current limit circuit

[jacky yang]

hi,everybody!i am a new member of this forum,and i do appreciate any technical support from you.recently,i tried to built a linear regulator with a pnp power transistor.as you know,this is a very traditional classical scheme which can provide a very low voltage dropout,and after some experiment i found its performance very well,then i would like to add a current-limiting function to this circuit by using a simple transistor and a current sense resistor.unforturenately,the current limmit circuit did not work correctly.when the load current reached the limiting point,the output voltage increased instead of decreasing.the circuit schmatic garaph is in the attachment file,is there anyone can offer me some knowledge about the circuit ?thanks !

 

[carbonzit]

There's a problem with your PDF; can't open it.

Can you use a less cumbersome format, since all you're giving us is a simple picture? GIF, JPEG, PNG, something like that.

 

[crutschow]

Your problem is that the current limit transistor TIS98 has its collector going to a lower (more negative) voltage than the base and emitter. An NPN transistor requires the collector voltage to be more positive than the base and emitter voltage for proper operation.

To remedy this, add a resistor (Rnew) between the base of Q16 and the collector of the TIS98 control transistor. Its value should be calculated to supply the required current to fully turn on Q16 at maximum output current.

Then change the current limit transistor to a PNP and move it and the 0.5Ω shunt resistor to the emitter side of Q16.

Connect the shunt resistor in series with the emitter of Q16 and the +12V line.

Connect the collector of the current limit PNP to the base of Q16, the base of the current limit PNP to the junction of the 0.5Ω resistor and the emitter of Q16, and the emitter of the current limit PNP to the +12V line.

With that connection, when the current limit PNP starts to turn on from over-current, it will start adding current to the base-resistor junction at Q16. This will reduce the current available to turn on Q16, thus reducing its drive current and limiting the output.

Edit: Here's a schematic of the changes:


You likely will also have to add a compensation roll-off cap between the op amp (-) input and its output, since the added gain of the two control transistors may make the loop unstable and cause oscillations.

 

[jacky yang]

THANK you very much,Carl! I have tested your recommanded current-limiting circuit,and it turns out that the circiut works well.thanks again!