grittinjames
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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.
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.
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.
I believe it's the base-emitter turn-on voltage of Q2 which is about 0.65V.what is this Vsense in the Equations?
I believe it's the base-emitter turn-on voltage of Q2 which is about 0.65V.
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 ..
If Vin-Vout is essentially zero in your circuit then that part of the equation goes to zero and can be ignored.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 ..
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.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..
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
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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.
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....................................
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 ...
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