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PTC cause instability in SMPS

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Lately I've encountered some problems with the use of PTC fuses in combination with buck and buck-boost converters. It seems they can cause instabillity on driving the gates. Does anyone else ever come across this? And does anyone have an explanation for it?

Switched it with a regular SMD fast acting fuse and the whole thing becomes stable instantly.

Without a schematic this question is quite meaningless. PTC fuse generally will have higher resistance than a regular fuse.

I would also guess it might have something to do with your PTC fuse having too much resistance, causing the input voltage to be unstable. Perhaps just adding some extra input capacitance can calm it down.

I would suggest some research on the Middlebrook Stability Criterion. 

Well any regulated output voltage switcher is a "negative resistance" device, i.e., when it sees input voltage drop, it increases input current, which causes more input voltage drop, increasing input current even more... and so on. When the voltage finally drops below Under Voltage Lockout (UVLO) limit, the switcher shuts down, voltage bounces back and here we go again with a relaxation oscillator, oscillating typically at some tens of Hz.

So you have to look at worst-case input current (not one calculated at the normal operating point) - i.e., calculate the input current at UVLO limit. Then pick a PTC polyfuse which has maximum resistance after ageing (for example, after 50 trips; exact rating depends on manufacturer) and calculate this doesn't cause excessive voltage drop.

Example calculation:
Buck outputs 10V 10A at 90% eff.
Input is 20V. Nominal current would be 1/0.9 * 10V/20V * 10A  = 5.6A
UVLO for the controller is set to 11V min, 12V typ, 13V max

Now assuming your 20V input normally goes as low as 15V.
You can't drop below the max UVLO (13V), otherwise the switcher could turn off.
So you can afford dropping 2V over the PTC (assuming other types of input resistance are negligible, i.e., good wiring and contacts).
At 13V, input current is 1/0.9 * 10V/13V * 10A = 8.5A.

R = U/I = 2V/8.5A = 0.235 Ohms

Then pick polyfuse with after trigging/ageing value of 0.235Ohms or less.


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