PSU help: HX620W vs. ST75F vs. ST75ZF vs. M12 SS-700HM

Zork said:

What I need to know is will any of these PSUs (listed below) fit the bill (even though they don't “officially” support nextgen Nvidia 8800GTX video cards in SLI)?

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Redbeard said:

Test system:
eVGA 680i SLI motherboard
2x 8800GTX reference cards from nVidia
Intel Kentsfield quad-core processor
Raptor X 150GB drive
Generic DVD drive
2GB PC2-8888 DDR2 memory

I haven't done any exact tests to determine how much power the system is drawing at full load, but the fan in the HX620 doesn't spin up to full speed, even under full load for an hour straight. (4 instances of CPU Burn-In in the background of 3DMark06 on loop).

I imagine that system will be drawing, say, anywhere from 38A to 45A on the +12V during full load.

For those wondering, I'm using a couple of customized SLI cables to do this, the PEG connector cables that come with the HX series do not, by default, have two PEG connectors on each cable. I've got a couple here that do.

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ellover009 said:

Just looked at the silverstone, looks sweet, just watch out for some units that had a wiring error and you need a longer case for this, otherwise this looks like a winner

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Zork said:

Man, this place is awesome! I really appreciate everyones help!!!




ellover009, which Silverstone are you referring to? Also, why would I need a longer case? Will it not fit in the case I ordered? Is the PSU simply longer than normal? If so, why is that an issue? The case I ordered is not a server, but it's still a decent size case -- isn't it?

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Zork said:

Hey Bbq, thanks for the info! And thanks again to everyone else as well; I really appreciate the help!!!

Ok, I think I'm still leaning toward the Corsar, but I thought I'd add another PSU to the list and get everyones thoughts:

OCZ GameXStream OCZ700GXSSLI ATX12V 700W: newegg.com , zipzoomfly.com

I've read lots of reviews on the Corsair, and they've all been very postive -- everyone says the PSU is awesome and everyone seems to love it (and that seems to be the going consensus here as well). However, I was just reading a review over at pcper.com and they only gave the Corsair a "Silver Award" while giving the OCZ the "Gold Award." Anyone have any thoughts/comments on the OCZ in general, as well is in comparison to the Corsair? The OCZ has 80 more watts (and I think 4 more total amps 54a over Corsairs 50a?) and is cheaper (though, not by much), but does not have modular cables. I really like the Modular cables on the Corsair, but I am mostly concerned with quality/performance and the abilty to run nextgen hardware. With the prices being so close, I wonder if I should consider the OCZ in my list of PSU candidates? Anyone like the OCZ over the Corsair, or should the Corsair still remain at the top of my list?

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ellover009 said:

My corsair 620 came in, dude your thinking too much about it, get the corsair and don't think about it, 5yr warantee is impressive. Corsair is made by seasonic, good stuff they even sprung for some extra quality in some components, I looked at the OCZ and I was not sold on it, This particular model does not have a MOV aka surge suppressor, if you don't have a power supply that corrects voltages or some reason insane ammount of power goes into the PSU it would suppress the extra power and possibly take the damage, since it does not have it if some electrical problem happens, it gets overfed and it can't compensate it's going to fry your pc to where it came from. Read it here, it says it's a shame no high end power supply should make it out without a MOV. http://www.hardwaresecrets.com/article/397/3
By the way nice case, it's pimp, you could fit anything in there. Good luck. So unless your going to get quad SLI you and quadcore you won't need more than 620watts.
There's a good review on the OCZ
http://www.extremeoverclocking.com/reviews/cases/OCZ_GameXStream_700W_1.html
But I'm not sold on it, I look for reliablitly, I like the hardware secrets review, they actually explain more and look into the build quality, sometimes when they send some of the units they are handpicked from the factory, it's all perfect and roses because it is a limited review, it's based on design, efficiency and does it deliver when it comes to pulling, they have yet to develop a test on life expentancy since they actually hold onto this units for limited time. Good luck.

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Metal Oxide Varistor Degradation
by Kenneth Brown


The purpose of this document is to provide an overview of the degradation process that can occur in metal oxide varistors (MOVs). MOVs are variable resistors primarily consisting of zinc oxide (ZnO) with the function of limiting or diverting transient voltage surges. MOVs exhibit a relative high energy absorption capability which is important to the long term stability of the device. The growing demand of ZnO varistors is due to the nonlinear characteristics as well as the range of voltage and current over which they can be used. This range is far superior to devices composed of other materials that were used prior to the development of MOVs.1

If MOVs are used within their well-defined specifications, degradation due to the environment is not likely. However, the environment that MOVs are used in is not well-defined. Low voltage ac mains are subject to lightning strikes, switching transients, voltage swells/sags, temporary overvoltages (TOVs) and other similar disturbances. Due to the variety of disturbances that MOVs are exposed to, degradation or failure are possible in many applications.

