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Identify the right power supply for test apps

07 Mar 2013  | James Niemann, Robert Green, Qing Starks

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Most electrical engineers believe they well understand power supplies as these are relatively simple, single-function DC devices designed to output controlled voltages. However, there is much more to power supplies than this description would suggest. Although a power supply's specifications describe its performance adequately for most applications, specifying every possible aspect of its performance (or any instrument's, for that matter) would be far too costly in terms of both money and time.

Although a review of a power supply's specifications should always be a part of the selection process, other characteristics should also be taken into consideration. From a user's perspective, what's important is understanding a supply's power envelope to ensure it will be able to deliver power at the voltage and current parameters needed for a specific application.

For developing, characterizing, and testing circuits that generate or measure low-level signals, selecting the design topology of the power supply and investigating its common-mode current can be essential to ensuring it doesn't interfere with circuit performance. Similarly, if developing a device with multiple isolated circuits, it's crucial that the power supply doesn't degrade the device's isolation. When using a power supply as an accurate voltage source for testing a circuit over its operating voltage range or as a calibrating source, it is essential to confirm that the supply's specified accuracy can be obtained at the input to the circuit under test. Applications like these require a detailed investigation of a power supply's characteristics.

Investigate the power envelope
The most significant decision is ensuring that sufficient power is available to energise the device under test (DUT). Although that might sound fairly obvious, be aware that different types of power supplies and sources have different power envelopes. One type of power supply has a rectangular power envelope (figure 1a) in which any current can be supplied to the load at any voltage level. This is certainly the most versatile power envelope. A second type of supply can have multiple rectangular envelopes for multiple ranges (such as the two-rectangular envelope shown in figure 1b). The advantage this type of power envelope offers is that it permits higher values of one parameter at the expense of the other parameter. For example, a supply with this type of envelope can output a higher level of current but at a lower maximum voltage.

Some supplies output a hyperbolic envelope, which offers a more continuous transition than a multi-range power supply. With this power envelope, one parameter is inversely proportional to the other (figure 1c). High power output supplies tend to have multi-range or hyperbolic envelopes. Take the time to investigate the type of envelope a specific application will require to ensure the supply selected can deliver the required power at the levels of voltage and current needed for testing.

Figure 1a: Rectangular power supply envelope. Any current level can be delivered at any voltage.

Figure 1b: Multi-range output. This characteristic allows higher voltages at lower currents and higher currents at lower voltages.

Figure 1c: Hyperbolic output characteristic. Maximum voltage and current follow a curve.

Determine the noise performance
When powering a circuit that operates at a very low voltage or a circuit that uses or measures very low currents (such as a transducer detector that must pick up milli-volt or micro-amp current signals), noise from external sources may cause problems. The power supply itself is one source of noise, which can be broken into two components: normal-mode noise and common-mode noise. Normal-mode noise is generated across the power supply's output terminals due to the supply's internal circuitry. Common-mode noise is earth-referenced noise originating from the power line and stray capacitance across the main transformer.

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