Path: EDN Asia >> Design Centre >> Power/Smart Energy >> Grasping linear power supply's data sheet (Part 2)
Power/Smart Energy Share print

Grasping linear power supply's data sheet (Part 2)

11 Jan 2013  | Robert Green

Share this page with your friends

In Part 1 of this article, we discussed an overview of several specifications of DC power supplies, including those related to accuracy and resolution, and to stability. Although this article focuses on DC power supplies, the output of these power supplies is not perfect DC. Some AC is to be expected on the output. For some applications, high AC on the output can produce unexpected circuit behaviour, so it helps to know the amplitude of the residual AC. In addition to AC noise, it may be useful to know the transient response of the power supply to changes in load and settings. For example, in automated testing, it is important to know when the power supply has settled in response to a change in settings. Therefore, Part 2 will focus on specifications related to a DC power supply's AC characteristics, as well as a variety of other performance considerations.

Specs related to AC characteristics
Spurious AC components on the output of a DC supply are known as ripple and noise, or periodic and random deviation (PARD). These terms are often used interchangeably. The term ripple refers to periodic AC on the output. When viewed in the frequency domain, ripple shows up as spurious responses. Unlike ripple, which is periodic, noise is random. Noise covers a broad spectrum, and, when viewed in the frequency domain, it manifests itself as an increase in the baseline (figures 1 and 2). Given that ripple and noise are usually lumped together and cannot be easily separated, I'll refer to the combined effects as PARD from this point forward.

Figure 1: This simplified drawing shows the concepts of periodic (ripple) and random (noise) distortion.

Figure 2: This noise measurement was taken with a 1X probe and a bandwidth of approximately 7MHz while the power supply was delivering full rated current.

PARD specifications must be defined with a bandwidth and should be specified for both current and voltage. Current PARD is relevant when using a power supply in constant current (CC) mode, and it is often specified as an RMS value. Because the shape of PARD is indeterminate, voltage PARD is usually expressed both as a root mean square voltage, which can provide a sense of the noise power, and also as a peak-to-peak voltage, which may be relevant when driving high impedance loads.

Because of the bandwidth consideration, PARD specifications are heavily dependent upon the measurement technique used to test them. The manufacturer's performance verification procedures usually include the procedure for checking PARD. It is important to consider the whole signal path used to verify ripple and noise specifications. For example, using a high bandwidth oscilloscope with a low bandwidth probe can make a supply's specifications seem more impressive than they actually are.

Another set of AC characteristics describes how quickly a power supply can respond to changes. Transient response specifications indicate how quickly the output settles to a stable DC value after a change in load or settings. Most power supplies have a large capacitance in parallel with their outputs to help deliver clean, steady DC. When this capacitance is placed in parallel with the load resistance, a time constant results and the size of the time constant varies with the load impedance. Because of the heavy dependence on the resistance of the load, response to setting changes must be specified for a specific load. It is common to see specifications for open circuits, short circuits, or specific resistance values.

Transient response can be tested by applying significant step changes both to the load impedance and to power supply settings, then measuring the time required to settle to a final value. The voltage transient response for all Keithley Series 2200 power supplies, for instance, is specified for three conditions: increasing load, increasing setting, and decreasing setting.

Table: Voltage transient response specifications for a Keithley 2200-32-3 power supply.


1 • 2 Next Page Last Page


Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.


Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Hardware development kit targets wearables, IoT
The LinkIt ONE is based on MediaTek Aster (MT2502) and integrates GSM, GPRS, Bluetooth EDR/BLE, Wi-Fi, GPS, SD Card, and MP3/AAC/AMR Audio features.

LTE module aims at automotive comm devices
SMIC activates 38nm NAND process technology


News | Products | Design Features | Regional Roundup | Tech Impact