# Over Voltage Protection at 28V

1. ## Over Voltage Protection at 28V

I am using the following circuit

The load will draw a max of about 10 amps at 24V

I want a voltage cut off when the input voltage crosses 28V DC or the fuse resistance will increase if the current crosses 10A. I am using PTC

I need to decide on part values.

The SCR I want to use is S602TSRP
TL431 - Voltage reference

Transistor and other components need to be decided

Is the concept I am trying correct for given ratings

2. ## Re: Over Voltage Protection at 28V

SCR crow-bar circuits have been used since long and are surely effective. The threshold would be more precise if R205 is connected to 24V node.

You should check that the PTC fuse can stand a full short without damage.

3. ## Re: Over Voltage Protection at 28V

Hi,

The SCR will be low impedance as long as current flows.

And a PTC fuse will never be that high impedance that an SCR releases.
So once it was triggered it will stay in "failure" mode.

Klaus

4. ## Re: Over Voltage Protection at 28V

So once it was triggered it will stay in "failure" mode.
Yes it requires power cycling, think this is intentional.

5. ## Re: Over Voltage Protection at 28V

I have changed the sch

I am using S602TSRP - SCR, TL431CDR and transistor BC856

I havenot used TL431 earlier, I found that the reference has to be set from 2.5 to 30V and the SCR should trigger when the supply reaches 28V

I am not clear on how the calculations for resistors are done. Its a voltage divider at TL431 but for what voltage should i design this so that TL431 operates when the inout is 28V.

If I am considering 2.5V as reference

If I use R7 as 10K and R9 as 980R. The out voltage when input is 28V will be 2.5V. Will this activa

•

6. ## Re: Over Voltage Protection at 28V

Originally Posted by tiwari.sachin
................
If I use R7 as 10K and R9 as 980R. The out voltage when input is 28V will be 2.5V. Will this activa
Yes, the voltage at the Tl431 control pin will reach 2.5V when the output is 28V with those resistor values, which will then cause the cathode (top) pin to start conducting current above that point, turning on Q1 and triggering the SCR.

7. ## Re: Over Voltage Protection at 28V

I am thinking of using a P MOSFET to cut off the supply when 28 V is reached.

This option should also work right? Seems ok

I ofcourse need to do my calculations yet. Choosing MOSFER with Vgs -4V and Id = 13 or 14A

•

8. ## Re: Over Voltage Protection at 28V

Originally Posted by tiwari.sachin
I am thinking of using a P MOSFET to cut off the supply when 28 V is reached.
............................
This option should also work right? Seems ok

I ofcourse need to do my calculations yet. Choosing MOSFER with Vgs -4V and Id = 13 or 14A
You want a MOSFET with a low ON resistance (no more than a few milliohms) to minimize any voltage drop and I²R power dissipation during normal operation.
The Vgs(th) is generally not critical in this application.

9. ## Re: Over Voltage Protection at 28V

I am thinking of using Si4401DDY which has following specs

0.015 at VGS = - 10 V when Id = 16.1 Amps
0.022 at VGS = - 4.5 V when Id =13.3 Amps

Max Vds = -40V

Vgs threshold is -1.2 to -2.5 (Max)

Max Vgs is 20V (This is a concern)

I am connecting 24V to source. This voltage maximum can go to 30V (Failure case)

gate is controlled via transistor to GND (When on gate voltage will be 0)

In this scnerio (Vgs = Vg -Vs = 0 - 24V which is greater than 20V given in datasheet

How can I control gate voltage so that it is less than 20V (Max Vgs) maybe somewhere around 10-12V (More acceptable value)

•

10. ## Re: Over Voltage Protection at 28V

How can I control gate voltage so that it is less than 20V (Max Vgs) maybe somewhere around 10-12V (More acceptable value)
Zener diodes have been invented for similar purposes...

You should add a few volts hysteresis to your circuit to enforce clear on or off state of the switch and avoid fast switching bursts that might damage MOSFET and load. A well calculated resistor between Q1 collector and TL431 control input will do.

