I’ve been thinking about bench power supplies and programmable dummy loads of late. Here is a collection of links on the subject.
Next level of sophistication is to use a computer power supply or wall wart to feed a linear regulator like a 78xx or LM317 adjustable with a pot. These designs usually use voltage and current meter modules for the display. This can give you a adjustable supply up to 30+ volts and a couple amps. Here are two examples.
There are two ways to go from here. The first is to drop the meter modules and control the supply with a microcontroller using some kind of DAC solution to generate a reference voltage supply. The other way to go is to design the linear regulator explicitly using a power transistor as a pass element controlled by an op amp negative feedback loop. With an explicit regulator, it is easy to do constant current/current limit. Dave’s many-part power supply design series (part 1 of many) over on the EEVBlog is a must watch. His design is based around the LT3080 but has a constant current control loop. Here is an example with an explicit regulator (part 2, part 3).
There are many more designs floating around the internets. Enough to drown in.
These designs are quite far from even a low-end precision bench power supply like the Rigol 832 or similar. These isn’t a lot of discussion of their accuracy, precision or transient response to varying loads.
Switching power supplies for computers generally have the following architecture: (1) EMI filtering to protect the grid from regulator noise, and vice versa, (2) power factor correction (PFC) which makes the power supply look like a resistive load to the grid, (3) an isolated switching converter controlled by something like the TI TL494 PWM Controller, and finally (5) output filtering. On Semi has a 255W ATX power supply reference design. Dave did a teardown of a 40A bench power supply that has a very similar design.
To test a power supply, you need a programmable dummy load, which can draw constant (or programmable) current or power. The Arachnid Labs Re:load 2 is a classic design using explicit linear regulation and is for sale on Tindie. It can handle up to 3A or 40V and dissipate 20W. However, the load is controlled by a pot and is not programmable. Kerry Wong build a similar device and has a new design which is microcontroller controlled. A dummy load necessarily has to dissipate the load as heat and the main challenge, besides precision regulation, is finding a big enough heat sink.
Finally, I’ve been reading The Fundamentals of Power Electronics and Power Electronics: Converters, Applications and Design to learn about switching converter design. More on this in future posts.