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What is a fold back current limit?

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fold back current limiting circuits

if the output of a system is a voltage output and if it is shorted then the system tries to bring the output to the desired voltage by sourcing as much current possible... this would lead to damaging of output stage...

in foldback limiting the current decreases as the output is shorted... this is achieved using a comparator like operation...
 

Foldback simply wants an output voltage term along
with the output current (or proxy). One simple approach
is to take the output voltage as the gate / base drive to
a NMOS or NPN cascode, in the final bias pilot path. Then
a stuck-low output will inhibit all (or most) of the bias -
you might split the path and wire-OR them at the high
side bias node, or you might use such a scheme to push
/ not-push a current into the overI (make it a sum of
the sensed current, and inverse-sensed-voltage).
 

It somewhat depends upon the circuit you are tying to limit. If you give more detail about that we may be able to give better suggestions.

Here's a reference that discusses such limiting.
 

crutschow said:
It somewhat depends upon the circuit you are tying to limit. If you give more detail about that we may be able to give better suggestions.

Here's a reference that discusses such limiting.
Click to expand...

basically i want to do is that i have a 28 volts bus and other systems that are acting as load. now i want to design a system that fold-back the current after it reaches a limit of 5 Ampere. i dnt want the other systems to be shutdown just to foldback the current to a suitable value that is not dangerous for the other systems. i think now u have a better picture of this..

THanks
 

You can use the circuit in Figure 1 that is to the left of the regulator in the article reference I posted. Then use the equations in the article to calculate the values you want for the limiting. Connect R4 to the drain output of Q1.

I noticed that the part number for Q1 appears to be for a PNP transistor, not a MOSFET. Pick a P-MOSFET with about a 10-20A, 50V rating. It may need to be mounted on a heatsink, depending upon the value of the foldback limit.
 

crutschow said:
You can use the circuit in Figure 1 that is to the left of the regulator in the article reference I posted. Then use the equations in the article to calculate the values you want for the limiting. Connect R4 to the drain output of Q1.

I noticed that the part number for Q1 appears to be for a PNP transistor, not a MOSFET. Pick a P-MOSFET with about a 10-20A, 50V rating. It may need to be mounted on a heatsink, depending upon the value of the foldback limit.
Click to expand...


i want to know that in my case i connect my load in parallel with the capacitor? the simulation results are missing in the attached file .. i try to simulate this but it doesn't satisfy me.. also i want to tell u that the part numbers of transistors and Mosfet's are interchanged.. kindly help me regarding this ...
 

Yes, the load would be in parallel with C1 (C1 is actually part of the regulator so it isn't necessarily needed for your application.

A P-MOSFET is shown in the schematic but the part number is incorrect. Do a search in a supplier website such as Digikey for a P-MOSFET that meets your needs.
 

crutschow said:
Yes, the load would be in parallel with C1 (C1 is actually part of the regulator so it isn't necessarily needed for your application.

A P-MOSFET is shown in the schematic but the part number is incorrect. Do a search in a supplier website such as Digikey for a P-MOSFET that meets your needs.
Click to expand...


what is this Vsense in the Equations?
 

crutschow said:
I believe it's the base-emitter turn-on voltage of Q2 which is about 0.65V.
Click to expand...

i have tried this circuit but it doesn't work .. m a little confused about fold back.. the equation 1 given in the reference says (Vin -Vout) . in my case they are same almost. if u have any circuit simulation kindly give me that ..
Thanks ..
 

basitccp said:
i have tried this circuit but it doesn't work .. m a little confused about fold back.. the equation 1 given in the reference says (Vin -Vout) . in my case they are same almost. if u have any circuit simulation kindly give me that ..
Thanks ..
Click to expand...

Thanks for the help.. i have implemented the same circuit and it works fine but the problem is that in my case vin - vout is zero or not? My input and output are almost same that is 28 volts. also i want to know that can it fold back current from 5Amp to 3 usi8ng the same configuration or i have to modify this thing? kindly tell me about this.

Thanks in advance..
 

basitccp said:
i have tried this circuit but it doesn't work .. m a little confused about fold back.. the equation 1 given in the reference says (Vin -Vout) . in my case they are same almost. if u have any circuit simulation kindly give me that ..
Thanks ..
Click to expand...
If Vin-Vout is essentially zero in your circuit then that part of the equation goes to zero and can be ignored.

Below is my LTspice simulation of the circuit with the load going from 0.1 ohm to 10 ohms. The foldback limit starts at about 1A for a short (not shown) and reaches a peak of 5A at a 5.3 ohm load.

The MOSFET is one that happened to be available in the model library. You can use any P-MOSFET that meets your current and voltage requirements. For 28V and 5A I would use one rated for at least 50V and 10A. A higher current transistor will have a lower ON resistance and thus lower voltage drop at the output.


Foldback Limit Ckt.gif

- - - Updated - - -

basitccp said:
basically i want to do is that i have a 28 volts bus and other systems that are acting as load. now i want to design a system that fold-back the current after it reaches a limit of 5 Ampere. i dnt want the other systems to be shutdown just to foldback the current to a suitable value that is not dangerous for the other systems. i think now u have a better picture of this..
Click to expand...
You should be aware that foldback limiting is to protect the power supply from excessive power dissipation due to a short, not to protect the other circuits. A high current won't damage the other systems, just perhaps the one drawing the excessive current.
 

crutschow said:
If Vin-Vout is essentially zero in your circuit then that part of the equation goes to zero and can be ignored.

Below is my LTspice simulation of the circuit with the load going from 0.1 ohm to 10 ohms. The foldback limit starts at about 1A for a short (not shown) and reaches a peak of 5A at a 5.3 ohm load.

The MOSFET is one that happened to be available in the model library. You can use any P-MOSFET that meets your current and voltage requirements. For 28V and 5A I would use one rated for at least 50V and 10A. A higher current transistor will have a lower ON resistance and thus lower voltage drop at the output.


View attachment 82506

- - - Updated - - -

You should be aware that foldback limiting is to protect the power supply from excessive power dissipation due to a short, not to protect the other circuits. A high current won't damage the other systems, just perhaps the one drawing the excessive current.
Click to expand...


Thanks.

i have simulated this thing .. you use R4 of 4.3 K and Rsc 0.15 ohm. i have calculated the values of these resistors using the equations. Rsc comes out to be 0.14 ohm and R4 is 10k. i simulated it at load values of 0.1 to 10 ohms. at 4.9 ohm it reaches peak value of 5.2 amp while at o ohm load it reaches 3.5 ampere current. it some how fulfills my requirements. i want to ask u a question that in this scenario with decreasing the load the current starts fioldbacking but voltage also decreases. is there any scenario possible in which my voltage remains constant while the current starts foldbacking from this 5.2 Amp to 3.5 Amp. kindly help me ...

Thanks...
 
basitccp said:
....................................
i want to ask u a question that in this scenario with decreasing the load the current starts fioldbacking but voltage also decreases. is there any scenario possible in which my voltage remains constant while the current starts foldbacking from this 5.2 Amp to 3.5 Amp. kindly help me ...
Click to expand...
That is impossible unless you increase the load resistance. You can't reduce the current without reducing the current with a fixed load. With a constant load the voltage has to drop if the current drops. Otherwise you would be breaking Ohm's law (and the Ohm's law police would be at your door :wink:).
 
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