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How to interpret a linear power supply’s data sheet, Part 2

-January 03, 2013

In Part I  of this article, I offered an overview of several specifications of DC power supplies, including those related to accuracy and resolution, and to stability. Although this article focuses on DC power supplies, the output of these power supplies is not perfect DC. Some AC is to be expected on the output. For some applications, high AC on the output can produce unexpected circuit behavior, so it helps to know the amplitude of the residual AC. In addition to AC noise, it may be useful to know the transient response of the power supply to changes in load and settings. For example, in automated testing, it is important to know when the power supply has settled in response to a change in settings. Therefore, Part II will focus on specifications related to a DC power supply’s AC characteristics, as well as a variety of other performance considerations.

Specs Related to AC Characteristics
Spurious AC components on the output of a DC supply are known as ripple and noise, or periodic and random deviation (PARD). These terms are often used interchangeably. The term ripple refers to periodic AC on the output. When viewed in the frequency domain, ripple shows up as spurious responses. Unlike ripple, which is periodic, noise is random. Noise covers a broad spectrum, and, when viewed in the frequency domain, it manifests itself as an increase in the baseline. See Figures 5 and 6. Given that ripple and noise are usually lumped together and cannot be easily separated, I’ll refer to the combined effects as PARD from this point forward.


Figure 5. This simplified drawing shows the concepts of periodic (ripple) and random (noise) distortion.


Figure 6. This noise measurement was taken with a 1X probe and a bandwidth of approximately 7MHz while the power supply was delivering full rated current.

PARD specifications must be defined with a bandwidth and should be specified for both current and voltage. Current PARD is relevant when using a power supply in constant current (CC) mode, and it is often specified as an RMS value. Because the shape of PARD is indeterminate, voltage PARD is usually expressed both as a root mean square voltage, which can provide a sense of the noise power, and also as a peak-to-peak voltage, which may be relevant when driving high impedance loads.

Because of the bandwidth consideration, PARD specifications are heavily dependent upon the measurement technique used to test them. The manufacturer’s performance verification procedures usually include the procedure for checking PARD. It is important to consider the whole signal path used to verify ripple and noise specifications. For example, using a high bandwidth oscilloscope with a low bandwidth probe can make a supply’s specifications seem more impressive than they actually are.
 
Another set of AC characteristics describes how quickly a power supply can respond to changes. Transient response specifications indicate how quickly the output settles to a stable DC value after a change in load or settings. Most power supplies have a large capacitance in parallel with their outputs to help deliver clean, steady DC. When this capacitance is placed in parallel with the load resistance, a time constant results and the size of the time constant varies with the load impedance. Because of the heavy dependence on the resistance of the load, response to setting changes must be specified for a specific load. It is common to see specifications for open circuits, short circuits, or specific resistance values.

Transient response can be tested by applying significant step changes both to the load impedance and to power supply settings, then measuring the time required to settle to a final value. The voltage transient response for all Keithley Series 2200 power supplies, for instance, is specified for three conditions: increasing load, increasing setting, and decreasing setting.


Table 1. Voltage transient response specifications for a Keithley 2200-32-3 power supply.

To ensure confidence in a test’s results and the repeatability of those results, a power supply that can accurately deliver the required power to the DUT is essential. A power supply that lacks sufficient accuracy or stability will have an effect on measurement results that will be indistinguishable from the effects due to the DUT’s actual performance. Temperature drift, sudden load changes, and fluctuating AC line voltage are just some of the factors that can cause inaccurate test results. An accurate power supply that is designed to cope with these variations and to provide the voltage or current specified consistently and accurately makes it possible to be confident about the accuracy of test results.

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