Current Limiting in Switching Power Supply

Thread Starter


Joined Mar 19, 2015
Hi there!
I am inspecting a welding power supply that can set its maximum output current. From the picture, the idea is quite simple. From the file "power circuit", when the current through the current sensing transformer (CT) is too high, the control circuit will limit this current, according to user's setting.
From the "current setting system" file, the CT outputs its sensed voltage to the negative pin of the op-amp: MC33074DG via a system of R, diode, and C.
The front panel volume allows the user to adjust the maximum output current. If the CT current is too high, the pin (8) becomes lower and V1 will be just around 3 diode drops above the ground (let's say 0.4*3 = 1.2 volts). Meanwhile, V3 will be more than 1.2V. So, this will make the reset (R) pin of the SR flip-flop high, pulling the output of the controller IC (TL2845B) to ground. This will be connected further to the IGBTs, so those IGBTs will be shutdown.

Here are my problems:
When V1 is low (at 1.2Volts), will it try to "force" the output of the TL2845B's amplifier to be 1.2Volts? The reason is that the amplifier receives 2.5V input and try to "force" its output the be a different voltage level.
Why this does not cause overheating?



Deleted member 440916

Joined Dec 31, 1969
You don't say what is overheating however I assume one or both chips as a consequence of nothing being in circuit to limit the current between the two.