How to identify the best power supply for a test application
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 energize 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.
Figure 1b. Multi-range output. This characteristic allows higher voltages at lower currents and higher currents at lower voltages.
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.
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.
For sensitive circuits, linear power supplies provide much lower normal-mode output noise than supplies designed using switching technology. The tradeoff is that linear power supplies have lower power-conversion efficiency than switching supplies and can be bulkier and heavier. Switching supplies typically offer more output power in a smaller enclosure. For noise-sensitive circuits, a linear supply can have just one-fifth to one-tenth of the noise (5mVp-p vs. >50mVp-p) of a switching supply. Whenever normal-mode noise is a crucial consideration, use a linear supply if possible.
Assess Common-Mode Noise Current
In addition to their lower normal-mode noise, linear power supplies also generally have lower common-mode noise than switching supplies. Common-mode noise is generated whenever changing voltages, such as AC voltages and transients (dv/dt) on either the primary or the secondary windings of an isolation transformer, couple current across the barrier. Any noise current generated on the primary (secondary) must return to the primary (secondary) in order to complete the circuit. Whenever this current flows through an impedance, a noise voltage is generated, which under some circumstances, can degrade load (or DUT) performance or cause load-monitoring measurement inaccuracies.
The magnitude of the noise term is directly related to the voltage rise time and the unshielded or stray capacitance of the power supply’s isolation transformer. Sources of common-mode noise include voltage transients from rectifier diodes (on the secondary) turning on and off and either the 60Hz line movement or the abrupt voltage transient common with a switching power supply’s primary circuit.
Figure 2 shows a simplified block diagram of a power supply. The quality of the transformer’s construction, including sufficient shielding between the primary and secondary windings, can minimize the stray capacitance between primary and secondary. With minimal coupling capacitance, the noise current flowing through the load won’t generally affect the load’s operation or impact measurements on the load. If the transformer’s primary and secondary aren’t sufficiently shielded from each other, then the coupling capacitance can be large and milli-amps of current can flow into the load, creating performance problems and load current measurement errors. For low power and sensitive components, modules, or end products, evaluate the power supply for low common-mode performance. For example, Keithley’s Series 2200 power supplies have common-mode currents of less than 10μA.