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TL431 linear power supply

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Kleinstein:
There still seems to be quite some ringing - so maybe C4 should be even larger. A suitable series resistor to C4 could also help.

iMo:
And with the current limiter, switching various R_Loads in/out.

iMo:
And with constant R_Load=10ohm (switched on/off) with various values of R2 (output voltage setting).

Kleinstein:
There are two small problems with the simple TL431 based regulator:
The first is relative obvious: one can not set a voltage below 2.5 V. This might be OK for a fixed supply to a circuit, but would be a slight limitation for a stand alone supply.

The second possible problem is that the loop gain and thus the suitable compensation tends to depend on the set voltage, if the voltage is set by adjusting the feedback divider. The way it is shown in the last simulation, by adjusting the lower resistor this is to a large part avoided, but at the cost of a rather nonlinear scale and even higher lower voltage limit.

So the circuit is nice and simple for a fixed supply, but not that suitable for a variable. For an adjustable supply it should be better to have a kind of OP to compare the feedback from a fixed divider with a variable set voltage.

For finding the right compensation from a simulation I found it rather useful to look at the output impedance: So similar to the load test with a constant current load a current sink (source) is used as a load, but this time in AC mode and first without much of the output capacitance. Looking than at the AC output voltage than directly gives the output impedance.  Ideally one want's a low output impedance of cause. The more important part is that the phase shift does not reach values above 90 degrees from a resistive load. Getting to close to an ideal inductance (90 degree phase) is also bad as this would cause much ringing if there is a corresponding capacitive load.

Some of the higher frequency problems can be fixed with those RC combinations at the output, but the lower frequency part has to come right from the regulator. Only at the very low end (e.g. below some 100 Hz) one can tolerate more phase shift, as it would take extreme, non realistic capacitors (e.g. C > 100 mF with ESR in the µOhms) to make the regulator unstable.

The nice point about looking at the output impedance instead of the more common loop gain is that a single simulation covers all possible load impedance cases. One still has to check different DC currents as the output stage can behave different at high and low currents. For the loop gain one would need to additionally test with different capacitors at the output to get the "whole" picture. Another point to check would be the hand over from current to voltage regulation and back.

iMo:
Bode

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