Zalman ZM300A-APF PSU

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June 12, 2002 -- by Mike Chin

Product: Zalman ZM300A-APF
Manufacturer: Zalman Tech Co. Ltd.
Supplier: Zalman Tech Co. Ltd.

Zalman of Korea needs little introduction to quiet PC enthusiasts. They burst on the scene with their amazingly ingenious "flower" or "fan" style heatsink some 18 months ago and have steadily expanded their range of products for quiet computing. Much of Zalman's focus has been on heatsinks based on variations of the original "flower" concept, for which they been awarded an impressive total of 22 patents thus far.

When their ST300BLP 300W power supply came on the market, it was hailed by many as a welcome addition to the growing range of quiet PC products. However, the response was not unanimous; some reported that while quieter than most PSUs, the ST300BLP was still not quiet enough.

Knowing that the ST300BLP has a thermistor-controlled fan, we suspected that it might feature a quieter fan than other PSU, but perhaps it still spins too quickly, too soon, like the vast majority of thermistor-controlled PSU fans. Silicon Acoustics, a great web retail shop for those seeking quiet PC products, kindly sent me a sample of the Zalman ST300BLP power supply to review some weeks ago.

The ST300BLP PSU was a bit of a disappointment. It is quiet compared to many PSU, but not quiet enough by Silent PC Review standards to truly justify being branded as a quiet product. While trying to write an honest review that would not discourage an innovative company that's making a difference in the noisy PC world, we heard about a new and improved PSU from Zalman. Direct contact with Zalman resulted in the prompt delivery of a review sample of their new ZM300A-APF. (Whew! Saved by product development!)


The suggested retail price of the ZM300A-APF is $74.95, the same as the earlier ST300BLP.

What do you get for your money? Firstly, a nice retail display box announcing their new slogan across the top: The Quiet Cooling Solution. Inside, there is a nice fold-out manual, the ZM300A-APF PSU itself in bubblewrap, and a handy little adapter that splits a Molex power connector into four 3-pin fan connectors, two white ones for 5V, and two black ones for 12V. They call it a Multi-Connector (ZM-MC1). It's a nice touch that will appeal to quiet PC enthusiasts who like to run 12V fans at 5V for minimal noise. With this handy little adapter, there's no need to hack around with wires and connectors and soldering irons. There's hardly even a need for something like a fan bus.

A welcome change is the wire fan grill instead of the stamped-out fan exhaust slots from the ST300BLP. Zalman presumably responded to market feedback, something worthy of praise in itself. The new PSU has a cover that features slots for air intake above the CPU area (in a tower case). This is a big change from their original PSU, which has no vent openings on the cover.

One other obvious change is the absence of an AC input voltage switch. This unit, like the Seasonic tested earlier, automatically adjusts for any AC voltage from 100V to 240V. The manual attributes this feature to its Active PFC circuitry. The original ST300BLP featured passive PFC.


Increasingly, switched mode power supplies (SMPS) are being designed with an active power factor correction (PFC) input stage. This is mainly due to the introduction of regulations aimed at restricting the harmonic content of the load current drawn from power lines. However, both the user and the power company benefit from PFC, so it just makes good sense.

Power Factor Correction (PFC) can be defined as the reduction of the harmonic content, and/or the aligning of the phase angle of incoming current so that it is in phase with the line voltage. Mathematically, Power Factor (PF) is equal to Real Power/Apparent Power. The basic concept behind PFC is to make the input look as much like a resistor as possible. Resistors have a power factor of 1 (unity). This is ideal, because it allows the power distribution system to operate at its maximum efficiency.

Non-PFC power supplies use a capacitive filter at the AC input. This results in rectification of the AC line, causes high peak currents at the crests of the AC voltage. These peak currents lead to excessive voltage drops in the wiring and imbalance problems in the three-phase power delivery system. This means that the full energy potential of the AC line is not utilized. Nonlinear peak currents also distort output voltage and create harmonic frequencies. There is now an international standard for controlling harmonics (IEC100-3-2) and PFC is mandatory for home appliances consuming 70W or more power in EU nations as of January, 2001 PFC circuits are classified into two types: active and passive.

Passive PFC uses passive elements such as a ferrite core inductor on the input source to create a countering reactance. While easily applied to the existing power circuitry without much modification, the power factor is low (60 - 80%), the AC input must be chosen (115VAC / 230VAC), and the harmonics produced from the difference between the capacitance and the inductance are hard to control. Significant electromagnetic noise can result with an 115VAC input source

Active PFC uses switching regulator technology with active elements such as IC, FET and diodes, to create a PFC circuit This circuit has a theoretical power factor of over 95%, reduces total harmonics noticeably, and automatically adjusts for AC input voltage. However, it requires a complex EMI filter and an input source circuit, and is more costly to build.

