by Kevin Cavell, Product Manager for Power Products, Agilent Technologies, Inc.
Before powering up your device for the first time, read your power supply’s data sheet! Going through the top specifications, knowing as much as you can about your device, will help you choose the right power supply.
When it comes time to power up a design for the first time, the most popular choice is a programmable power supply. There are many vendors to choose from, as well as many different product lines from each vendor. To sort through which one might be the best for your design, you will have to turn to the power supply’s data sheet.
Within the data sheet you will find detailed information about how the power supply will perform under different conditions. But before diving into the details, the most important thing is to know as much as you can about your design.
No standard for data sheets
Unfortunately, there is no standard established for power supply data sheets. Descriptions of the specification, as well as the specifications themselves, are determined by the manufacturer and will vary from one manufacturer to the next. It is also at the manufacturer’s discretion whether or not a particular characteristic is even specified. Some vendors specify more than others. Comparing two data sheets, Power Supply A and Power Supply B, you will see the differences between the power supplies.
DC output ratings
The most basic information expected to be seen on a dc power supply’s data sheet should tell you whether it will meet your power needs. What voltage, current, and power requirements does your device have? If your device’s needs do not fall within these parameters, you will need to continue your power supply search.
Auto-ranging power: When V x I > P
In most cases, a quick check of the voltage, current, and power in the dc output ratings section is enough before moving on to the other specifications. However, you should quickly check to see if the product of the maximum voltage and current ratings are equal to the power. If it is equal, then the power supply has a rectangular output characteristic. If it is greater, than the power supply has an auto-ranging output characteristic. Auto-ranging is outside the scope of this article. Figure A and B show a graphical representation of the two types of outputs.
Output noise/Output ripple and noise/Periodic and random deviations (PARD)
The output noise specification, (Table 2) is labeled in a few different ways on a data sheet depending on the vendor. Output noise refers to the deviations of the dc output voltage from its average value over a specified bandwidth. It is typically measured in RMS (Root Mean Square) and peak-to-peak (p-p). It is important to note that there are two types of noise to consider: normal mode and common mode. If the specification does not explicitly state that it is common mode noise, then it is a specification of normal mode noise. Normal mode noise is the voltage deviation on the positive output terminal with respect to the negative output terminal.
The most important noise specification is the peak-to-peak voltage noise. If the peak-to-peak voltage noise specification indicates that the deviations are large, they could damage or destroy a sensitive device, such as a voltage-controlled oscillator (VCO). The RMS measurement is not an ideal representation of the noise and should not be considered as a good representation of a power supply’s noise performance. Fairly high output noise spikes of short duration could be present despite a low RMS noise specification since they do not appreciably increase the RMS value. Be alert to a power supply specification sheet that only specifies the RMS noise value. Also make sure you note the bandwidth over which the measurement was made. It should be in the range of 20 Hz to 20 MHz. Poor noise specifications can be hidden by changing the bandwidth over which is it measured.
Common mode noise
Common mode noise, (Table 3) is the deviation that appears on both the positive and negative output terminals of the power supply with respect to ground. It is not usually specified, but if it is high, it may be the unknown culprit of many problems. Since the current flowing from one of the output terminals to ground is easiest to measure, the mode noise is specified in amperes.
Load regulation/Load Effect
Load effect, or load regulation, (Table 4)describes how much of the dc output voltage will change from its steady state (programmed) value due to a change in the load from open circuit to a resistance value that yields maximum rated output current, or vice versa. A small number indicates the power supply will not deviate too much from its programmed value when large load changes occur. The power supply will deviate beyond the load effect specification for a short period of time. But, under steady state conditions, it will not deviate more than the indicated load effect specification.
Load Transient Recovery Time/Transient Response Time
If your device requires fast pulses of current to power, such as a mobile phone, (Table 5) then the power supply powering the design will be subjected to large load transients. When this occurs, the voltage may deviate greatly from the programmed value for a period of time. For the amount of time it takes the power supply to recover after a fast load transient, refer to the Load Transient Recovery Time specification. This specification indicates how quickly the voltage would return to within a settling band around the programmed value.
Source Effect, Line Effect, Source Regulation, Line Regulation
A dc power supply is essentially an ac to dc converter (Table 6). It takes the ac voltage from the plug in the wall and converts it to the programmed dc value. For this specification, “Source” and “Line” both refer to where the power supply derives its power, the ac line voltage. Source effect is the change in the steady state value of the dc output voltage due to a change in the ac input voltage over a specified range, which is usually the rated low-line to high-line and vice versa. For Power Supply A on a 120 Vac line, low-line is 90 Vac and high-line is 132 Vac.
If you use the power supply in an environment that has very stable ac line voltage, then this specification may not matter to you. However, if your ac line voltage is not “clean” and fluctuates, this specification is important because it lets you know how much the output might vary due to these fluctuations.
Perhaps you set the output of a programmable power supply to a specific voltage of 10.000 V; what is the expected output? Will it be 10.000 V or 9.900 V? The programming accuracy specification is where to look for this information (Table 7). This specification is sometimes shown as a percentage of full-scale voltage, such as “0.1% of Vmax” (Power Supply A with LAN interface), or as a percentage and an offset, such as “0.06% + 25 mV” (N6756A). The latter gives a better representation of actual results throughout the entire voltage range. Both represent a “±” accuracy band. However, the “±” is typically not shown.
We can see that the “percentage and an offset” specification is a much better representation of accuracy. It also shows that the vendor has spent extra time verifying the accuracy across the entire range of output voltages. At 60 V, both power supplies, yield an error band of 0.1%. However, the comparison is much different at low voltages, such a 0.5 V. Here the former yields a 12% error, while the latter yields a 5% error.
One final thing to look for is whether or not the programming accuracy is dependent on the interface being used, such as GPIB, USB, or LAN (Table 8). Many vendors will have different specifications based on the interface. For example, Power Supply A has a programming accuracy of 0.1% of Vmax when using the Ethernet interface and 0.2% of Vmax when using the GPIB interface. Power Supply B’s specifications are not affected by the interface used.
Measurement accuracy and Readback accuracy
If the power supply you are evaluating has a built-in measurement system, then there will be a measurement accuracy specification (Table 9). Similar to programming accuracy above, this can be represented as a percentage of full-scale or as a percentage and an offset. Again, the latter shows a better representation of accuracy throughout the power supply’s entire range. Also similar to programming accuracy, the measurement accuracy may depend on the interface being used to acquire the measurement. So, make sure you understand which interface you will use prior to making a final determination of the specification.
In our example, Power Supply A’s measurement accuracy is specified as being the same as the programming accuracy. Power Supply B’s measurement accuracy is 0.05% + 25 mV, so slightly better than its programming accuracy. Each of these examples has only one measurement accuracy range. There are some precision power supplies available that have multiple measurement ranges. The accuracy in these cases will depend on which measurement range the power supply is using to make the measurement. Measurement accuracy bands can be calculated similar to the programming accuracy bands shown in Table 7.
There are many other power supply specifications that can be discussed. However, the ones discussed here are the top specifications that should be considered when you power an electronic device with a programmable dc power supply. Remember that it is most important to start by knowing your design and how you plan on powering and testing it. What voltage, current, and power will be required? Is your device’s power consumption dynamic? Will you make measurements with the power supply? Once you know the answers to these questions you will be ready to start evaluate a power supply’s data sheet and be on your way to making the right choice for powering your design.