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R&S®Essentials | Power supplies fundamentals

Understanding Derating Curves

Author: Paul Denisowski, Test & measurement expert

The following will explain what power supply derating curves are and how to interpret them.

Benchtop power supplies have three primary specifications: maximum output voltage (in volts), maximum output current (in amperes) and maximum output power (in watts). These are usually specified per channel for both single and multichannel power supplies.

When operating a benchtop power supply, the user normally configures both – the desired output voltage and the output current limit, or the maximum amount of current that can be drawn from the power supply. The latter is usually for safety reasons and/or to protect the device under test. The output power is the product of the configured output voltage and the amount of current being drawn by the load (P = V * I).

What are derating curves used for?

Mathematically, if both voltage and current can be varied then there are many ways to produce the same output power. For example, 40W could be produced by using 40V and 1A, 20V and 2A, or even 100V and 0.4A.

In practice, all power supplies have limits on the output voltage and current they can produce. In many cases, maximum current is the limiting factor. For example, it’s relatively easy to design a 40W power supply that delivers 1A at 40V, but it’s not quite as easy to build a supply that provides 40A at 1V.

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In addition, it might also be possible to only use certain combinations of voltages and currents to achieve a maximum output power of 40W.

Since voltage and current can take on a wide range of user-configured values, it would be difficult to express all allowable combinations of voltage and current in tabular form. Therefore, this data is usually represented graphically.

What is a derating curve?

Derating curves are used to graphically represent the combinations of voltage and current supported by a power supply. Below is an example of a typical derating curve for a 160W power supply.

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Any combination of voltage or current that falls on or below this curve (in the colored area), is supported by the supply. Derating curves are normally included in the power supply specifications or manual or can be found on the manufacturer’s website. Whenever using a power supply, it’s always a good idea to check the derating curve to ensure that the desired combinations of voltage, current, and power are supported.

How to read a derating curve?

Below there is an example of a classical derating curve. This supply has a maximum power of 160W, a maximum current of 10A and a maximum voltage of 30V. It is important to remember that for most power supplies, the maximum output power is not maximum voltage times maximum current.

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On classical derating curves, there is usually a curved region bounded by something called the “maximum power output curve”. As the name implies, all points – that is, voltage and current combinations – along this curve will yield the maximum output power, in this case 160W.

Although many power supplies have “classic” derating curves similar to the one above, other derating curve shapes are also possible. Here, the power supply has a maximum output power of 40W and instead of a curve, it has a maximum power output line.

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As before maximum power output is achieved when the combination of output voltage and output current lines along this line, as shown in the examples.

Autoranging: FlexPower

Elementary power supplies often operate in a single range only. Single-range power supplies deliver maximum power Pmax only at the maximum rated voltage Vmax and current I max. The left chart in the figure below shows the output characteristics of a single-range power supply. Multi-range power supplies have wider output voltage and current ranges. The middle chart shows a dual-range power supply. These power supplies generate a much higher Vmax or Imax within the same maximum power limit as a single-range power supply. Both voltage and current outputs have two operating ranges within the same Pmax power envelope. Autorange power supplies have an infinite number of ranges. Rohde & Schwarz autorange power supplies use the FlexPower technology (see right chart in the figure on next page).

The Rohde & Schwarz FlexPower feature makes it possible to test a wide range of product families with a single power supply. This substantially extends flexibility, saves space and simplifies the test setup. FlexPower power supplies are also great when you need high voltage and current, but not a high output power. Using a single-range power supply to meet this need is much more expensive than using a FlexPower power supply.

Maximum power of a DC power supply in a voltage versus current representation

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Summary

The primary specifications for benchtop DC power supplies are maximum voltage, maximum current, and maximum power.
Maximum output power is usually less than the product of the maximum voltage and maximum current.
Derating curves are a graphical way of showing which voltage and current combinations are supported. Allowable combinations lie on or below this line.
Many derating curves also contain a maximum power output curve (or line) for determining which voltage and current combinations can be used to produce maximum output power.
Derating curves are useful when determining if a power supply can meet voltage, current, and power requirements.

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