Revere polarity protection using PTC/Shunt Diode?

[treez]

Hello,
We have had some 40W LED driver boards made for us (vin = 24v). However, they are dissipating 1W in the sense resistor and so we need to re-do them, since its too much heat, especially since we have 20 of these all running in the same enclosure.

Anyway, we looked over the circuit and saw the attached diode and PTC circuit which appears to be for reverse polarity protection.
Please can you advise if this really is for reverse polarity protection?

Surely, if reversed polarity occurs, then in that short interval before the PTC has become high resistance, a massive fault current will flow and damage the circuit anyway?

 

[FvM]

I don't see an indication that R1 is a PTC fuse. If it is, which type?

I basically agree with your doubts about reverse voltage protection safety. We can expect that for peak currents above 10 or 20 A, the reverse voltage exceeds the forward voltage of ICs directly connected to the input voltage and destroy it instantly. If there are additional current limiting means, the circuit might withstand considerably higher peak currents.

In other words, you need to analyze the connected circuit in detail, know the supply voltage impedance and the PTC characteristic.

A PMOSFET as reverse voltage protection will probably achieve lower voltage drop and more safety.

 

[Venkadesh_M]

PDS5100 is a high speed Schottky Diode having reverse voltage of 100V and 5A forward current,

Clearly its for reverse voltage protection, If you are not dealing with any inductors in the circuit.

The Fault current is not only depend on the resistor, but also depend on the supply ability which powers a 40W load in 24V voltage, so it might has its maximum ability upto 2A. In that case nothing will happen to the diode and resistor.

But in case of high current source scenario it will at least burn the fusible resistor or fuse and will save the 40W of LEDs.

 

[treez]

The PTC has a marking on it of "TP", that's all we know about it, and that its 1206 in size.
Anyway, I agree it looks suspect, if doint it that simple were possible then everyone would do it.

The schematic above shows a resistor because I don't have a PTC component for the program.
This in fact looks like another bogus subcircuit. -should be using PFET as FvM says.
The 24v supply is PSP-500-24 by meanwell, (500w at 24v, offline)

 

[FvM]

The PTC has a marking on it of "TP", that's all we know about it, and that its 1206 in size.

I would expect that you have more complete information about a product designed for you?

I'm no aware of 1206 size PTC fuses being able to handle several A with 24 V rated voltage.

Marking TP could mean it's Littelfuse 468 3A, a regular slow-blow fuse rather than resettable PTC.

 

[treez]

I would expect that you have more complete information about a product designed for you?

...no , they ripped us off....they withheld all detail, so if we need to modify , they want us to go back to them.
It uses mlx10803, and burns up a watt in its source sense resistor...talk about rubbish design!.
The normal input current is 2 amps......it looks like a 1206 resistor, but has "TP" written on it, so it must be a PTC.....?....unless its an NTC...but I doubt it.

btw, Is there a difference between a "resettable fuse" and a PTC?

 

[FvM]

Resettable fuse and PTC fuse are synonyms related to this question.

I hear from your last response that you don't anything about the part, just assume that it could (should) be a PTC fuse.

In this case you should consider Littelfuse 468 3A as the most likely guess. Other suggestions with same marking are welcome. https://www.littelfuse.com/~/media/...s/fuses/littelfuse_fuse_468_datasheet.pdf.pdf

 

[D.A.(Tony)Stewart]

Current sensors are always designed for 50 or 75 mV drop max for thermal reasons. If 24V@40W then Imax= 1.67A and 50mV shunt would be 0.05V/1.67=0.03 dissipating, I^2R=50mW

Obviously this circuit uses a PTC and would only be used for OVP and your supply is over-voltage!! It is not intended to be used as an LDO. The zener is too low and would get too hot thus design is NG.
If you need reverse protection use a 0.37V 3A , 30V $0.10 power Schottky diode
if you need OCP use a 1.6A,hold, 30V PTC

 

[treez]

sorry , when I spoke of current sense dissipation I was not talking about the dissipation in the PTC...my mistake...I strayed off topic..sorry.