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0-30V/10A Adjustable power supply - Part 3 (Limiting dissipation)

In the previous part, I described modifying the tranformer to supply about 23V AC. After the smoothing cap, that would leave
(23-1.4) * 1.4 = 30.24V to apply to the pass transistors. Now we need to have a look at the total dissipation. I would like this power supply to deliver at least 24V of regulated DC, so the 30V at the smoothing cap is fine as the regulation circuit also needs some over voltage (about 4V in this case). Now let's do the math on an example. When supplying 12V at 10 amps, about 18V at the same current will be dissipated in the pass transistors. That is 180W we are talking about ! There are a couple of ways to deal with that problem. Increasing heatsink area can be used to offset dissipation problems, but that is a wasteful and inefficient approach. Instead, the regulator can be placed in a switched-mode loop that servo-controls the voltage across the regulator. In this arrangement, the regulator functions normally while the switched-mode control loop maintains the voltage across it at a minimal value, regardless of line or load changes. Allthough this approach is not quite as efficient as a classical switching regulator, it offers lower noise and the fast transient response of the lineair regulator.
Note : The circuit shown next is not a complete design, merely a means of explaining how an opamp as comparator can be used to keep the voltage difference across the regulator constant, thereby minimizing dissipation in that regulator circuit.


Above figure details a DC driven version of the circuit and find it's place between the bridge rectifier and the smoothing cap. The regulator functions in the conventional fashion, supplying an output at 10A capacity. The components shown form the switched-mode dissipation limiting control. The loop forces the potential across the regulation circuit to equal the 3.7V value of combined zener diodes D2 and D3. When the input of the regulator circuit decays far enough, the LT1018 output switches low, turning on T1. This allows current flow from the bridge to the smoothing capacitor, raising the regulator's input voltage. When the regulator circuit input value rises far enough, the comparator goes high, T1 cuts off and the capacitor ceases charging.

The 1N4003 damps the flyback spike of the current limiting inductor. R1 (4k7) ensures circuit start-up and the 68pF-1MOhm combination sets loop hysteresis at about 80mVpp. This free running oscillation control mode substantially reduces dissipation in the regulator circuitry, while preserving its performance. Despite changes in the input voltage, different regulated outputs or load shifts, the loop always ensures the minimum possible dissipation in the regulator.

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