11. ## Re: Over Voltage Protection at 28V

I have added a resistor in the following sch

At say at MIN INPUT : 22V, gate will have 14.966 and Vgs = 14.966-22V = -7.034 and MOSFET is ON
At INPUT:24V, gate will have 16.327 and Vgs = 16.327-24V = -7.67 and MOSFET is ON
At say at MAX INPUT : 28V (or 27.9), gate will have 19.048 and Vgs = 19.048-28V = -8.95 and MOSFET is ON

At INPUT is 28V (+ few mV maybe) or more, the control pin on TL431 will be at 2.5V (Vref of TL431), PNP will turn ON, Voltage at gate will go to 0 and Vgs = 28V-28V and MOSFET will be off

Why and how are we talking about connecting extra zener?? what voltage??

12. ## Re: Over Voltage Protection at 28V

Using a voltage divider to reduce the gate voltage is an option, a zener diode has the advantage of giving constant Vgs for optimal MOSFET operation independent of supply voltage.

I keep my point that a hysteresis would be preferred.

13. ## Re: Over Voltage Protection at 28V

Originally Posted by tiwari.sachin
I am thinking of using Si4401DDY which has following specs

0.015 at VGS = - 10 V when Id = 16.1 Amps
0.022 at VGS = - 4.5 V when Id =13.3 Amps

Max Vds = -40V
.................
That device will dissipate 1.5W@10A so it may need a small heatsink since the case thermal resistance to air is a maximum of 50C/W.

14. ## Re: Over Voltage Protection at 28V

Alternately if you use a MOSFET with less than 10mΩ ON resistance you shouldn't need a heatsink.

15. ## Re: Over Voltage Protection at 28V

And if you anyway have a MOSFET in your power path, consider replacing the fuse with active current sensing and cut the power using the MOSFET. A MOSFET is way faster than a fuse.

16. ## Re: Over Voltage Protection at 28V

Originally Posted by ArticCynda
And if you anyway have a MOSFET in your power path, consider replacing the fuse with active current sensing and cut the power using the MOSFET. A MOSFET is way faster than a fuse.
Depends upon the MOSFET and the maximum short circuit current available, as to whether the MOSFET will open before the fuse.

17. ## Re: Over Voltage Protection at 28V

I have changed the sch a bit. Removed TL431 and replaced with a NPN and a zener of 28V

Attachment 134134

FVM: I dint understand how zener should be used. D17 that I have connected is it right? and how would that be help ful

Crutschow: I shall have values such that Vgs is about -10V (plus minus as the input voltage can vary). The MOSFET Rds is 8mOhm and -10V

Considering this, the power would be around 1.25W at 12Amps

Max Vds is 30V and since Vds can go upto 28 (although this is rare), I am more towards choosing MOSFET with max Vds around 40V but cannot due to cost issues

Is 30V good enough??

I think the Power dessipation with about 1.5 sq cms PCB heat sink on all 6 layers should do the trick

- - - Updated - - -

How can I calculate power dessipation of the 28V zener by the reverse current when the input crosses 28V so that the diode doesnt fail (as someone can just plug in 30V and leave). In that case MOSFET can go off but the zener shouldnt get damaged and shouldbe within its thermal limits

- - - Updated - - -

I think MMSZ5255B-7-F (28V Zener) should do the work

Below is its spec

- - - Updated - - -

Simulations are not giving correct results. Can I even use a 28V zener like that??

I kept this video as reference

Updated SCH

•

19. ## Re: Over Voltage Protection at 28V

Originally Posted by tiwari.sachin

Updated SCH
At 30V the D18 zener dissipation would be 28V times its current.
The zener current is about (2V-0.7V) / 1kΩ + 2v / 470Ω = 1.3mA + 4.26mA = 5.6mA.
The zener power dissipated would then be 28V * 5.6mA = 157mW.

D17 is in backwards.

When connected correctly it's dissipation would be ((24V-10V) / 10kΩ) * 10V = 14mW.

Yes, you can use a Zener like that. But note that the threshold is 28.7V due to the 0.7V drop of Q4's base-emitter.

Eliminate R93. It serves no useful purpose.

How does the simulation not work?

20. ## Re: Over Voltage Protection at 28V

I have used R93 so as to control Vg and keep Vgs around -10V in the input range of 21.6 to 26.4. R93 and R92 act as the voltage divider to control Vg

When you say zever is backwards. Do you mean that I should connect anode to gate and cathode to source

gate ----|>----- Source

How does this work. I am confused on the working concept of D17

--[[ ]]--