Power supply protection circuit

[kopytko]

Hi,

I created the circuit to protect power supply:



I noticed that when the current is limited by the circuit (short circuit appears), a noise appears. I fixed it by adding a 4.7uf capacitor between the resistor R56 and transistor, but im not sure if it is good idea ?
What You think about it ?

Thank you!

[mij59]

Try by adding a 10K resistor between the wiper of RV4 and pin 2 of the LM358, add a capacitor between pin 1 an 2 of the LM358.
Choose the  smallest value of the capacitor  where the circuit is still stable.

In your circuit diagram there is a short between GND and -12VA .

[kopytko]

Hi,

thank You for Reply. I tested yours solution and it seems to work. I selected 2.2nF capacitor. But I am not sure if I understand this solution, can You explain it for me ?

In your circuit diagram there is a short between GND and -12VA .

  My bad, it is two different LM358.

Thanks!

[mij59]

Unfortunately I can not give you an simple explanation, its gets quite mathematical.   
In circuits with feedback the stability of the circuit is defined by the closed loop gain, lots of information can be found on the internet e.g. on Youtube.
The closed loop in your circuit is made up of current measurement (U2B),  compare the current measurement with the set value  by VR4,(U2A), and the output actuator Q18.
For the circuit to be stable the closed loop gain needs to be less than 1 at a phase shift of -180 degree.

[blackdog]

Hi kopytko,


mij59 is correct in its statement, but i want to ad something...
A opamp is to be designed for a minimum gain, most of them are 1x stable, but say a NE5534 is minimum 3x and a LT1226 is minimum of 25x stable!

Every element with gain/phaseshift insite the opamp loop, wil create's instability.
In your design its the FET and the U2B opamp, de gain/faseshift of your FET depend on the load en you have a extra gain of 5 from the U2B opamp.
This is asking for troubles 

Place the extra resistor as mij59 explaned between the -input of U2A and the wiper of the pot and the compansation capacitor between the output en the inverting input of U2A.
And then test it with different load's for its stability!
If the capacitor that makes it alwas stable is 1,2nF, than use a 1.5nF to be shure that is also stable on different temperatures.

The bigger the compensation capacitor the slower the circuit wil be, and i do not know how fast it has to be 

Kind regarts,
Blackdog

[kopytko]

Hi,

thanks for your reply. I tested the solution and at the moment I have this circuit:

What you think about it ? Maybe in future I connect for each rail two parallel mosfets.

[mij59]

Hi,

thanks for your reply. I tested the solution and at the moment I have this circuit:

What you think about it ? Maybe in future I connect for each rail two parallel mosfets.

Looks OK.

Use one LM358 package  per channel.
Paralleling mosfets is not a good idea, load sharing is not guaranteed.
R11 and R19 can be omitted.

[kopytko]

Hi,

thank you for help. Now is time for PCB.