Circuit Details:
The voltage regulation is carried out by the adjustable linear regulators LM317/LM337.Adjustment of the output voltage is done by a trim-pot R10/R20. The way the LM317 works is by regulating the voltage between the ADJ and OUT pin to 1.2V. The 1.2V induces a "programming current" of 5mA (1.2V/240Ohm) on R4 which is placed between Vo and Vadj.This current flows through the adjustable R10 and works to change the voltage at the ADJ pin. Then Vout = Vadj + 1.2V. In order to allow adjustment down to 0V, Vadj is allows to swing down to -1.25V (clamped by a TLV431). The cap C3 (1uF) stabilizes the voltage at the ADJ pin for a smoother regulated output voltage.
Standard design equations for operation from Vo= 1.25V ...(Vi-2V).
Vo = Vadj +Vref = Vadj + 1.25V
Iprog = 1.25V/R1
Vo = (Iprog +50uA) x R2
Vo ~= (1.25V/R1) x R2
Design equations for operation Vo= 0V .. (Vi-2V) requires a negative supply and a clamp:
Vo ~= (1.25V/R1) x (R2 + R3) + -5V
When the zener clamps the Vadj voltage is -1.25V. Therefore Vo = +1.25V - 1.25V= 0.
The current is measured and transduced into a voltage by a shunt resistor and the LT6105 high side amplifier with a gain of x50. The nice thing about this amplifier is, due to its wide common mode range, can be used symmetrically for the postive as well as negative power supply outputs.
Overcurrent, or constant-current mode works by shunting the programming current away from R10 through a low resistance MOSFET. The sensed current is compared to the desired over-current detection trip-point (set by a trim-pot) and when it exceed this, the MOSFET is turned on. For resistive loads, the shunting of the Vadj voltage will also reduce the Vout,which in turn will lower the current. Therefore the collapsing Vout will reduce the current and the system should stabilize at the current limit point. Note that that is not the case for constant power loads.Hysteresis (positive feedback on comparator U3) is added to prevent oscillations when the operation is near the trip point.
Since I will be using isolated AC/DC converters, the outputs of the bench power supply are isolated. To prevent floating signifcantly above earth ground I added transient voltage protectors (TVS) at 40V breakdown to each output. Note that "ground" output is also floating -- it is not tied to earth ground except through a 1M bleeder resistor.
Since I was using LM317, I also used the same for a temperature controlled fan. A NTC thermistor is used to sense the temperature and modify the voltage at the ADJ pin of an LM317. This LM317 directly drives a 5V DC fan. If temperature exceeds 50C, then output to fan is 4.16V and 75C 5.10V.