I was just reading a LinkedIn message regarding methods that could be used to load-test a power supply.
The original poster wanted a method to do some active load testing inexpensively. He wanted to avoid buying any expensive test equipment (active load box, network analyzer). This seemed like a common problem. I needed to do this on one of my basement lab projects and (since I have never won the lottery) I wanted to do my testing on the cheap. Mostly, I needed to know if the control loop in my supply had sufficient phase margin that it wasn't going to ring like the local neighborhood kids on one of their candy-selling marathons whenever the load conditions changed.
For the particular specs with my supply (5V at 2A), I knew that if I loaded it at around 10 percent of full load, then switched to full load, and then switched back, I could simply observe the output voltage to see what happened. I built the following circuit on a scrap of perf-board using a medium power NPN transistor. I used a TIP41B because I had some, but those are not recommended for new designs. Almost any decent NPN power transistor or N-channel power FET will work in this application.
I set the '555 up as a free-running square wave generator (well, within a few percent of being a 50 percent duty cycle square wave). Frequency is just below 100Hz. The actual frequency doesn't matter as much as the rapid rise and fall of the waveform. This is the way the circuit was used: I connected the 2.49Ω resistor (LOAD SINK) to my power supply being tested. I also had an additional 27Ω resistor connected to the power supply. When the power transistor is on, load current was about 2A; when off, around 200mA. I monitored the voltage at the LOAD SINK terminal with my scope. The SYNC terminal is used if I needed a separate sync input to the scope for more accurate triggering. More on that in a minute.
What I was looking for is excessive ringing in the output voltage as the high-current draw snapped on and off. If the voltage just overshot slightly and then quickly settled to a final value, then my loop compensation was probably just right. If the phase margin was insufficient, the ringing would last a while. That SYNC input comes in handy if the output is ringing excessively. It's tough to set a good, consistent trigger point on such a waveform. The SYNC output provides a clean, square waveform with no ringing.
I couldn't produce quantitative evaluations with this gadget (I could not specify precise values of phase margin), but it worked well enough to do a quick evaluation of power supply prototypes.