Mixed Mode Bench PSU Delivers High Performance

If you have an electronics bench, it follows that you will need some form of bench power supply. While many make do with fixed-voltage supplies it’s safe to say that the most useful bench power supplies have variable voltage and a variable current limiter. These are available in a range of sizes and qualities, and can be had from the usual online suppliers starting with a surprisingly small outlay.

There is however a problem with inexpensive bench power supplies. They are invariably switch-mode designs, and their output will often be noisy. Expensive linear supplies provide a much more noise-free output, but do so at the expense of excessive heat loss when regulating a high voltage drop.

One solution is a mixed-mode design, in which a switch-mode supply does the hard work of reducing the voltage most of the way, and a linear regulator drops the last couple of volts to provide a noise-free output. [Andrei] shows us his design for just such a mixed-mode supply, and it’s one you can have a go at building yourself.

His primary supply is an off-the-shelf switcher that turns mains AC into 24 V DC. This then feeds an LTC1624 buck converter that brings the voltage down to about 1.2 V above the final output voltage, this is in turn fed to a parallel pair of LT3081 linear regulators that deliver the final noise-free output. There is an INA260 for voltage and current measurement, and an Arduino with LCD display as a user interface. His prototype has been nicely constructed using a four-layer PCB, though he suggests it could be made on stripboard with the appropriate SMD adaptors. The cardboard chassis he’s used looks slightly alarming though.

We’ve covered numerous bench power supplies here over the years here at Hackaday. If it is an author’s favourite you are seeking though, take a look at the 723.

11 thoughts on “Mixed Mode Bench PSU Delivers High Performance

    1. what do you think is going to happen with the cardboard? the autoignition temperature of cardboard is drastically higher than the temperature at which most of the regulators involved in that circuit are going to shutdown to protect themselves. the cardboard itself is a decent electrical insulator.

      1. With a catastrophic failure a lot of heat can be produced. A metal case woukd better contain the fire. Some psus can supply 30V at 5 or 10A, that’s enough to cause fire, melt tin and aluminium etc. Before a fuse blows. And if something live touches a well grounded/earthed case will blow a fuse or trigger earth leakage switch. Rather that than something hanging loose but insulated by cardboard, ready for a catastrophic failure.

      2. Lack of shielding.

        Even if he isn’t doing this for a radio project RF emitted from that switcher could couple into his power leads or other parts of the device he is powering. That may or may not matter for every purpose but considering clean power is one of the main reasons to build such a supply in the first place I would think that shielding would be a must!

  1. I’m no power supply expert, but the LTC1624 has a fixed switching frequency of 200 KHz, and the LT3081 has the lowest ripple rejection at that frequency. It doesn’t look like the best pair choice for smoothing the line.

    1. Yeah, this. Most linear regulators have very good load regulation but poor supply-rejection, i.e. they respond very well to changes in current demand from the load but do a poor job of attenuating noise from the first-stage switching regulator.

      Designing a good hybrid PSU is a more interesting problem than most people realise at first glance, and it’s real easy to end up with an output that’s not much cleaner than that of a well-designed standalone switcher, and with poorer stability because there are now at least two interacting feedback loops.

  2. There is a very similar, though different, circuit in the LT3081 datasheet from Linear Technology. FYI for anyone interested in comparing the two implementations.

  3. Well, cardboard box and expensive parts is not something to see everyday.
    As pointed out, the LT3081 has poor rejection at the switching frequency and more than that, it is very expensive compared to a opamp + tranzistor that is more feasible for a bench supply. Actually, the good old LM317 is better at rejecting, if one can spare a little loss in efficiency.

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