Designing Switching Power Supplies with APx

Created on 2011-06-17 23:14:00

Although primarily intended for audio, designers of switching (switch-mode) power supplies are finding the APx525 Series with the BW52 High Bandwidth option to be superior to their scope-based FFTs and spectrum analyzers for circuit analysis. The BW52 option extends the measurement range to over 1 MHz.


Switching supplies can be tricky to design. Because of the sharp switching cut-off, they can generate significant harmonic and intermodulation content. Analysis in the frequency domain is necessary in order to optimize input/output filtering, efficiency, feedback loop stability, input/output regulation, reliability, and other design elements.

APx500 display of a switching power supply, linear x and y axes. Any combination of linear or log scales may be selected, and multiple windows with different scales can be open simultaneously.

APx500 display of the same switching power supply, log x and y axes.

Two major advantages of the APx instruments with BW52 are the high bit depth and high frequency resolution. The 24-bit extremely low distortion AD converters in the APx instruments provide a much wider dynamic range and lower noise floor than the 8-bit converters in scopes or spectrum analyzers. Selectable averaging (up to 1000 averages) also helps to reduce random noise from obscuring low level detail. Additionally, most spectrum analyzers only offer a 50 Ω input impedance and a very low maximum input voltage tolerance. An external attenuator can be used to raise the input impedance and multiply the voltage tolerance, but the consequence is to degrade the noise floor even further.

Frequency resolution at maximum FFT length is 2.38 Hz from DC to 1 MHz. This makes it possible to see 50/60 Hz power line noise and intermodulation, switching frequency drift and instability, and other artifacts that would be obscured or be impossible to identify with the much lower resolution typical of scopes and spectrum analyzers.

FFT of the same supply, using a scope.

For switching power supplies used in audio equipment, the APx525 Series with BW52 has the additional advantage of being able to make both power supply and audio measurements with a single instrument. Since leakage of switching power supply noise into audio circuits is a major concern in these devices, it is often necessary to have a high-bandwidth view of the audio output. Likewise, by generating an audio signal with the APx instrument and observing the power supply, any resulting intermodulation products on the supply rails can be seen.

APx500 FFT of a Class D amplifier with 10 kHz audio stimulus. Note the intermodulation products centered around the switching frequency and its harmonics.

Being completely software controlled, many advanced measurement, automation, and reporting capabilities are built into the APx500 software that can be advantageous in switching power supply design. Measurement Recorder, for example, can make a continuous real-time plot of switching frequency, DC voltage, and audio THD+N simultaneously over a specified time period. Built-in automation with GPI control, as well as external automation via LabVIEW drivers and a .NET API, are included. And, the automated reporting allows you to save results to a Word or PDF document, so that you can compare FFTs and other measurements to past results when you make design or production changes.