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FSP Twins 500W Redundant PSU Review

The FSP Twins series combines the usability of a normal ATX PSU and the advanced features of a redundant server unit. The Twins 500W we're evaluating today addresses users that need an ultra-reliable PSU and are willing to pay for it.

Efficiency, Temperature, And Noise


Our efficiency testing procedure is detailed here.

Using results from the previous page, we plotted a chart showing the Twins 500W's efficiency at low loads, and loads from 10 to 110 percent of its maximum-rated capacity.

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Under light loads, the combined power loss of both modules leads to low efficiency. If there was an option to completely deactivate one of the modules instead of having both of them working in parallel, those figures would be significantly better.

The main advantage of having both modules active at the same time is improved efficiency under higher loads. They also endure less stress, since only a fraction of their maximum power output is delivered.

Efficiency At Low Loads

In the following tests, we measure the Twins 500W's efficiency at loads significantly lower than 10 percent of its maximum capacity (the lowest load the 80 PLUS standard measures). The loads we dialed were 20, 40, 60, and 80W. This is important for representing when a PC is idle, with power-saving features turned on.

Test #12V5V3.3V5VSBDC/AC (Watts)EfficiencyFan SpeedFan NoisePF/AC Volts
11.208A0.491A0.479A0.195A19.6447.926%7870 RPM50.6 dB(A)0.738
22.447A0.991A0.991A0.391A39.7764.635%7853 RPM50.8 dB(A)0.919
33.686A1.487A1.505A5.070A59.8571.806%8002 RPM52.2 dB(A)0.949
44.915A1.999A1.989A0.790A79.7676.071%8062 RPM52.2 dB(A)0.958

Our measured efficiency is disappointing under light loads. To make matters worse, the fans exceed 50 dB(A).

FSP Guardian Screenshots

You'll find screenshots of FSP's Guardian software below, which we captured under light loads.

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5VSB Efficiency

The ATX specification states that 5VSB standby supply efficiency should be as high as possible, recommending 50 percent or higher with 100mA of load, 60 percent or higher with 250mA of load, and 70 percent or higher with 1A or more of load.

We take four measurements: one each at 100, 250, and 1000mA, and one with the full load the 5VSB rail can handle. 

Test #5VSBDC/AC (Watts)EfficiencyPF/AC Volts
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Having both 5VSB circuits operating in parallel prevents high efficiency scores, as you can see in the table above.

Power Consumption In Idle And Standby

In the table below, you'll find the power consumption and voltage values of all rails (except -12V) when the PSU is idle (powered on, but without any load on its rails), and the power consumption when the PSU is in standby mode (without any load, at 5VSB).

Mode12V5V3.3V5VSBWattsPF/AC Volts
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Phantom power increases, and this PSU is way above the ErP Lot 6 2013 directive's maximum allowed 0.5W.

Fan RPM, Delta Temperature, And Output Noise

Our mixed noise testing is described in detail here.

The first chart below illustrates the cooling fan's speed (in RPM), and the delta between input and output temperature. The results were obtained at 36°C (96.8°F) to 45°C (113°F) ambient temperature.   

The next chart shows the cooling fan's speed (again, in RPM) and output noise. We measured acoustics from one meter away, inside a small, custom-made anechoic chamber with internals completely covered in sound-proofing material (be quiet! Noise Absorber kit). Background noise inside the chamber was below 18 dB(A) during testing, and the results were obtained with the PSU operating at 36°C (96.8°F) to 45°C (113°F) ambient temperature. 

The fans inside of the power modules generate a bearing noise, which is responsible for this strange-looking graph.

The following graph illustrates the fan output noise over the PSU's operating range. The same conditions of the above graph apply to our measurements, though the ambient temperature was between at 28°C (82.4°F) to 30°C (86°F).  

This definitely isn't a silent PSU, as you can see in the graph above. Even under low load on the +12V rail, fans in the modules spin very quickly once the load on the DC-DC converters exceeds a combined 100W. Fortunately, most real-world workloads won't stress the minor rails as much as we do.

The last graph illustrates the fan noise over the PSU's operating range. The same conditions of the above graph apply to our measurements. 

The fans spin quickly, even under light loads. This is necessary, given their small diameter, to provide sufficient airflow.

Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.