Tips on Buying Power Supply

Ordinary people without knowledge about power supply may find difficult on how to select the right power supply for their application. In this article I will share the tips on buying power supply. Most of the time ordinary consumers only consider the basic requirements such as output power, output voltage and input voltage. There are also other people who are more particular in the price rather than the feature of the power supply. Below are the very important guides on buying power supply so that you will be guided. These are the top things to consider on buying power supply, of course there are some features that nice to have but in this article I will just share to you the basic guides on buying power supply.

Here are the Important Tips on Buying Power Supply

1. Allocate Money

The first thing to consider is your budget. Table 1 below is the average price of a power supply within the given power. Platinum, Gold and Bronze are power supply classification in terms of efficiency level by 80 plus.

Tips on buying power supply

Table 2 is the efficiency level classification for basic, bronze, silver, gold, platinum and titanium. By the way, in table 1, silver, basic and titanium prices are not included. Silver average price is in between bronze and gold. Basic 80 plus is a bit cheaper than bronze while titanium is the most expensive to all.

Tips on buying power supply

2. Know Your Load

You need to estimate the total wattage of your load. Do not buy a power supply which is rated 1000 W but your load is only 100 W. It is a waste of money and also you cannot maximize the efficiency. Do not also buy a power supply with a power rating just the same with your load e.g. 90-100% load utilization. Although by design there should be no problem because power supply engineers are designing their product above full load for margin. However, you cannot maximize the efficiency of your power supply. Power supply efficiency is at maximum at 50-70% load.

Tips on buying power supply

Figure 1 is an actual efficiency data of a medium power PSU. The efficiency at light load is low. The efficiency at higher loads above 70% is also low. The best load to operate the power supply is at 50-70% because the efficiency is at maximum.

So if an advertisement says that a power supply has an efficiency of 94%, this is only true at a single point and most likely at the half load.

3. Select the Highest Efficiency at the Same Price Level

Supposing your budget is 250 USD. Select the power supply with the highest efficiency at this particular price. To maximize this efficiency, operate the power supply to the load with the highest efficiency as shown in Figure 1.

To appreciate the efficiency factor, let us consider below example:

PSU 1: 800W, Efficiency – 85%

PSU 2: 800W, Efficiency – 94%

PSU 1 Total Power Consumption at Full Load


PSU 2 Total Power Consumption at Full Load


For PSU 1, the total power charged to you by the electric company is 941.176W but the actual you are consuming is only 800W. 141.176W is an additional loss due to low efficiency. For PSU 2, the additional loss is only 51.064W. A lower additional loss corresponds to a savings in monthly electric bill.

4. Input and Output Specifications

Consider a power supply with a wide input range e.g. 100-240V. This is very important to avoid abnormalities during line transients. Consider the input frequency range suited for your application. If you are going to use a DC input, buy a power supply with a DC input voltage capability (e.g. 127-300VDC).

Tips on Buying Power Supply
Figure 2

Select a power supply with the same output voltage with your requirement. If you want to use the power supply for different loads with closer input voltages, you can select a power supply with a trimmable output. Some commercial power supplies support output trimming of +/-15%. If your application is PC, you should consider a PC suitable power supply. Usually you should select a multiple output power supply. Consider also the output connectors or terminals. Select the one which is more convenient to your end.

5. Output Connectors/Terminals Configuration

Tips on Buying Power Supply
Figure 3

You need to consider this also. Select the best suitable for your application. For PC for example, you must consider Figure 3. For other applications (multi-purpose), you may consider the terminal configuration same with Figure 4. With Figure 4, you can easily connect a wire on the screw terminal going to your load.

Tips on Buying Power Supply
Figure 4

For laptop uses, you must consider this one

Tips on Buying Power Supply
Figure 5

6. Select Power Supply with Safety Certificates

Tips on Buying Power Supply
Figure 6

This is very important thing to consider. A power supply without safety certificate may cause fire and electric shock. Safety agencies control the creapage and clearance between power devices. Inadequate creepage and clearance may cause electric arcing that might trigger fire incident.

Power supply without safety certificate may have higher leakage or touch current. What is a touch current? This is the current that will flow to your body when you touch the chassis or enclosure of the power supply. A poorly grounded (earth connection) power supply usually has higher touch current in which you can really feel. Table 3 below lists the human body perceptions with respect to a particular current level.

