Reverse-Flow Protection Mosfet Control?

[M de L]

TL;DR Summary

Can anyone please explain how this circuit's reverse-flow protection Mosfet is turned on and off?

Hi,

Attached is a circuit diagram for a solar powered, synchronous Buck converter. It appears to have and extra Mosfet, Q1, that is intended to prevent the flow of current, out from the battery, and back toward the unlit solar panel, via the body diode of Q2. Q2, being the regulator's high-side Mosfet.

I'm okay with how the synchronous Buck converter works, including the bootstrap drive from the Mosfet driver chip. What I don't get, is how Q1 is turned on and off, from the high-side Mosfet's gate drive signal.

Can anyone please explain this to me? Is this a conventional, and reliable configuration?

M.

 

[Baluncore]

What I don't get, is how Q1 is turned on and off, from the high-side Mosfet's gate drive signal.
Can anyone please explain this to me? Is this a conventional, and reliable configuration?

The picture is a bit blurry, but I believe the designer of that circuit knew exactly what they were doing. The clue is in an N-channel device, weirdly, (though not irrationally), connected to the positive supply input.

Q1 appears to be there to protect the circuit from a reversed polarity supply, in effect, an efficient "idiot diode". The gate resistor can be a few kΩ, as it does not need to change the gate voltage of Q1 rapidly.

If polarity is OK, the Q1 (normally reversed) body-diode will turn on, then the MOSFET will begin to conduct due to the gate signal, effectively shorting the body-diode to reduce Q1 device power and voltage drop.

If polarity is reversed, Q1 will operate normally, by turning off. The body-diode will be reverse biased, as expected. The circuit is then protected from the reversed input connection.

It may at first seem counterintuitive, but that is a "more than reliable" connection of a MOSFET, that protects the following circuit from reversal of the input connections.

 

[M de L]

Ah, Thank you, Baluncore. Q1's value as a reverse polarity protection hadn't occurred to me. Okay, how it turns off during reverse polarity connection seems to make sense. Very clever.

How Q1 then turns on, depending on the gate signal through D1 is a mystery? I get that Q1 can conduct current from source to drain when turned on (opposite to how it's usually used), but how we arrive at a sufficiently positive voltage, from the gate to the source, to turn Q1 on, defies my understanding?

Suppose the high-side Mosfet gate drive signal is low, relative to the half bridge's switched pole. Is it true that current cannot then flow backwards from the battery, to a non-illuminated solar panel? It would just be very convenient if the reverse polarity protecting isolator, Q1, could double as an isolator to prevent any reverse current flow, through the high-side Mosfet's body diode as well?

 

[Averagesupernova]

If possible I'd like to see a clearer image. Thank you.

 

[M de L]

Hey, Averagesupernova. Sorry, I thought that was the best image resolution available. Seems there's a pdf:

https://content.instructables.com/FQO/PS5A/I6B1RI4U/FQOPS5AI6B1RI4U.pdf

From a possibly abandoned project at:

https://www.instructables.com/ARDUINO-SOLAR-CHARGE-CONTROLLER-Version-30/

 

[M de L]

Okay, seems the part of the circuit of interest, doesn't actually work. From the Instructables article:

"The other problem was that MOSFET Q1 ( in V-3.0) conduct even when there is no solar input."

Identical 'protective' mechanisms appears in a number of similar solar charger projects, so maybe it's been propagated.

Yet other projects leverage an anti-reverse current flow control Mosfet, gate-driven by an isolated DC supply that can be switched. Maybe that's the way to go, and there are no easy shortcuts?

 

[DaveE]

Yes, a better schematic would be nice. It looks to me like this one has Q1 drawn incorrectly. It looks like it's drawn as a P-Channel MOSFET, But I agree with @Baluncore's description since the source is on the wrong side. So, anyway, we'll just chalk this up to poor documentation, which is common on DIY style web sites.

As an aside, this is a good description of "normal" reverse polarity protection schemes. It all seems sort of confused in this example. Just pick a version yourself a stick it in place of that stuff.

So, the answer to your question "how does Q1 get the bias voltage needed to turn on?" is that the half-bridge gate drive IC provides this with a bootstrap circuit. This is a capacitor charge pump like a voltage multiplier. This is also needed to turn on the high side MOSFET (Q2?). Read the data sheet for the IC, it should explain it.

 

[DaveE]

there are no easy shortcuts?

If you mean that you have to understand each part of your circuit, then, yes, I agree. If you mean reverse polarity protection isn't easy, then I think your wrong. You can figure it out.

 

[Averagesupernova]

I question the functionality of Q2 specifically. Doesn't the gate-source voltage need to be several volts apart before the mosfet turns on hard? The way it is, the gate can go no higher than the drain unless I've missed something. This implies the source will never make it all the way to the drain voltage meaning heat dissipation in Q2. Have I missed something here?

 

[DaveE]

Have I missed something here?

https://www.ti.com/lit/an/slua887a/slua887a.pdf?ts=1707908052489

 

[Averagesupernova]

Thanks @DaveE. I was wondering about the D2-C7 pair formed a voltage doubler but I expected a larger capacitor. Guess the .1 uF is adequate. As well as the little 1N4148 diode.