Hei.
Couldn't find topics with similar keywords, maybe there exists smth similar?
But nevertheless, I've been crunching my head around a certain power input protection. Don't have much PCB space to use (as usual
).
My
idea is to have a transient overvoltage protection on the main DC power line - to dissipate exess transient voltages so they wouldn't go over 50V.
Constant power @42V is expected (for higher Vin I can expect more current consumption. But I don't expect functionality above 50V constant DC).
Input range(7-10V)
12V - 42V (Buck converter can go up to 55V, but I use 50V input caps).
System (accumulated from different chips' datasheets) could consume
2.5W max peak (assuming ALL of the chips consume max amount of current at the Same time - unlikely, but that's the top-notch max I could put together).
I assume system to run on much-much lower current
at nominal ~250mW or smth like that (~20mA @12V / ~6mA @42V).
I have sketched a power input circuit and would like to discuss ideas - the good and the bad about current approaches and how would it be good to solve such a situation.
(For me input voltages go over what I can test in real life right now, so I've mostly thought about these things only in theory).Circuit: (any way to embed images into posts from attachments?)
My trail of thought:Imagine, someone hot-plugs in 42V battery pack or smth similar with high inrush current capability.
I imagine having some voltage peaks from such connection - how big exactly -
beats me. I've used 55V overvoltage as an example.
So I focus currently keeping everything under 50V.
1) First thing to clamp the voltage is some 48V TVS (D2).
The resettable fuse (F1) would be a current limiter for the TVS and in the end - against dead short in the system. - Although at it's values (140mA Ihold, 300mA Itrip, 60V), this PTC can provide protection from low voltages as then currents can reach that high. With higher voltages the current consumption is not so high and PTC fails to protect.
At worst (if transient ever goes that high) the voltage after this point is 77.4V (clamping voltage of TVS). I've assumed it probably won't go that high and took the breakdown+margin so ~55V after this point.
2) D1 would be protection against applied reverse voltage.
But to me, it seems I can still short the input from TVS and thus should use bidirectional TVS ?.
3) Since I still might have voltage over 50V I should provide means to supress this energy a bit further. For that I have used a Zener (D3).
Problem with the zener is that I need a certain current, to get it to regulate @47..48V and still not to blow it and also provide sufficient current to the system (due to limiting series resistor R1).
After some trial and error I've taken some intermediate consumption value that I think would be grossly over what system would need at worst case - maybe (not gonna go for the max peak) - and also protect the device. So I've thought ~1.2W would be OK to provide for the system.
So I've found a Zener that is 1.5W. When voltage is over 50V, Zener becomes the load - basically. So I've sized the series resistor R1 so that it limits the zener's current near it's maximum.
Assuming 55V overvoltage (which in this case can be constant).
1.5Wz / 47Vz = 31.9mA -> 30mA, just to be more 'safe'.
In R1 point of view, (55Vin - 47Vz) / 0.03A = 266.6 Ohm -> 270 Ohm.
And thus power through R1 -> 8Vr*0.03A = 0.240W -> 1206 / 1210 SMD package will do.
4) Ok awesome, but will the resistor limit my system ?
Assume we have expected max input voltage, how much can the resistance value allow current for the system ?
42Vin / 270 Ohm = 156mA. @42Vin, system might ask for ~30mA max. This will dance on the limit of R1 power capability, but should manage.
12Vin / 270 Ohm = 44.4mA.
BUT, assuming that at lower voltages, to keep up the demand of 1.2W from the system (
that might happen, but might not, as this could also be gross over estimation), the buck converter draws more current from the 12V
and thus exeeding my resistor's capability. But does live on my nominal 250mW @12V assumption..
Am I screwed?
Are there any better ways to achieve that kind of supply range and consumption demand or should I consider these things somehow differently?Thanks, if you've reached so far, much appreciated!