OK. You're probably asking, What is Active Power Factor Correction? The Seasonic PSU featured it, too, but it was never explained. How does Active PFC differ from Passive? If these questions come to mind, please read the sidebar on the right. This explanation is culled from Zalman and Seasonic's literature, as well as this PDF document at General Semiconductor's web site. BTW, here's the short and simple on PFC: The basic concept behind PFC is to make the input look as much like a resistor as possible. (A resistor is the simplest electrical load.)

The fan used in the ZM300A-APF is model MGA8012HB by Protechnic Electric, a brand I've not seen before. Judging by its 0.24A current rating, it should be similar in RPM and CFM to the fans found in most PC power supplies. I believe the B at the end of the model designation stands for ball bearing, as opposed to sleeve. At startup, it is quiet.

A quick listen to the fan held in free air outside the PSU revealed that it has a smooth quality when driven at 12V. As the voltage is dropped, overall noise comes down steadily, with little of the bearing chatter that prevents some fans from achieving quiet low speed performance.

Under the cover, the most notable aspect of this PSU are its two large heatsinks. With a "L" cross-section, raised ridges and numerous circular holes, these heatsinks are just about as large in surface area as any I've seen in a computer power supply.

Note the black colored leads on the top of one of the heatsinks. It leads to a thermistor hot-glued to the heatsink.

Yes, like the original Zalman ST300BLP, the Seasonic, the Enermax and numerous other PSUs, the Zalman ZM300A-APF fan speed is thermistor controlled.


As noted in the review of the Seasonic SS300-FS, most thermistor fan controlled PSUs usually start out fairly quiet, but the fan(s) quickly speed(s) up to become quite noisy, even with a minimal load and modest environmental temperature. The Seasonic was the first PSU I'd seen that does not exhibit this behavior. The Zalman ZM300A-APF is the second. True to the manufacturer's claims for its CNPS (Computer Noise Prevention System) products, this PSU runs very quietly.

Zalman states that its CNPS products cut traditional computer system noise of 30dB or higher to 20dB or lower, which is below what a normal user can usually notice. However, no information about noise measurement standards is given, so the description is of limited usefulness. For the ZM300A-APF, a graph plotting the speed of the PSU fan and the temperature of its heatsinks is provided in the manual. It was scanned and made prettier for this show-and-tell:

Don't be surprised if it looks familiar. Replace the horizontal scale with Watts, and the vertical scale with Fan Voltage, and you'd get a graph similar to that provided for the Seasonic PSU. The Zalman ZM300A-APF manual states,

When the temperature inside the power supply is 55° C or less, the cooling fan will operate in Quiet or Silent mode.

Referring to the graph, it appears Zalman's definition of Silent is when the fan is spinning at ~1350 RPM or less, while Quiet is when the fan is spinning under 1500 RPM.

Regardless of semantics, this graph suggests that Zalman has struck upon a similar approach to PSU cooling as Seasonic: Use a quiet fan that is capable of fairly high airflow, run it at minimal speed through low and medium loads, and allow it to speed up only when HS temperature reaches a certain minimum level. According to the above graph, that minimum is about 42-43° C, very similar to the Seasonic. For the vast majority of PC systems, the fan in this PSU should run at minimal speed (and noise) under most conditions.

If this approach to PSU thermistor fan control is a trend, it's one quiet PC enthusiasts should be pleased to see!


The published 75% efficiency at full power is noteworthy. In response to a direct inquiry, Zalman stated that at 122W and 120VAC, the efficiency is 72-73%. The small drop is normal for switch mode power supplies; they are most efficient at full power. These figures are impressively high, as most PSU efficiency is specified to be 65% to 70% at full power.

The efficiency rating tells us how much energy is wasted as heat by a PSU. At full power, the Zalman generates 100W of heat while delivering 300W of electrical power. A typical 65% efficiency PSU would generate 161.5W of heat delivering 300W. That is a huge difference in the amount of heat for a PC case. At the more realistic power level of 122W, the Zalman generates 50W of heat. Assuming that the efficiency of a typical PSU drops to 62%, it would generate 75W of heat. 25W is still a very significant amount of heat.

In practical terms, a less efficient PSU produces more heat, and thus makes all components run hotter. To maintain safe component temperatures with a less efficient PSU normally requires higher airflow in the case, which means more fan noise. To state the obvious, conversely, a higher efficiency PSU should allow for adequately cool components with lower airflow and lower fan noise.

The DC output specifications are consistent with those for most quality 300W PSU. (They are, in fact, identical to those for the Seasonic SS300FS.)

DC Output

Ripple & Noise

Output Load Current

Combined Power














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