Zalman ZM770-XT Power Supply Review
By Gabriel Torres on December 15, 2009
Zalman ZM770-ZT is a 770 W power supply featuring modular cabling system and a heatpipe to cool down the components from the secondary. Let’s see if this is a good unit.
Traditionally power supplies from Zalman are manufactured by FSP, but this particular unit is manufactured by Enhance Electronics.
The power supply glows blue when turned on, thanks to four LED’s placed on the heatsinks.
Zalman ZM770-XT is a long power supply, being 7 1/8” (180 mm) deep, using a 140 mm fan on its bottom and active PFC circuit, of course.
The reviewed power supply features a modular cabling system. Three cables are permanently attached to the power supply (main motherboard cable, EPS12V/ATX12V and six-pin video card power cable) using thick 16 AWG wires and thick nylon sleevings. These cables are 18 7/8” (48 cm) long. The cables from the modular cabling system are flat using 18 AWG wires and measure 19 ¾” (50 cm) between the power supply and the first connector and 5 ¾” (14.5 cm) between connectors.
The cables included are:
ZM770-XT comes with an impressive number of SATA power connectors, but we don’t understand why Zalman only put two video card power connectors on this unit. Users looking for a power supply on this power range will certainly run SLI or CrossFire configurations, and this unit doesn’t provide direct support for two video cards that require two power connectors each (e.g., two GeForce GTX 260). The only way you have to install two video cards with two power connectors each is by installing adapters on the peripheral power plugs, which is far from being the ideal solution, especially when we are talking about a power supply that costs almost USD 200.
Now let’s take an in-depth look inside this power supply.
We decided to disassemble this power supply to see what it looks like inside, how it is designed, and what components are used. Please read our Anatomy of Switching Power Supplies tutorial to understand how a power supply works and to compare this power supply to others.
This page will be an overview, and then in the following pages we will discuss in detail the quality and ratings of the components used. The first thing we notice was how the connectors from the modular cabling system are installed to the main printed circuit board. Instead of using a bunch of wires like it usually happens on power supplies with this kind of system, it is soldered to it through some pins. This is a terrific solution, as it reduces the number of wires inside the unit and thus improves airflow.
As we have mentioned in other articles and reviews, the first place we look when opening a power supply for a hint about its quality, is its filtering stage. The recommended components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor), and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components, usually removing the MOV and the first coil.
This power supply is flawless on this stage, with two Y capacitors and two X capacitors more than the minimum required, plus an X capacitor after the rectifying bridge.
In the next page we will have a more detailed discussion about the components used in the Zalman ZM770-XT.
On this page we will take an in-depth look at the primary stage of Zalman ZM770-XT. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses two GBU1006 rectifying bridges connected in parallel in its primary, each one being able to deliver up to 10 A at 100° C. At 115 V this unit would be able to pull up to 2,300 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,840 W without burning these components. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.
On the active PFC circuit two SPW32N50C3 power MOSFET transistors are used, each one capable of delivering up to 32 A at 25° C or 20 A at 100° C in continuous mode (note the difference temperature makes), or up to 96 A in pulse mode at 25° C. These transistors present a resistance of 110 mΩ when turned on, a characteristic called RDS(on). This number indicates the amount of power that is wasted, so the lower this number the better, as less power will be wasted thus increasing efficiency.
This power supply uses two electrolytic capacitors to filter the output from the active PFC circuit. The use of more than one capacitor here has absolute nothing to do with the “quality” of the power supply, as laypersons may assume (including people without the proper background in electronics doing power supply reviews around the web). Instead of using one big capacitor, manufacturers may choose to use two or more smaller components that will give the same total capacitance, in order to better accommodate space on the printed circuit board, as two or more capacitors with small capacitance are physically smaller than one capacitor with the same total capacitance. Zalman ZM770-XT uses two 270 µF x 420 V capacitors in parallel; this is equivalent of one 540 µF x 420 V capacitor. These capacitors are Japanese, from Chemi-Con and labeled at 85° C.
In the switching section, two STP25NM50N power MOSFET transistors are used, using the traditional two-transistor forward design. Each one is capable of delivering up to 22 A at 25° C or 14 A at 100° C in continuous mode, or up to 88 A at 25° C in pulse mode, with an RDS(on) of 140 mΩ.
The two rounded components attached on the top of the heatsinks are two blue LED’s that glow when the unit is on.
Notice that Zalman painted the aluminum heatsinks from this power supply with a copper color to mimic the appearance from this material.
The primary is controlled by the popular CM6800 PWM/PFC combo integrated circuit.
Now let’s take a look at the secondary of this power supply.
This power supply has eight Schottky rectifiers attached to the secondary heatsink.
The maximum theoretical current each line can deliver is given by the formula I / (1 - D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Just as an exercise, we can assume a typical duty cycle of 30%.
The +12 V output is produced by four 40CPQ060 Schottky rectifiers connected in parallel, each one supporting up to 40 A (20 A per internal diode at 120° C, 0.49 V voltage drop). This gives us a maximum theoretical current of 114 A or 1,371 W.
The +5 V output is produced by two STPS60L45CW Schottky rectifiers connected in parallel, each one supporting up to 60 A (30 A per internal diode at 135° C, 0.50 V maximum voltage drop). This gives us a maximum theoretical current of 86 A or 429 W.
The +3.3 V output is produced by another two STPS60L45CW Schottky rectifiers, giving us a maximum theoretical current of 86 A or 283 W.