MOVs perform their intended function reliably and experience low failure rates when applied within their specified limits. For an MOV to operate without failure or degradation it must quickly dissipate absorbed energy and return to its standby operating temperature. The ability to dissipate energy to the environment will depend on the design of the environment itself—ambient temperature, ventilation, heat sinking, other component population and density, proximity of heat sources, weight of PCB conductor traces, presence of thermal cutoff devices, etc. Degradation and catastrophic failures may occur if an MOV is subjected to transient surges beyond its rated values of energy and peak current.

The life of an MOV is defined as the time required reaching a thermal runaway condition. The relationship between ambient temperature and the life of an MOV can be expressed by Arrhenius rate equation,

t = t0exp[Ea-f(V)]/RT

where :

(t) = the time to thermal runaway,

t0 = constant,

R = constant,

Ea = activation energy,

T = temperature in Kelvin,

and f(V) = applied voltage.

Most Arrhenius rate models impose increased voltage and/or elevated temperature to accelerate the reaction rate (i.e., degradation or time to thermal runaway) and do not adequately address the detrimental effects of surge history.2 Surge history, especially transient surges beyond rated maximums, are perhaps the greatest single contributor to reductions in varistor voltage, increased standby leakage current, and ultimate thermal runaway. When increased voltage is applied for durations longer than microseconds, physical and chemical changes occur within the many boundary layers of a multi-junction MOV device. As with single-junction semiconductor devices, these changes occur on electronic and atomic scales at rates determined by the diffusion rates of structural defects—electrons, electron holes, and interstitial vacancies and ions. The MOV’s joule heating increases rapidly and exceeds the MOV’s ability to dissipate heat causing a thermal runaway condition and ending the MOV’s effective life.

Metal Oxide Varistors Description
MOVs are bipolar ceramic semiconductor devices that operate as nonlinear resistors when the voltage exceeds the maximum continuous operating voltage (MCOV). The term varistor is a generic name for voltage-variable resistor. The resistance of an MOV decreases as voltage magnitude increases. An MOV acts as an open circuit during normal operating voltages and conducts current during voltage transients or an elevation in voltage above the rated MCOV.

Modern MOVs are developed using zinc oxide due to their nonlinear characteristics and the useful range of voltage and current is far superior to silicon carbide varistors. The characteristic feature of zinc oxide varistors is the exponential variation of current over a narrow range of applied voltage. Within the useful varistor voltage range, the voltage-current relationship is approximated by the expression:2

where:

I = current in amperes,

V = voltage,

A = a material constant, and

a = exponent defining the degree of nonlinearity.

MOV Failures
MOVs have a large, but limited, capacity to absorb energy, and as a result they are subject to an occasional failure. The significant MOV failure mechanisms include: electrical puncture, thermal cracking, and thermal runaway, all resulting from excessive heating, in particular, from non-uniform heating. Non-uniform joule heating occurs in MOVs as a result of electrical properties that originate in either the varistor fabrication process or in the statistical fluctuations of properties that generally occur in polycrystalline materials.6

There are three basic failure modes for MOVs used within surge protective devices.3

1. The MOV fails as a short circuit.

2. The MOV fails as an open circuit.

3. The MOV fails as a linear resistance.

Note: Small-diameter MOVs that initially fail short circuit are likely to fail as an open circuit due to the absorption of large continuous current within the MOV.

The short-circuit failure of an MOV is usually confined to a puncture site between the two electrodes on the disk. Large fault current can create plasma inside the ceramic, with temperatures high enough to melt the zinc oxide ceramic. This failure mode can be caused by long-duration overvoltage, such as switching from a reactive load or thermal runaway of the MOV connected to the ac mains.

Open circuit failures are possible if an MOV is operated at steady state conditions above its voltage rating. The exponential increase in current causes overheating and eventual separation of the wire lead and disk at the solder junction.

Degradation of MOVs
It is well-known that MOVs experience degradation due to single and multiple current impulses. The test results documented in Mardira, Saha and Sutton show that MOVs can be degraded from an 8/20us surge current at 1.5 times the rated MOV surge current. A 20 mm MOV with a 10 kA surge current rating will be degraded if a 15 kA single pulse surge current is applied.5

When MOVs degrade they become more conductive after they have been stressed by either continuous current or surge current. MOVs generally experience degradation due to excessive surges exceeding the MOV’s rating while in operation. However, many MOVs show no signs of degradation when operated below a specified threshold voltage. The degradation of MOVs is primarily dependent on their composition and fabrication, as well as their application or duty.

