March, 2015
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tech.com
Probing Power Supplies for Use with Test Equipment
By Paul Kingsepp, Product Manager, SL Power Corp., Ventura, CA T
est-equipment designers face many chal- lenges when choosing an optimum power supply for their designs. A basic starting
point usually involves power capacity, in terms of voltage and amperage, cooling requirements, size, and regulatory certifications. If the criteria for choosing a power supply simply involved these parameters, an engineer’s job would be relatively easy with many available options from which to choose. However, there are additional parameters
which must be considered when selecting a power supply for use in test equipment. Electrical noise, in its many forms, is a major concern because noise can severely degrade instrument performance. The environment in which test equip- ment will be used also plays a strong role in the selection of a power supply for that test equipment. Electrical noise consists of ran-
dom or undesired fluctuations in elec- trical signals or a voltage source. It can appear in either differential mode (across power or signal lines) or com- mon mode (between a power or signal line and a common ground) noise. Radiated noise is a form of electro- magnetic interference (EMI) trans- mitted through the air by cables and components with AC voltages or cur- rents. The radiated coupling can be very local; it can occur, for example, between a transformer and a nearby wire or printed-circuit-board (PCB) trace, and become conducted noise. Differential-mode noise (known
as ripple and noise) can range from a few microvolts in linear type power supplies, up to hundreds of millivolts in switching types. The values for the levels of this noise are normally speci- fied by the manufacturers in both root-mean-square and peak-to-peak noise, although the peak-to-peak noise is generally 10 times higher than the root-mean-square (rms) noise, and is potentially more trouble- some to sensitive electronic circuits. Minimizing the effects of this type of noise requires additional filtering, but since this noise is known when choos- ing a power supply for a test instru- ment design, selecting a proper power supply can go a long way towards minimizing the noise.
Common-Mode Noise Common-mode noise is caused
by current flowing from primary to secondary windings through parasitic capacitance in the transformer, then through system ground. If not proper- ly filtered, such noise can interfere with the system resolution. Common- mode noise (between power or signal lines and ground) on the output of a power supply is often overlooked and rarely specified on power-supply datasheets. This may be due to lack of knowledge by power-supply manufac- turers concerning its impact on cer- tain product performance levels, or that it may not be an issue for the types of applications targeted by their products. The power-supply output, in some applications, is often shorted to earth or chassis in the end applica- tion. In this case, the common mode noise will likely not be an issue. However, it can be instructive to
know and understand the common- mode currents that flow in the circuit while making their way to ground. If the output of the power supply is con-
See at NEPCON China, Booth A-1G68 TU425 series power supply.
nected to chassis/earth ground somewhere on the load board (with the electronic circuitry), there can be common-mode currents flowing through the board. In addition, if the PCB layout is not opti- mized, current can flow through sensitive parts of the circuitry and cause malfunctions.
Switching Power Supplies All switching power supplies exhibit some
level of common-mode noise, the extent of which is dependent on many factors including power-sup- ply topology, filter designs, and input-to-output isolation. Since most power-supply manufacturers
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