All these numbers are theoretical. The real amount of current/power each output can deliver is limited by other components, especially by the coils used on each output.
As mentioned earlier, the heatsinks are made of aluminum, even though its color reminds copper. The rounded component shown in Figure 11 is another LED that glows blue when the unit is on.
The outputs are monitored by a PS232 integrated circuit, which supports the following protections: over current (OCP), under voltage (UVP) and over voltage (OVP). Any other protection that this unit may have is implemented outside this integrated circuit.
Electrolytic capacitors from the secondary are from Teapo and labeled at 105° C.
In Figure 13, you can see the power supply label containing all the power specs.
This power supply has four virtual rails, distributed like this:
This distribution is also listed on the power supply label (not shown in Figure 13), which is great.
This distribution is perfect since it keeps the CPU (ATX12V/EPS12V connectors) and each video card power cable on separated rails.
Now let’s see if this power supply can really deliver 770 W.
We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology.
First we tested this power supply with five different load patterns, trying to pull around 20%, 40%, 60%, 80%, and 100% of its labeled maximum capacity (actual percentage used listed under “% Max Load”), watching how the reviewed unit behaved under each load. In the table below we list the load patterns we used and the results for each load.
If you add all the power listed for each test, you may find a different value than what is posted under “Total” below. Since each output can vary slightly (e.g., the +5 V output working at +5.10 V), the actual total amount of power being delivered is slightly different than the calculated value. On the “Total” row we are using the real amount of power being delivered, as measured by our load tester.
The +12V1 and +12V2 inputs listed below are the two +12 V independent inputs from our load tester. During this test the +12V1 input was connected to the power supply +12V3 and +12V4 rails while the +12V2 input was connected to the power supply +12V1 rail.
5.5 A (66 W)
11 A (132 W)
17 A (204 W)
22 A (264 W)
28 A (336 W)
5 A (60 W)
10.5 A (126 W)
16 A (192 W)
22 A (264 W)
27.5 A (330 W)
2 A (10 W)
4 A (20 W)
6 A (30 W)
8 A (40 W)
10 A (50 W)
2 A (6.6 W)
4 A (13.2 W)
6 A (19.8 W)
8 A (26.4 W)
10 A (33 W)
1 A (5 W)
1.5 A (7.5 W)
2 A (10 W)
2.5 A (12.5 W)
3 A (15 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
% Max Load
Ripple and Noise
Zalman ZM770-XT can really deliver its labeled wattage at high temperatures.
Efficiency was high when we pulled between 40% and 60% from its labeled wattage (i.e., between 308 W and 462 W), being between 84.8% and 85.2%. At light load (20% load, i.e., 154 W) and 80% load (i.e., 616 W) efficiency was still pretty good at 83%. At full load (770 W) efficiency dropped to 80.5%, still above the 80% mark.
This unit is 80 Plus Bronze certified, meaning that it should present 82% efficiency at full load. This didn’t happen because differently from Ecos Consulting we test power supplies at high temperatures, and efficiency drops with temperature (read our Can We Trust the 80 Plus Certification? article for more details).
Voltage regulation was the highlight from this power supply. All voltages stayed at a maximum of 3% from their nominal values (i.e., voltages closer to their official values than required, since the ATX specification allows them to be up to 5% from their nominal values (10% for the -12 V). This included the -12 V output, which usually doesn’t like to stay at such tight tolerance.
Noise and ripple were inside the allowed range (maximum of 120 mV for the +12 V outputs and 50 mV for the +5 V and +3.3 V outputs, peak-to-peak figures), however the -12 V output was touching its limit at 115.4 mV during test five.
Let’s see if we can pull more power from Zalman ZM770-XT.
We overloaded Zalman ZM770-XT to the maximum listed in the table below. If we increased one amp on any output noise level at +12 V would increase above 120 mV, making the unit to work outside the correct range. During this overloading the -12 V output was already outside range, with a noise level of 134.4 mV (the maximum allowed is 120 mV). Voltages stayed inside a 3% margin from their nominal values, which is terrific.
30 A (360 W)
30 A (360 W)
15 A (75 W)
15 A (49.5 W)
3 A (15 W)
0.5 A (6 W)
% Max Load
Zalman ZM770-XT power supply specs include:
* Researched at Newegg.com on the day we published this review.
Zalman ZM770-XT is not a bad power supply, providing high efficiency between 83% and 85% if you pull up to 80% from its labeled power (i.e., up to 616 W) and terrific voltage regulation, with all voltages closer to their nominal values than required. The addition of a heatpipe and a modular cabling system that is soldered directly on the printed circuit board (instead of using wires for its connnection) were good ideas.
This power supply, however, comes with only two connectors for video cards, preventing you from installing two video cards that require two power connectors each without using adapters.
The problem with this unit is that it is very expensive for what it is. At USD 193 we simply can’t recommend it, as you can find better products costing less. You can even get a Seasonic X-Series 650 W – the best power supply we’ve ever tested – costing less. Some examples of better options include Nexus RX-8500 (USD 150, 850 W, modular cabling system, four cables for video cards), Cooler Master Silent Pro M 850 W (USD 180, 850 W, modular cabling system, six cables for video cards) and Corsair HX750W (USD 165, 750 W, modular cabling system, higher efficiency, four video card cables), just to name a few. Even the overpriced Ultra X4 850 W (USD 200, 850 W, full modular cabling system, six video card cables) is a better option than Zalman ZM770-XT.