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Over Voltage Protection not working with load when it draws more current

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tiwari.sachin

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I am using the following circuit for over voltage protection

1.png

The zener breaks down at approx 9.5V

When I connect the load and only when the load draws more current (arount 4 to 5 Amps maybe- Its a fast changing load and hence finding exact current is difficult as I have only a basic multimeter), the zener breaks down and the MOSFET goes off.

How can I stop this from happening.

What parameters of the zener or mosfet I will have to look for to make sure that the over voltage works fine.
 

Hi,

Isn't this exactly what you want?

Overvoltage at C6/ zener swcitches OFF the load...

It's difficult to suggest something as long as we (you) don't know the load behaviour.

****
You say 4..5A but you use 100nF / 22uF capacitors only.
You know it just takes 22us to charge/discharge the 22uF capacitor with 5A by 5V.
I'd expect about 1000uF capacitors in parallel to fast ceramics capacitors.

Klaus
 

Hi tiwari.sachin,

If you circuit couldn't supply enough current to the load, automatically the voltage may drop.
But that is not a problem since that is not going o effect your zener breakdown if you see that staright forward.

ut if your voltage is dropping for the initial surge current, it tries hard to maintain the intended voltage from then on,m and once the initial surge current vanishes, the voltage suddenly may increase even more than double the rated voltage which can exceed your over voltage criteria and hence causing zener breakdown

So two things you may have to rectify

* Make sure your circuit can supply enough peak current to the Load

* Add a freewheeling circuitry to bypass surges to your zener and Mosfet


All the best !!!
 
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Hi,

Isn't this exactly what you want?

The load is a thermal printer and it draws high current only when the complete line is black. This is where the zener breaks down.

****
You say 4..5A but you use 100nF / 22uF capacitors only.
You know it just takes 22us to charge/discharge the 22uF capacitor with 5A by 5V.
I'd expect about 1000uF capacitors in parallel to fast ceramics capacitors.

I donot have space to put a 1000uF. It works fine with normal text print as the current drawn at 8.8V is relatively less

- - - Updated - - -

* Add a freewheeling circuitry to bypass surges to your zener and Mosfet

How can i connect this?
 
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Hi,

The load is a thermal printer and it draws high current only when the complete line is black. This is where the zener breaks down.
I doubt that the zener becomes conductive at high printer current. This makes no sense.

I think it´s more likely that the power supply fails to supply this high current, like already suggested by bala0x07.

Klaus
 

KlausT

I removed the zener and tested and its working fine (even with complete black line print).
 

Hi,

you need an oscilloscope to see what´s going on.

Klaus
 

Put a 1K and 1uF capacitor lowpass filter in series to the zener (not hard connected to the supply line) to supress spikes that might pass through it - that's the freewheeling circuit bala0x07 mentioned before. If the power supply is a switching mode one, it could have such a high voltage spikes at 4 or 5A.

It makes no sense that the zener breaks down with a lower voltage.
 

BrunoARG;

Are you reffering to something like this

2.png

And yes, it is a SMPS

- - - Updated - - -

KlausST,

I will get some measurement data and put it here soon
 

Hi,

I don't know what's the idea of the OP behind this overvoltage protection is.
The RC now allows overvoltage for a short period of time.
But how much time is allowed?

Klaus
 

BrunoARG;

Are you reffering to something like this

View attachment 142667

And yes, it is a SMPS


Exactly. I would recommend 1K and 330nF. It allows fast spikes to go through the supply to the load, but will prevent long-term overvoltages (for example, using a wrong transformer/supply). What's the MOS ON resistance? Maybe with its value and a capacitor, you can calculate a filter these dangerous spikes I mentioned before, with 1K and 330nF it will take about 1,5ms to react, say 2ms. With a 20milli ohms RDSon and a 47uF capacitor (C69) the overvoltage shouldn't be longer than 1ms. A higher RDS will filter the spikes better.
 