Degraded MOVs were found to have smaller average grain size and change in the diffraction peak position compared to a new sample.5 The non-uniform temperature distribution in the material is due to the development of localized hot spotting during the current impulse and the dissolving in some other phases.

In high current conditions the zinc oxide junctions of the MOV begin to degrade resulting in a lower measured MCOV or turn-on voltage. As the degradation continues, and the MOV’s MCOV continues to drop until it conducts continuously, shorting or fragmenting within several seconds.

One of the key parameters related to measuring degradation of a varistor is leakage current. Leakage current in the pre-breakdown region of an MOV is important for two reasons:

1. Leakage determines the amount of watt loss an MOV is expected to generate upon application of a nominal steady-state operating voltage.

2. The leakage current determines the magnitude of the steady-state operating voltage that the MOV can accept without generating an excessive amount of heat.

The total leakage current is composed of a resistive current and a capacitive current. The resistive component of current is thermally stimulated and is significant, since it is responsible for the joule heating within the device. The capacitive current is a function of the MOV’s capacitance value and the applied ac voltage. If an MOV is subjected to an elevated voltage at a specific temperature, the internal current increases with time. Conversely, if the MOV is subjected to an elevated temperature at a specific applied voltage, the internal current increases with time. This phenomenon is accelerated by higher operating stress, and is further aggravated by elevated temperatures. The life of an MOV is primarily determined by the magnitude of the internal current and its increase in temperature, voltage, and time. As the current increases, the amount of heat (if not allowed to dissipate) can rapidly raise the temperature of the device. This condition may result in thermal runaway that can cause destruction of the MOV.

Tests were performed to induce thermal runaway. Photo 1 is a 40 mm MOV with an MCOV rating of 130 volts ac. During the test 240 V ac were applied at 15 amps and the MOV ignited.

MOVs exhibit greater power dissipation at higher temperatures given a fixed voltage. This characteristic can lead to thermal runaway. If the increase in power dissipation of the MOV occurs more rapidly than the MOV can transfer heat to the environment, the temperature of the MOV will increase until it is destroyed.

MOVs degrade gradually when subjected to surge currents above their rated capacity. The end-of-life is commonly specified when the measured varistor voltage (Vn) has changed by + 10 percent.4 MOVs usually are functional after the end-of-life, as defined. However, if an MOV experiences sequential surge events, each causing an additional 10 percent reduction of Vn, the MOV may soon reach a Vn level below the peak recurring value for the applied Vrms. When this state is reached the MOV draws in excess of 1 mA of current during each half-cycle of the sine wave voltage, a condition tantamount to thermal runaway. In nearly all cases, the value of Vn decreases with exposure to surge currents. The degradation manifests itself as an increase in idle current at the maximum normal operating voltage in the system. Excessive idle current during normal, steady-state operation will cause heating in the varistor. Because the varistor has a negative temperature coefficient, the current will increase as the varistor becomes hotter. Thermal runaway may occur, with consequent failure of the varistor.

Littelfuse publishes varistor pulse rating curves that are shown in figure 3. The pulse rating curves plot the maximum surge current versus the impulse duration in seconds. It is noted that stresses above the conditions may cause permanent damage to the device.

Power Dissipation Ratings
If transients occur in rapid succession, the average power dissipation is the energy (watt-seconds) per pulse times the number of pulses per second. The power generated must be within the specifications shown in the chart above. Operating values must be derated at high temperatures as shown in figure 2. Note the rapid drop in rated value at temperature greater that 85C.

Varistors can dissipate a relative small amount of average power compared to surge power and are not suitable for repetitive applications that involve substantial amounts of power dissipation.

In the ANSI/IEEE C62.33 (1982) Standard for Surge Protective Devices the following is stated: "Single and lifetime pulse current ratings are appropriate tests of varistor surge withstand capability. In the absence of special requirements, energy ratings are recommended for use only as supplements to the predominant current ratings, and for application problems, which are more conveniently treated in terms of energy."7

Mean Time Before Failure (MTBF)
MTBF is a measure of the typical number of hours that a varistor will continuously operate, at a given temperature, before a failure will occur. Accelerated aging test techniques are used to understand and minimize the MOV degradation process.

To obtain MTBF value, accelerated aging testing techniques are used to acquire the necessary data accurately and reliably in a short period of time. The following is a brief explanation of how an accelerated aging test is perfomed:

1. Obtain 60-90 MOVs of the same production run.

2. Initially test the varistor voltage @ 1 mA, and the leakage current at the rated dc working voltage.

3. Place an equal count of 20-30 varistors in three separate temperature chambers that have the temperature set at 85°, 105°C, and 125°C.

