Friday, September 18, 2015

Buying guide for computer power supply unit (PSU)

Time flies. My Dell Studio XPS 8100 is 5 years old now. It is still working fine, except of some occasional auto restarts during cold boot up.

I decided to replace its stock Chicony CPB09-001A 350 Watt power supply (PSU) as an attempt to fix this auto restart problem. So far, this problem doesn't occur again after I replaced the PSU.

After all, the PSU is a very important component of the computer that converts the AC current from wall socket into DC current of various voltage and distribute them to power up all the electronic parts of the computer. A faulty PSU might fry up the motherboard, introduce bad sectors and/or damage the harddisk, introduce error to memory, burn up the graphics display card, etc.

Not all branded PC can swap in with power supply of other brand, because of proprietary cabling or setting. Luckily, this Dell Studio XPS 8100 uses a standard ATX power supply, and therefore can fit in with another standard ATX power supply, regardless of the brand.

I looked for a low cost but reliable PSU, so that the computer can continue to serve me for another 3 years or so.

Initially I wanted to buy the Corsair VS450, but there was no stock in the shop I went to. I ended up purchased a Cooler Master B500 ver.2 to replace the stock PSU in my desktop.

I have made some study on how to choose a suitable PSU for my computer before I went to the shop, and would like to share the knowledge here with you.

Choosing Suitable PSU Wattage

The first thing to consider for a PSU is how many Watt of PSU is suitable for the PC. Choosing a lower wattage PSU increases the risk of system to become unstable and louder noise from PSU. Choosing a higher wattage PSU decreases efficiency at lighter loads, and the price is higher too.

You can refer to my article about calculating the recommended power supply wattage for desktop computer. However, this time, I used another website at to calculate for the wattage recommendation.

Typically, the CPU and the graphics display card (with GPU on it) are the 2 components in the PC that consume the most power, and high end GPU can consume much more power than the CPU.

For normal PC with normal CPU and one normal graphics display card, a high efficiency power supply of 400-500 watt should be sufficient. You might need more wattage if you have high end GPU, or multiple graphics display cards in the same PC.

PSU Efficiency and Certification

What is the efficiency of power supply? Well, when the PSU converts the AC current to DC current, certain portion of the electric will be lost in the form of heat.

If a power supply is 80% efficient, it can convert 100W of AC current into 80W DC, dissipating the remaining  20W in heat.

Therefore, a high efficiency power supply will waste less electric in the process of converting the AC to DC, produces less heat, and requires less cooling fan speed (and hence runs more silently).

There is a certification body called 80 Plus that will test and confirm if a model of PSU always has an efficiency of at least 80%.

There are several certification grades, from just 80 Plus, to bronze, silver, gold, platinum, and titanium. The higher the grade, the more efficiency is required, and normally the more expensive will be the PSU.

The Cooler Master B500 ver.2 is 80 Plus certified. You can click here for its 80 Plus verification and testing report.

Normally, the PSU has the highest efficiency when it is loaded with around 50% of its capacity. It has lower efficiency at lower load (eg. when the computer is at idle state) or higher load (eg. when all the CPU cores, GPU and memory are at high utilization state).

The 80 Plus test result of Cooler Master B500 ver.2 shows its efficiency rate as below:
  • 20% load - 82.75% efficiency, power factor of 0.97
  • 50% load - 84.55% efficiency, power factor of 0.99
  • 100% load - 81.51% efficiency, power factor of 0.99
You can find the list of all other brands and models of PSU that are 80 Plus certified in their website.

Beside 80 Plus, this Cooler Master B500 ver.2 is also ecoPSU certified. You can click here for its ecoPSU  Level 85 verification and testing report.

The test result with ecoPSU shows even better efficiency than 80 Plus.

Note that both the 80 Plus and ecoPSU also define the minimum power factor requirement in their certification. What is power factor then?

Power Factor Correction

The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit.

In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.

The PSU has power factor correction (PFC) circuits to increase its power factor. The PFC circuits type is either passive, active, or dynamic. Low grade PSU uses passive PFC, better one uses active PFC, and dynamic PFC is just too expensive for use in computer PSU.

Therefore, "Active PFC" is probably one of the keyword you search for when considering a PSU. The Cooler Master B500 ver.2 uses active PFC circuits.

The +12v Rail

The Cooler Master B500 ver.2 has a single +12v rail with maximum rating of 38A, 456W. This is the most important rail that supply electric to the high power consumption components of the computer, including the CPU and GPU.

Some PSU has dual or multiple +12v rails to split out the current, especially those PSU with high +12v rail maximum rating of above 40A, so that you can use one rail for CPU, and another for GPU, to balance up the load on the +12v rails.

The Cooler Master B-series of PSU used to have dual +12v rails, but version 2 has switched to single rail design. You can click here to find out the reason why Cooler Master made the change.

Modular vs Non-modular

The Cooler Master B500 ver.2 is non-modular. All its cable rails are attached with the PSU and cannot be detached from its outlet. A PSU with modular cabling system provides a detachable cabling system, offering the ability to remove unused connections, which can reduce clutter and improve air flow in the computer.

Modularity isn't my main concern, as the chassis of Dell Studio XPS 8100 is pretty spacious.

Lifespan and MTBF

The life span of a PSU is usually specified in mean time between failures (MTBF). The Cooler Master B500 ver.2 has an MTBF of 100,000 hours.

This should mean that the PSU has a probability of more than 75% to survive more than 3 years (and therefore has about 25% of chance to fail within 3 years). Also, it has around 25% probability to last as long as 100,000 hours.

Protection Circuits

Modern PSU has protection circuits to prevent the computer components such as the motherboard and harddisk from being fried due to excessive current drawn, or overheat, etc.

These are the keywords you can look for a PSU protection circuits:
  • SCP - short circuit protection
  • OPP - overpower/overload protection
  • OCP - overcurrent protection
  • OTP - overtemperature protection
  • OVP - overvoltage protection
  • UVP - undervoltage protection
The Cooler Master B500 ver.2 has SCP, OPP, OCP, OTP and OVP. Its manufacturer doesn't mention about UVP for it.


The Cooler Master B500 ver.2 has the following connectors, which are sufficient for my Dell Studio XPS 8100 PC.


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