There was a time when test equipment manufacturers published service manuals that included schematics and detailed descriptions of the operating principles of the instruments. Perusing these is still a great way to get a master class in precision design, or to pick up some really interesting circuit ideas. After all, imitation is the sincerest form of flattery.
Here is one of the latter from the classic Agilent (was HP, now Keysight) E361xA series of 60W bench power supplies. They are obsolete now but had some impressive specifications with the E3616A (35V at 1.7A) having better than 200µVrms noise from 20Hz to 20MHz and 0.01% line and load regulation. It is also passively cooled which means no audible noise either.
The way to minimize the power dissipation in a series-pass regulator is to use a pre-regulator of some sort to limit the voltage drop across the series element. This is often done with a switch-mode pre-regulator, but this introduces electrical noise. The designers of the E361xA power supplies used a very different approach to achieve the impressive noise figures of this supply.
The block diagram in Figure 1 shows the “Rectifier Filter Preregulator” block connected to the power transformer in the upper left. This cleverly switches several taps on the transformer’s main secondary winding to provide four different DC voltages which are then fed to the series regulator. This tap-changing is accomplished by thyristors (called SCRs in the manual).
Just as a reminder, a thyristor is a four-layer semiconductor device (PNPN). You can think of a thyristor as a diode that you can turn on with a gate terminal is brought positive with respect to the cathode. Once on, the thyristor will remain in conduction (even in the absence of a gate signal) until the forward current drops to zero. It always blocks current in when reverse biased.
Figure 2 is a simplified version of the circuit. The transformer has three secondary windings labelled W1, W2 and W3. If the all the thyristors are off, D1 to D4 form a standard bridge rectifier and the rectified voltage will be determined solely by W. If thyristors TH1 and TH2 are on, D1 and D2 will never be forward biased and the output voltage will be proportional to W1 + W2.
Similarly, if TH3 and TH4 are on D3 and D4 will be out of the picture and the output voltage will be proportional to W1 + W3. You can see where this is heading. If all four thyristors are on, none of the diodes will conduct and the output voltage will be proportional to W1 + W2 + W3. By choosing the voltages of W1 and W2 to be equal and W3 to be twice as big, we get for evenly spaced DC voltages. Nice.
The Thyristors are driven by opto-coupled TRIACs (at least in later models) as shown in Figure 3. TRIACs are similar to thyristors but conduct in both directions (a feature not used in this circuit, given the series diode). They are triggered optically for isolation and stop conducting at the same time the thyristor does when the current drops to zero at the end of the mains half cycle.
The control circuit monitors the unregulated voltage and the output voltage and switches tap to make sure there is sufficient headroom for the series-pass regulator, but the unregulated voltage is no higher than necessary, minimizing the power dissipation in the series-pass MOSFETS. This is a low-noise solution since switching occurs only when the tap actually changes. The dissipation in the thyristors themselves is a little higher than that of an equivalent diode but obviously manageable in this application
You could achieve the same end with relays, but you would have to put up with the clicking of relays as the voltage changed. I think this is a neat solution and uses a device not often used these days except in very high-power applications.
Agilent Technologies. “Agilent E361xA 60W Bench Series DC Power Supplies Operating and Service Manual.” Agilent technologies, April 2000. https://www.manualslib.com/manual/2875/Agilent-Technologies-E3614a.html.
Keysight. “Agilent E3616 60W Bench Power Supply.” Keysight. Accessed March 15, 2021. https://www.keysight.com/au/en/product/E3616A/60w-power-supply-35v-17a.html. Agilent Technologies. “E3620A and E3630A Non-Programmable DC Power Supplies,”
“Thyristor.” In Wikipedia, December 31, 2020. https://en.wikipedia.org/w/index.php?title=Thyristor&oldid=997500487.
“TRIAC.” In Wikipedia, January 11, 2021. https://en.wikipedia.org/w/index.php?title=TRIAC&oldid=999740245.