Exactly. I would recommend 1K and 330nF. It allows fast spikes to go through the supply to the load, but will prevent long-term overvoltages (for example, using a wrong transformer/supply). What's the MOS ON resistance? Maybe with its value and a capacitor, you can calculate a filter these dangerous spikes I mentioned before, with 1K and 330nF it will take about 1,5ms to react, say 2ms. With a 20milli ohms RDSon and a 47uF capacitor (C69) the overvoltage shouldn't be longer than 1ms. A higher RDS will filter the spikes better.

I was trying different RC combinations. It did finally work for 1K and 2.2uF.

Also tried 1K and 4.7uF and it worked well with that too

Capacitor value any less than 1uF was a issue.
 

I was trying different RC combinations. It did finally work for 1K and 2.2uF.

Also tried 1K and 4.7uF and it worked well with that too

Capacitor value any less than 1uF was a issue.
´
Suggests that the supply voltage overshoots during load steps. You may want to check if the peak voltage is tolerable for your load. Otherwise you need a more stable power supply.

Or is it so that the load can feed reverse current?
 

We are doing some changes in the software to control the current drawn. So far good. The voltage isnt shooting much but the quality of printing has degraded which we are working on. lets see how that goes

meanwhile, since the given circuit is over voltage protection, we were testing by giving input voltage upto 15 to 24V or so (this is a random input voltages used just for tests)

One thing that I observed is the TVS diode (D26), heats way too much that soldering pad also comes out. This happened at 18V. I am using SMBJ11CD-M3/H

Link: https://www.mouser.in/ProductDetail...GAEpiMZZMvxHShE6WhpuzO7rIln7zj6%2bVMl5NBvCLk=

The purpose of this was for ESD Protection.

TVS diode standoff voltage is 11V but giving a constant 18V input to it (DC JACK). I am not sure if TVS can tolerate that as its meant for small duration high pulses, but I guess user if plugs in a random adaptor beyond what we supply than it can be a issue as TVS section completely burns off.

Wondering how this can be handled :thinker:
 
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The TVS diode data sheet gives you all necessary information to calculate a continuous input voltage rating of your circuit. 18V is surely beyond the limit.

The bad thing is that it's difficult to protect the diode against this kind of overload, even with a fuse. There's a good chance to damage the diode, resulting either in a complete short, or high leakage currents below the standoff voltage.

TVS diodes must never be used with enduring input above the standoff voltage.
 

The TVS diode data sheet gives you all necessary information to calculate a continuous input voltage rating of your circuit. 18V is surely beyond the limit.

The bad thing is that it's difficult to protect the diode against this kind of overload, even with a fuse. There's a good chance to damage the diode, resulting either in a complete short, or high leakage currents below the standoff voltage.

TVS diodes must never be used with enduring input above the standoff voltage.


If not TVS then what else can I use to protect the circuit from ESD and also can handle such voltages?

We sure will give the voltage information on manual but the DC Jack used will be standard size and there are good changes that users can plug any random adaptors. (We have had these complaints earlier)
 

You need to choose the TVS according to expectable enduring input voltage, whatever it is. Transients can be higher and will be safely absorbed by the TVS.

The confusion with your design is that you originally specified 9V input and then applied 15 - 24V.
 

Hi,

We sure will give the voltage information on manual but the DC Jack used will be standard size and there are good changes that users can plug any random adaptors.
My recommendation:
Good, clear description is necessary. But don't feel responsible for every mistake a user may cause.
Here this overvoltage protection is just a benefit for the users, for you it's just effort, for your company this means:
* increased development cost
* increased device cost because of the additionally installed electronic parts
* decreased - user paid - repair cost. Make the one responsible that caused the fail.
* decrease sell, when user destroyed the device
* and - last but not least - there still will be users that mistreat your device..you can't make your device 100% fool proof.

Better sell the correct power supply with your device.

Klaus
 

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