.

I did a simplification forgetting for now of the first MOSFET (Q2), trying to make working at least the second one.

As per previous comment I checked the Gate Voltage, it seems to be good:

Yellow: Gate Pin
Blue: Batt+ (12,9v when disconnected)
Purple: Supply (14,8v when disconnected)
In all screenshot, battery are absorbing 2,5A no matter of Gate Status:

Pin on OpAmp Input Set to GND:

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Pin on OpAmp Input to PWM 50% duty @ 100Hz:

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Pin on OpAmp Input to 3v3

Now I'm confused and I can't understand why it's not working (I used a new just unpacked MOSFET, just for avoid bad part issues (I buy it on Mouser so not fake for sure).

Answer 1

LM2904 is not a gate driver, rather an opamp. You can't control the back to back N-MOSFET switch in a such way. You would need a gate driver with a charge pump. By the way, they should have a common source, not drain.

The only solution is to replace the Q2 with a diode.

EDIT:

N-MOSFET transistor has in-built intrinsic diode. As you may see the M1 will still conduct even if the transistor is OFF. That's why the promo schematics shows two back to back transistors, a workaround to eliminate the diode conduction.

^{simulate this circuit – Schematic created using CircuitLab}

For controlling the N-MOSFET you have to change Vgs gate-to-source voltage. One way is to connect both transistors with common source.

^{simulate this circuit}

As you might see the S potential has to be different from GND, it has to be floating. So you would need a isolated dc/dc converter or a gate driver IC with a charge pump.

The second setup is a common drain as from promo. You can see that with your opamp circuit you can control just the lower transistor M1, for the upper M2 you do need an isolated dc/dc converter or charge pump IC gate driver.

^{simulate this circuit}

Conclusion:

There is no simple remedy. Further doing PWM without an inductor (buck converter) makes no sense. Why would you like a PWM?