Tips on Buying Power Supply

Safety regulated power supply has touch current of less than the threshold of perception which is very safe to human body. On the other hand non-safety regulated may have a touch current higher than the threshold of pain.

Some safety standards and approvals:

  1. ES60601-1 – Safety of medical electric equipment
  2. CSA-C22.2 No.601-1 M90 – Medical equipment
  3. European Community Safety investigated and marketed by TUV or VDE to EN60601-1 latest amendment with risk management file
  4. UL60950-1 – Safety of information technology equipment including electrical business equipment
  5. CSA C22.2 No. 60950-1 – Safety of information technology equipment including electrical business equipment
  6. European Community Safety investigated and marketed by TUV or VDE to EN60950-1
  7. CE Mark LVD
  8. CCC or CQC

7. With Fan or Without Fan

Power supply with a fan as a cooling system is noisy. If the fan noise can disturb you, you may consider a convection cooled power supply. A convection rated power supply may be expensive than with force air cooling but it is relatively quiet since it has no fan.

Tips on Buying Power Supply
Figure 7

Example in the above power supply features, you can operate the PSU without turning on the fan continuously for 400W. Look for the same specification that matches your power requirement.

8. Consider EMC Compliance

Tips on Buying Power Supply
Figure 8

Power supply is composed of power devices which are switched on and off. The on/off action radiates and conducts noise that will interfere with your other devices or appliances. It is important that you will select a power supply with EMC certificates to avoid interference that will cause noise to other systems. A non-EMC compliant power supply will make your TV reception blurry; can make your PC monitor to flicker and many more.

When you walked in a rough area and then you touch the exposed part of the power supply such as connector or programming header, the electrostatic charged build up on your body will be discharged to the power supply and this will damage it if there is no protection ready for this. For an EMC compliant power supply you can assure that it will not damage because this is regulated with EN 61000-4-2, ESD up to 4 kV contact and 8kV discharge. Moreover, an EMC compliant power supply can withstand electrostatic discharge, flicker, dips and interrupts and surge.

Some EMC regulating bodies are FCC, CISPR 22, EN55022, IEC60601-1-2 etc.

9. Select a Power Supply with Longer Holdup Time

A power supply with a longer holdup time will not turn off with fast line sag or under voltage. When you are living in the areas where there are dominant reactive loads, the line voltage may dip in a fast manner when these loads are turned on. The dipping might happen at few milli seconds but if the power supply you buy has no holdup time capability, if you use this in your PC, every time the line sags your PC will turn off and you lost your unsaved work. This is very disgusting.

Another importance of a longer holdup is that when you turn off the power supply intentionally or by accident or fault, the system can still save your work or data within the holdup period. This is applicable to a smart system though.  A common holdup time is 20msec.

10. Ripple

Select a power supply with a small ripple. Higher ripple will cause nuisance to your delicate or sensitive system or even destroy them. An ideal ripple should only be a maximum of 1% of the output voltage of the power supply. So for 12V output, the ripple must only be 120mV peak to peak maximum.

11. Select Power Supply with Protections

Tips on Buying Power Supply
Figure 9

Common protections to consider are over current, short circuit protection, over voltage protection and over temperature protection. Without over current protection the power supply will be damaged when you accidentally operated in higher load than specified. This is a common protection though.

In the event of accidentally shorting of the power supply output terminals, the power supply must not get damaged and should back to normal operation after the short has been removed. This is the purpose of the short circuit protection.

Over voltage protection is very important to have on a power supply. This protection will shuts down the power supply when it detects a higher output voltage and the system which use the power supply will not be damaged.

Since a power supply is comprised of power devices, the heat generated is high. In case of over temperature the power supply must turn on itself to cool down. This is the purpose of the over temperature protection.

12. Power Factor

This is a nice to have. Consider a power supply with a higher power factor. In electrical circuits power factor is defined as the phase difference between the voltage and current. For resistive device or circuit the power factor is one which means the voltage and current is in phase. For reactive circuits on the other hand the power factor is less than unity. When the power factor of a power supply is low the current drawn from the transmission line is high. This higher current corresponds to a higher transmission loss and usually electric companies charge this to their consumers. That is why I included this as one of the guides on buying power supply. For household consumption, most likely you will not get questioned for using devices with lower factor but big establishments or companies will.

Power factor is the principle used by the advertised power savers. These advertisements are exaggerated and can be considered a scam because they are claiming that the consumer will directly experience lower power consumption using the product which is not the case. Read this article for more information on power savers.

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