In this picture, Oscium’s iMSO-104 oscilloscope paired with an iPad 2 measures the output voltage of a switching power supply. The measurement is made using iMSO-104’s analog input channel. DC Coupling is used and the time scale has been expanded to capture a couple periods of the signal. iMSO-104 requires a minimum of two full waveform cycles to perform the advanced statistical and analysis functions like Pulse Width, Duty Cycle, RMS, Mean, and of course - Period and Frequency.
This class of power supply offers circuit designers a power-efficient alternative to linear regulators. Switching supplies can achieve efficiency ratings near 80%, which might be an important feature for battery or solar powered applications.
Switching Power Supplies With Analog Circuits
The waveform in the display shows the fundamental switching frequency and harmonics, and other spurious signal content. For heavier loads the regulator’s internal circuitry adjusts the frequency and duty cycle of the output in order to maintain a constant voltage.
A cautionary note: The use of switching power supplies with sensitive analog circuitry requires more noise suppression than if the circuit included only digital components.
Consider the voltage reference in an analog-to-digital converter (ADC) and how it would be affected by the alternating currents generated by the switching supply. To make accurate, precise conversions, ADCs require rock solid voltage references. It’s easy to overlook this side effect and wonder why the output of the ADC flickers instead of settling.
Another application that can be problematic for designers is powering an amplifier stage directly from a switching regulator. Without the necessary precautions, the amplifier’s output may contain artifacts of the power supply’s switching frequency. One way to check for this condition is with a digital oscilloscope and looking at the FFT of the amplifiers output. If the regulator is switching at 10 kHz, for example, then the oscilloscope would show a 10 kHz signal superimposed on the amplifiers output.
Wall Warts Make Inexpensive Power Supplies for Electronics Projects
For electronics enthusiasts, hobbyists, or even seasoned electrical design engineers facing budgetary constraints, the AC/DC power supplies that plug into the wall can be an inexpensive way to provide continuous power to projects. HAM radio operators refer to these plug-in supplies as “wall warts” because they resemble a bump-outgrowth from the otherwise flat surface of a wall. Each wall wart has the numbers for output voltage and maximum sourced current printed on it somewhere. With some ingenuity these power supplies can be outfitted with the necessary cleanup circuitry and used with analog circuits.