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How to optimise power supplies for test apps

Posted: 04 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:power supplies? device under test? switching supply? Common-mode noise? remote sensing?

Most electrical engineers believe they have well understood power supplies because these are relatively simple, single-function DC devices designed to output controlled voltages. However, there is much more to them than this description would suggest. Although a review of a power supply's specifications should always be a part of the selection process, other characteristics should also be considered.

Investigate the power envelope
The most significant decision is ensuring that sufficient power is available to energise the device under test (DUT). Different types of power supplies have different power envelopes. A power supply with a rectangular power envelope as shown in figure 1a, the most versatile type, allows supplying any current to the load at any voltage level. A supply with multiple rectangular envelopes for multiple ranges (such as the two-rectangular envelope shown in figure 1b), permits higher values of one parameter at the expense of the other parameter, so it can output a higher level of current but at a lower maximum voltage. Supplies that output a hyperbolic envelope offer a more continuous transition than a multi-range power supply, with one parameter inversely proportional to the other (figure 1c). High power output supplies tend to have multi-range or hyperbolic envelopes.

Figure 1a: Rectangular power supply envelope.

Figure 1b: Multi-range output.

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

Determine the noise performance
Noise from external sources may cause problems when powering a circuit that operates at a very low voltage or a circuit that uses or measures very low currents. The 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 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 but have lower power-conversion efficiency 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
Linear power supplies 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. Whenever this current flows through an impedance, the noise voltage generated can degrade load (or DUT) performance or cause load-monitoring measurement inaccuracies. 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 minimise 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 milliamps 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. Keithley's Series 2200 power supplies have common-mode currents of less than 10?A.

Figure 2: Normal-mode and common-mode noise currents.

Check isolation from Earth ground
One further indication of the quality of a power supply is the isolation of its output is from the power line. A power supply with high isolation further minimises noise on the supply's output. A good level of isolation impedance includes parameters greater than 1G in parallel with less than 1nF and shielded well enough to support less than 5?A of common-mode current. Unfortunately, few instruments meet or exceed these guidelines.

Low frequency 60Hz designs may meet the common-mode current specification but fall short of the DC resistance and capacitance figures; switching designs may have low capacitance and higher DC isolation but excessive common-mode current. In some applications, the DC isolation resistance and capacitance are more important than common-mode current. One case in which the high impedance is important is when a supply is powering a circuit driven by a linear amplifier. In this situation, the power supply is part of the load on the linear amplifier and a large power supply capacitance can create stability problems for the amplifier.

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