How do you specify a DC power supply?

Every automated test system that tests electronic circuit boards, modules or equipment needs one or more DC power supplies. Specifying the right supply involves more than just finding one that can supply the right current and voltage. So, the question is, how do you specify a DC power supply, anyway?

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To answer this question, let's first take a look at the types of power supplies that are available and their various specifications. Then, we'll consider the features that you need to consider when selecting a power supply for an automated test system.

Linear or switching supply?

DC power supplies come in two basic flavours: linear or switching. Linear power supplies offer low ripple and noise specifications and have fast transient behaviour.

Linear supplies are, however, inefficient and generate a lot of heat. They are also quite heavy.

As a result, most engineers find them desirable only at lower output power levels (typically less than 500W). Most linear DC power supplies are benchtop supplies.

If your system requires higher output power, then using switching supplies is a better option. They provide higher power density than linear supplies.

By using switching supplies, you can have 12 DC outputs, providing up to 4,000W of power in the same rack. Switching supplies are easier to control than linear supplies and cost about the same per channel.

Load regulation, line regulation, and stability

Perhaps the most important specifications are load regulation, line regulation, and stability. Load regulation is a measure of how well a supply maintains its output voltage when the output current changes. Normally, this effect should be very small (less than 0.01% of set output voltage).

Line regulation is a measure of how well the supply maintains its DC output voltage or current as a function of AC input line voltage. This specification is important when the input line voltage is not stable.

Stability is a measure of a supply’s long-term output voltage or current drift.

Figure 1

Figure 1

Transient response

Transient response is a measure of how well a supply copes with changes in current demand or how well the supply follows changes in the load impedance. This is an important specification for many applications. In general, linear supplies offer better transient response than switching supplies.

Slew rate

The next specification to consider is the DC output voltage slew rate (rise and fall time). To improve ripple and noise specifications, DC programmable power supplies have output filters that use large capacitors that store a lot of energy.

It is mainly the charge and discharge time of this filter, combined with the current demand of the DUT that determines a supply’s voltage slew rate. The voltage slew rate is mostly independent of the connected DUT.

Parallel, series operation

To supply a higher output voltage than is possible with a single supply, DC power supplies can be connected in series. All you have to do is to connect the positive terminal of one supply to the negative of another.

There are some limitations, however. Every programmable power supply has voltage isolation specifications, one for the negative to chassis isolation and one for the positive to chassis isolation. Ensure these voltages are not exceeded.

Local and remote sensing

Many DC supplies can be configured for either local or remote sensing. For a more accurate output voltage setting, remote sensing should be used. In this mode, you sense the voltage where the power supply connects to the load. This method compensates for the voltage drop across the leads.

Constant-current mode

Although most supplies are used in constant-voltage mode, many applications call for using a DC power supply in constant current mode. When operated in constant-current mode, accurate current control is the most important specification. Output voltage ripple and noise are not as important as output current ripple and noise.

Analogue programming

DC programmable power supplies typically provide a standard and isolated analogue interface. Through the analogue interface a supply’s DC output voltage, current and over-voltage-protection (OVP) can be set.

These values are controlled by supplying a voltage signal, a current signal, or by connecting a resistor to the analogue input. For example, you might use the analog output of a PLC to control the output voltage of a power supply.

Digital control and measurement

In general, the output voltage and current of a programmable supply is set most accurately, with the highest resolution, through its digital interface. As mentioned earlier, DC supplies typically offer many different interfaces, including RS-232, RS-485, USB, GPIB, Modbus-TCP, Modbus-RTU, and Ethernet.

In addition to the hardware, most DC power supply companies also offer the software you need to integrate your DC supply into your system. For example, we provide IVI drivers with each supply, and the supplies are programmed using standard SCPI commands.

Accessories and support

The availability of accessories could also be an important consideration in your choice of power supply. For example, if you plan to rack mount the power supply verify that one is available for your supply. Purchasing an off-the-shelf rack-mount kit is always less expensive than making one yourself.

As you can see, there are many parameters to consider when selecting a DC programmable power supply for an automatic test system. Electrical specifications are perhaps the most important, but you also need to consider the form factor, control needs, and even what accessories are available. By taking all of these into account, you’ll make the best choice for your application.

Bill Martin is sales/applications engineer at Ametek Programmable Power


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