4. Apply rated volts ac to the devices.

5. Every 100 hours remove varistors from testing chambers and measure the varistor voltage @ 1 mA, and the leakage current at the rated dc working voltage.

6. If the leakage current is greater than 100 uA (an arbitrary failure point) then remove the device from test and record the number of hours before failure.

7. Continue test until all devices have failed, or enough data has been collected to allow an accurate curve fit of the data.

8. Input data into a data analysis program and extrapolate the time before failure at other temperatures.

The amount of time required to perform this test can be long. Typically Maida tests its MOVs for 10,000 – 15,000 hours (416 – 625 days) before the test completes. The criteria used to signify a failure or the time between testing is arbitrary. The values shown in the procedure are what Maida uses to run its test. Other values can be used for these parameters if required.

Using the Arrhenius model, the data collected is imported into a spreadsheet and then exported into a curve-fitting program. Using the equations of the Arrhenius model, the MTBF for a given temperature is plotted and printed.

Accelerated testing has been used in reliability prediction models. Accelerated testing allows accurate reliability and failure rate estimation in a relatively short period of time. Failure rates obtained by subjecting electronic components to highly accelerated testing conditions are used to estimate failure rates under normal operating conditions.

Studies have shown that failure of many electronic components, and varistors in particular, are due to chemical degradation processes, which are accelerated by elevated temperature. The Arrhenius model has found wide application in accelerated testing technology. The Arrhenius model is applicable if:

1. The most significant stresses are thermal.

2. The expected mean life is logarithmically related to the inverse of temperature.

The model is generally described by the following equation:

ML = e A+B/T

where:

ML: Mean life

A,B: Empirically derived constants from the life test data. The constant’s values depend on the characteristics of the material tested and the method.



T: Absolute temperature in Kelvin

The expected mean life (ML) of a varistor under normal operating temperatures is calculated using the above equation. The constants A and B are calculated from the (ML vs. temp.) graph developed during the accelerated testing experiment. The following two equations make calculating A and B easier:

B = (ln ML1 / ML2 ) ( 1 / T1 – 1 / T2 )-1

And,

A = ln (MLI) – B / TI

T1 and T2 are high temperatures used during the accelerated testing, and ML1 and ML2 are the corresponding mean lives obtained from the accelerated test.

A varistor normally operates under 40°C, a standby current value less than 50 uA and a voltage (10-15%) less than the MCOV.

Mean life of an electronic component is the expected mean or average life of the component. Mean life is estimated by testing a sample of components for a period of time, then:

The number of "varistor hours" on test at any time can be computed by adding the lives, in hours, of the varistors that have failed up to the moment of estimation, to the lives, in hours, of the varistors observed that have not failed. The greater the number of item hours (testing time), the more confidence in the resulting estimates of mean life.

Figure 3 is an example of the MTBF Analysis completed recently for varistor Style D69ZOV251RA72.

The vertical axis (ML) is a label that signifies the mean life (or the average time before failure) of an MOV expressed in hours. The horizontal axis (1/TEMP IN K) is a label of the temperature expressed in the reciprocal of the temperature in Kelvin. As the reader can see from the example the ML, at 0.00299-1 (61.5°C or 334.5°K), equals 1e+06 or 1 million hours. The ML, at 0.0023-1 (161.8°C or 434.8°K), equals 100 hours.

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Zork said:

I'm wondering, should I get any kind of device (digital multimeter?) for testing my PSU when I get it before I hook it up to all my components? Can a bad/defective PSU potentially harm the hardware?

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Spectre said:

Can it.......yes. Is it worth the effort and cost to get a good multimeter for one PSU....not really.

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Zork said:

Why is it not worth the "cost?" In one of the HX620W reviews I read (here), one of the devices the reveiwer used was a Mastech MS8230B Multimeter -- which can be found on amazon.com here for $14.95. That doesn't seem too constly to me; is it not a good one? If not, what is considered a good mutimeter? Can someone give me more specific information; perhaps a recomendation for what device(s) to get if I were to go that route?

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ellover009 said:

What he meant is that if you had a faulty unit you would notice something is not right. Not worth the time and effort unless you know how to do the test properly.

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Ockie said:

http://deadeyedata.com/index.php?main_page=product_info&products_id=95


And enter coupon code GRAND and you get another 10% off.

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Zork said:

I'm wondering, should I get any kind of device (digital multimeter?) for testing my PSU when I get it before I hook it up to all my components? Can a bad/defective PSU potentially harm the hardware? I'm kinda a newb when it comes to PSUs, and since this is my first build from scratch, I want to take every precaution. So, does anyone recommend any kind of device, such as a multimeter, to test my PSU before it goes in my rig? Any info on the matter would be greatly appreciated. Thanks!

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