Electronics > Projects, Designs, and Technical Stuff

Implementing a simple current limit for a basic follower

(1/3) > >>

Shay:
Hello,
Consider this voltage "regulator" output stage. it is a part of a low noise regulator that is more like a reference instead of a regulator, as there is no error amplifier feedback system.

[attach=1]

Quick brief description of the circuit:

V1 is the input supply. V2 is what sets the output voltage minus Vbe of Q1. R3 is a minimum load, and C2 is output capacitance for noise and stability.

Q1, Q2, Q3 form a szilaki pair with a pnp darlington pair, a hybrid of both. R4+R7 is for current balancing between Q2 and Q4.



R8, R5 is the basis of current limiting. if the voltage difference across R7 is enough to make Q5 start conducting, it means there is an overcurrent.



I am stuck on the next steps. How can I utilize this to reduce the voltage?

I tried attaching the collector to a N-MOSFET gate, and the drain to Q1 base, and source to GND. but this caused a lot of oscillations, which I am not sure how I can fix.



Would like to hear your opinions.

Thanks.

mikerj:
There is no circuit visible in your post.  You need to attach the image to your post rather than linking to an external website.

Shay:

--- Quote from: mikerj on March 25, 2023, 12:29:24 pm ---There is no circuit visible in your post.  You need to attach the image to your post rather than linking to an external website.

--- End quote ---

Thanks. I think I fixed it

CountChocula:
I can see your circuit, but I agree that you should attach it here and not use an external service, or a lot of people won't be able to see it because of their browser or network settings (plus, the external service may just disappear or be unavailable).

You should do your current sensing after your pass elements—add a separate resistor between the emitter of Q1 and R3, then use Q5 in the same configuration to turn on an NPN transistor that steals current away from the base of Q1. Try this simulation of the circuit I attached below, for example (I tried to keep the same designators as in your original).


—CC

Shay:

--- Quote from: CountChocula on March 25, 2023, 02:27:08 pm ---I can see your circuit, but I agree that you should attach it here and not use an external service, or a lot of people won't be able to see it because of their browser or network settings (plus, the external service may just disappear or be unavailable).

You should do your current sensing after your pass elements—add a separate resistor between the emitter of Q1 and R3, then use Q5 in the same configuration to turn on an NPN transistor that steals current away from the base of Q1. Try this simulation of the circuit I attached below, for example (I tried to keep the same designators as in your original).


—CC

--- End quote ---
Thank you for you response.
I am trying to stay away from putting any additional resistance on the output side. As this circuit doesn't have any voltage correction (e.g feedback) it will cause additional voltage drop under load, which contributes to more ripple. This is why the resistors are on the input side to the regulator.

Navigation

[0] Message Index

[#] Next page