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R&S®Essentials | Power supplies fundamentals

Understanding benchtop power supplies

Author: Paul Denisowski, Test & measurement expert

A fundamental understanding of dc power supplies and their basic functions is necessary for correct operations. The following will provide a general technical introduction to benchtop DC power supplies and explains the most important functions, parameters, and features of power supplies in modern test and measurement applications.

Most electronics are powered by direct current (DC), and therefore a DC power source or “supply” is required for almost all design, testing, and debugging of modern electronics. Although it is possible to use a fixed, non-configurable source of DC power, so-called benchtop or bench power supplies are normally used for this purpose.

There are many different models and configurations of benchtop DC power supplies, but there are some common requirements for almost any type of supply:

1. A variable, high-precision output:

Voltage typically up to about 30V max
Currents typically up to about 10A max

2. A high-quality bench power supply output: low noise and “ripple”

3. A low recovery time after a sudden change in load impedance

4. Multiple channels that can be used independently or combined

Basic specifications of benchtop power supplies

The most basic specifications of a power supply are the maximum output voltage (in volts), the maximum output current (in amperes), and the maximum output power (in watts). These are usually specified per channels.

For example, this channel has a max voltage of 30V, a max current of 10A, and a max power of 160W. It is important to note that max. power is less than max. voltage times max. current. Here, the maximum channel power output is 160W, and not 300W.

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The combinations of voltage and current supported by a bench dc power supply are often shown using a “derating curve”. Any combination of voltage or current that falls on or below this curve, (in the colored area) is supported by the supply.

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Derating curves are normally included in the power supply specifications or manual or can be found on the manufacturer’s website.

You can learn more about derating curves here.

Configure voltage and current on a benchtop DC power supply

The user of a bench power supply configures two things: the desired output current limit and the output voltage. Output current is a function of the load resistance, while output power is the output voltage times the output current.

The supply output voltage is usually kept constant, but in some cases the output voltage may be dynamically changed or varied. The three most common ways in which this is done are a ramp, an arbitrary sequence, and using an external source.

Ramp output

As the name implies, a “ramp” is a continuous rise or “ramping up” in the output voltage. The output voltage starts at zero and then rises to a defined voltage over a user-defined ramping time after which the voltage remains constant.

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Arbitrary output

Unlike ramp, which linearly increases voltage from zero to a defined value, an arbitrary sequence switches the DC power supply between different discrete voltage levels or current thresholds. These levels each have user-defined values and durations, and this sequence may be repeated multiple times. The sequences are sometimes defined directly on the power supply itself but are often loaded into the supply by means of an externally created file.

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Analog / modulation output

On some supplies, it is also possible to control the channel output using an external input voltage – this “analog” or “modulation” input is fed into the supply using a separate connector. The output voltage (or sometimes the output current limit) is controlled or regulated by this analog input. This can also be used to create “modulated” output voltage waveforms such as the sine wave shown here.

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Readback function

Regardless of the output type, many benchtop power supplies will show two values for voltage and current. The first set of values are the values entered by the user. The second set of values are the measured or sensed values of what the supply is currently outputting. This function is often called “readback”.

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In normal operation, the readback voltage value should be close to the configured value. The current readback value will depend on the load resistance, since it is the load resistance that determines how much current is being drawn from the supply. It is important to remember that the “current” value entered by the user is an upper limit or max value – it doesn’t “force” the supply to output a given current.

In addition to simply displaying the values, readback is also used in three other areas: remote sense, protection functions, and in determining constant voltage or constant current mode.

Remote sense

Power supply lead cables can reduce the voltage seen by the load because they have a non-zero resistance. In many cases this can be ignored, but it can be significant for small load resistances or high current.

Remote sense enables bench DC power supplies to overcome voltage drop caused by the supply leads. Learn more about remote sense here.

Protection functions

Protection functions are designed to protect the load from excessive voltage, or “overvoltage”, from excessive current or “overcurrent” and from excessive power or “overpower”. All three of these functions disable the supply output when a threshold is exceeded. Often, there is also some kind of visual and/or audible indication when protection is activated. In most cases, the supply output must then be manually restarted.

Protection functions are normally configured and enabled/disabled on a per channel basis, although overcurrent protection may be linked to other channels – that is, if one channel’s “electronic fuse” is tripped, all of the channels linked to it are also disabled. And overcurrent protection may also have a delay at startup, to avoid having the output disabled due to high, but short-duration, inrush currents.

Finally, the supply itself may have an over-temperature protection function that shuts the supply down if the supply’s internal temperature becomes too high.

Constant voltage and constant current modes

The third main use of readback is related to something called “constant voltage” and “constant current” modes. Excessively and/or unexpectedly high current can occur when load resistance decreases. Since high current often will damage or destroy the load, it is important to prevent this situation from occurring.

One way to avoid this issue is to reduce the voltage such that the current falls below the threshold. This mode of operation is called “constant current mode”.

Learn more about constant voltage and constant current here.

Summary

Benchtop DC power supplies are widely used in the design, test, and debugging of almost all electronic devices.
The key specifications for any bench power supply are the number of channels and the maximum output voltage, current, and power.
It’s also important to choose a supply with a clean, high-quality output and good precision / accuracy.
In most cases, the output of a DC power supply is constant, but many supplies support ways to generate a variable output, such as ramp and arbitrary waveforms.
Readback shows the measured output values, and enables functions such as remote sense, different protection types, and switching between constant voltage and constant current modes.
Additional functions found in some benchtop power supplies include serial and parallel operation, the ability to function as an electronic load, and battery simulation.

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