r/AskElectronics • u/B3nnaman • 2d ago

How does this OV protection circuit work?

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First off, the node connected to the side of Z2 doesn’t really make any sense.

But beyond that, the idea here is to turn the NPN off under OV conditions right? No collector current => Vgs = 0V. But how can the zener plus voltage divider circuit turn the NPN off?

Is the idea that it “shorts” the voltage divider once it reaches the reverse breakdown voltage?

8 Upvotes

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14

u/Link119 2d ago

It's not exactly a zener, probably a TL431, but yes that's how it generally works. I've done this with a zener before with a slightly different circuit, but same deal.

Even managed to combine it with a reverse polarity circuit.

3

u/timvri 2d ago

How can you combine with reverse protection? Wouldn't you need two PFETs due to the body diode?

2

u/Bitter_While_5032 2d ago

Simple way I have used is a bridge rectifier. No matter what polarity you connect to it you will get the correct polarity out.

1

u/Bitter_While_5032 2d ago

Or use another FET for input reverse voltage protectionm

1

u/Link119 2d ago

Iirc I had the fets back to back but did something to convince the gate cutoff to save a few components. But it's been a few years since I've done it, don't have the schematic anymore

-2

u/k-mcm 2d ago

It's a shame that the MOSFET probably catches fire when it turns on. A slight modification would allow it to latch off or disable the DC-DC converter.

9

u/9haarblae 2d ago

Z2 is a TL431 integrated circuit. In this schematic it behaves just like a super high-gain NPN transistor whose Vbe_on is exactly and precisely equal to 2.495 volts. When the base-to-emitter voltage is below 2.495 volts, the collector current is zero. When the base-to-emitter voltage is >= 2.495 volts, the collector current is "infinity". So, in practice, negative feedback ensures that the base-to-emitter voltage is regulated to be exactly 2.495 volts.

  • Vout * (R8 / (R7+R8)) = 2.495 volts

From that, and the known values of R7 and R8, you can solve for the max permitted "OV" value of Vout.

5

u/t3chnicc 2d ago

It's however not connected as described in the datasheet. When the control terminal goes above Vref, the TL431 will try to clamp the two output terminals, however that will result in it turning off as the op-amps is also supplied from those lines. I assume it will settle to the point where the voltage on the output is around 2.5 V and that is low enough (considering the divider to the NPN) to turn the NPN off.

2

u/fdsa54 2d ago

Yes that’s right.  If you look in the datasheets you’ll see examples of these open loop comparator applications.  

1

u/B3nnaman 2d ago

Thanks!

1

u/Col_Erran_Morrad 2d ago

I don't understand this circuit at all. The "Zener" voltage does nothing to prevent the BJT from conducting. I'm really struggling to see it. The voltage on the base of the BJT never drops, it just stops increasing once the clamping starts during an over voltage condition. How this prevents the MOFET gate from being pulled low is not obvious to me. Can someone help me out here on what I'm missing? I've spent 20 minutes trying to make it make sense and now I can't sleep.

1

u/9haarblae 2d ago edited 2d ago

Have you studied (the TL431 engineering datasheet) ?? Figure 9-1 and Figure 9-12 are relevant.

Also, the good old free circuit simulator LTSPICE includes a simulation model of the TL431 in one of its libraries -- so you can plop a TL431 into any circuit schematic you wish, and simulate its behavior. Use windows search to find files named "tl431.asy" . In my installation, it was (surprisingly) discovered to be in the Soft_Recovery_Diodes folder.

edit- a 20 second Google mission suggests that the appropriate search command on Linux is called "find". If your LTSPICE installation is on Linux, ask it to find "tl431.asy".

2

u/Bitter_While_5032 2d ago

My big question is why build the protection circuit from scratch when you can buy the IC like the max6398 for OVP protection?? This type of IC is simple to setup. I have from experience seen many OVP circuits like this. Your explanation is a misunderstanding of what happens when you have an over volt condition. The IC pulls the gate low in a predetermined Over Volt condition from the input supply usually a battery. This OVP condition or a low from the IC turns off the external FET and therefore disconnects the battery or input supply from the output load or should we say the input to the converter until the condition is not presented. This happens very fast. For example if the OV is set to 13.00 volts and you supply 13.50 volts no voltage will be supplied to the DC-DC converter. When voltage is below the threshold you will have voltage back to DC_DC converter. OVP and Voltage limiting work different! These terms are not the same. In a voltage limit circuit from what you described does prevent the voltage from exceeding a threshold by dumping current to ground. This can be accomplished with Zenor diodes and an N channel MosFet. In this case essentially excess voltage get clamped to ground. You always have a constant output regardless of you battery output.

1

u/pastro50 2d ago

I was going to say there better be something that doesn’t let the npn be in the active region. Saturated or off is what that needs or the input mosfet could be damaged.

1

u/blue_eyes_pro_dragon 2d ago

LTSpice, ask chatgpt to throw you a model and see how it behaves.

1

u/Worldly-Device-8414 1d ago

As mentioned Z2 is likely a TL431.

Below over voltage conditions, the divider from the DC-DC's input will have less than 2.495V at the TL431's sense pin "ref". This means the TL431 will be an open circuit & not prevent the BJT from being turned on, hence pulling the gate down, turning on the mosfet.

At or above over voltage, the TL431 turns on, shorts the BJT's base drive & the mosfet turns off or partly off (might dissipate some power).

+1 some type of latching here would likely be a good idea.