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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.

Protection Features And Transition Time

Check out our PSUs 101 article to learn more about PSU protection features.  

Our protection features evaluation methodology is described in detail here.

Protection Features
OCP12V1/2/3: >41A 5V: 28A (140%) 3.3V: 28.7A (143.5%) 5VSB: 10A (303.3%), 4.655V
OPP608.094W (121.6%)
OTP✓ (105°C @ Secondary Side)
SCP12V: ✓ 5V: ✓ 3.3V: ✓ 5VSB: ✓ -12V: ✓
PWR_OKOperates properly
SIPSurge: MOV Inrush: NTC Thermistor & Bypass Relay

The over-power protection feature is set at a normal level, while OCP is quite high on all rails (especially at 5VSB where it should be set much lower). This is the first time we've seen a 10A 5VSB rail.

Although FSP doesn't mention it, OTP is present and properly configured. This protection is essential, especially in server-oriented PSUs with high power density. Moreover, there is fan failure protection, which we confirmed works correctly. In case the fan breaks or is simply removed, the PSU shuts down.

As expected, there is short-circuit protection on all rails, and the power-good signal is accurate, since it drops before the rails go out of spec.

Since this unit consists of two power modules, one of which fully takes over in case the other breaks down, we had to run some tests to check the transition time and how smooth this transition actually is on the critical +12V output. This proved to be a tricky task. However, our scope has an advanced set of trigger options that we fully utilized. For the following tests, we applied full load (500W) to the PSU.

This is the moment that we cut power to the first module. As you can see in the screenshot, the total time between dropping power and the second module stabilizing the +12V rail is 70.2ms. In order to achieve a smoother transition, FSP used large bulk caps. In this case, the long hold-up time serves up a tremendous advantage.

The peak voltage overshoot is 40.6mV, while the voltage drop reaches 203.7mV. This means that the +12V rail stays within spec during the transition period.

We notice the first effect on the +12V rail's output 26.6ms after removing power from the first module. This is very close to the power module's hold-up time, and we believe this is the moment that the second module kicks in.

The peak of the voltage overshoot happens 39ms after we remove power from the first module.

The voltage drop starts 48.8ms after the power cut. Naturally, the second power module needs some time to adjust its operation to the transient load.

The peak of the voltage drop is 53.8ms after removing power to one of the modules.

The transient recovery time is 21.4ms. This is normal, and as you can see from the scope screenshot above, ripple at +12V slightly increases afterwards because the module has to deliver 500W on its own now instead of roughly half that amount.

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