Testing Power Supplies (EE Tip #112)

How can you determine the stability of your lab or bench-top supply? You can get a good impression of the stability of a power supply under various conditions by loading the output dynamically. This can be implemented using just a handful of components.

Power supply testing

Power supply testing

Apart from obvious factors such as output voltage and current, noise, hum and output resistance, it is also important that a power supply has a good regulation under varying load conditions. A standard test for this uses a resistor array across the output that can be switched between two values. Manufacturers typically use resistor values that correspond to 10% and 90% of the rated power output of the supply.

The switching frequency between the values is normally several tens of hertz (e.g. 40 Hz). The behavior of the output can then be inspected with an oscilloscope, from which you can deduce how stable the power supply is. At the rising edge of the square wave you will usually find an overshoot, which is caused by the way the regulator functions, the inductance of the internal and external wiring and any output filter.

This dynamic behavior is normally tested at a single frequency, but the designers in the Elektor Lab have tested numerous lab supplies over the years and it seemed interesting to check what happens at higher switching frequencies. The only items required for this are an ordinary signal generator with a square wave output and the circuit shown in Figure 1.Fig1-pwrsupply

You can then take measurements up to several megahertz, which should give you a really good insight for which applications the power supply is suitable. More often than not you will come across a resonance frequency at which the supply no longer remains stable and it’s interesting to note at which frequency that occurs.

The circuit really is very simple. The power MOSFET used in the circuit is a type that is rated at 80 V/75 A and has an on-resistance of only 10 mΩ (VGS = 10 V).

The output of the supply is continuously loaded by R2, which has a value such that 1/10th of the maximum output current flows through it (R2 = Vmax/0.1/max). The value of R1 is chosen such that 8/10th of the maximum current flows through it (R1 = Vmax/0.8/max). Together this makes 0.9/max when the MOSFET conducts. You should round the calculated values to the nearest E12 value and make sure that the resistors are able to dissipate the heat generated (using forced cooling, if required).

At larger output currents the MOSFET should also be provided with a small heatsink. The gate of the FET is connected to ground via two 100-Ω resistors, providing a neat 50-Ω impedance to the output of the signal generator. The output voltage of the signal generator should be set to a level between 5 V and 10 V, and you’re ready to test. Start with a low switching frequency and slowly increase it, whilst keeping an eye on the square wave on the oscilloscope. And then keep increasing the frequency… Who knows what surprises you may come across? Bear in mind though that the editorial team can’t be held responsible for any damage that may occur to the tested power supply. Use this circuit at your own risk!

— Harry Baggen and Ton Giesberts (Elektor, February 210)

Member Profile: John Peterson

John Peterson

John Peterson

Location: Menlo Park, CA

Education: BS and MS, University of Utah

Occupation: Software Developer

Member Status: John has been a subscriber since 2002.

Technical Interests: His interests include user interfaces for embedded systems, field-programmable gate array (FPGA) development, and embedded Internet development.

Most Recent Embedded Tech-Related Purchase: John recently purchased a power supply for one of his designs.

Current Projects: He is currently working on a custom light controller for strings of progammable LED lights.

Thoughts on the Future of Embedded Technology: John feels that smartphones have raised everybody’s expectations for how we interact with everyday things (e.g., cars, appliances, household control, etc.). “Either the phone becomes the interface (via the network) or the gadgets need touchscreen displays,” John said.

Engineer’s “Harlem Shake” Meme

In an blog posted today on the Phoenix New Times site, Troy Farah asks: “Harlem Shake vs. Gallon Smashing Prank: Which Meme Will Destroy America First?” Well, both have caused a lot of problems for smashers and shakers in the United States. We read a recent report about the possible legal issues facing some gallon smashers. And CNN.com posted a story on March 1 about the FAA’s probe into a recent “shake” on a plane. With negative results such as these, it’s clear that the Smash and the Shake are bidding for the most tile of “most destructive.”

Where does the engineering community stand on these pranks? Well, we have not seen an electrical engineer, robot, or microcontroller-based system smashing a gallon of milk to get a laugh. (Thankfully! We don’t endorse it.) But we did recently seen an engineer’s take on the Harlem Shake.

And so the meme continues.

Be sure to check out Dave Jones’s EEVblog video about the rocker.