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Additional technical assistance: Peter McFarland and Jeremy Price.
Correctly testing power supplies is a complex procedure and KitGuru have configured a test bench which can deliver up to a 2,000 watt DC load. Due to public requests we have changed our temperature settings recently – previously we rated with ambient temperatures at 25C, we have increased ambient temperatures by 10c (to 35c) in our environment to greater reflect warmer internal chassis conditions.
We use combinations of the following hardware:
• SunMoon SM-268
• CSI3710A Programmable DC load (+3.3V and +5V outputs)
• CSI3711A Programmable DC load (+12V1, +12V2, +12V3, and +12V4)
• Extech Power Analyzer
• Extech MultiMaster MM570 digital multimeter
• SkyTronic DSL 2 Digital Sound Level Meter (6-130dBa)
• Digital oscilloscope (20M S/s with 12 Bit ADC)• Variable Autotransformer, 1.4 KVA
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DC Output Load Regulation
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Combined DC Load |
+3.3V
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+5V
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+12V
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+5VSB
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||||
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A
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V
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A
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V
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A
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V
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A
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V
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75W
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1.02
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3.36
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1.46
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5.00
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4.89
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12.07
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0.50
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5.02
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150W
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2.44
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3.35
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3.16
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5.00
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9.71
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12.03
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1.00
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5.02
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250W
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3.66
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3.33
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5.18
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4.98
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16.54
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12.01
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1.50
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5.00
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| 300W |
4.95
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3.31
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5.70
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4.97
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20.50
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11.94
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1.50
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5.01 |
Load regulation is really quite impressive and all output maintain close to reference results.
| be quiet! TFX Power 2 300W Gold | Maximum Load |
| 333 watts |
We managed to push the supply to 333 watts before it would shut down safely.
Next we want to try Cross Loading. This basically means loads which are not balanced. If a PC for instance needs 500W on the +12V outputs but something like 30W via the combined 3.3V and +5V outputs then the voltage regulation can fluctuate badly.
| Cross Load Testing | +3.3V | +5V | +12V | -12V | +5VSB | |||||
| A | V | A | V | A | V | A | V | A | V | |
| 285W | 2.0 | 3.35 | 2.0 | 5.00 | 19.5 | 11.93 | 0.2 | -12.06 | 0.50 | 5.01 |
| 141W | 13.0 | 3.30 | 10.0 | 4.92 | 4.0 | 12.07 | 0.2 | -12.04 | 0.50 | 5.01 |
Our first test placed a 19.5A load on the +12 V output with a light load on the remaining voltages. The supply delivered excellent results across the output range.
We then used an oscilloscope to measure AC ripple and noise present on the DC outputs. We set the oscilloscope time base to check for AC ripple at both high and low ends of the spectrum.
ATX12V V2.2 specification for DC output ripple and noise is defined in the ATX 12V power supply design guide.
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ATX12V Ver 2.2 Noise/Ripple Tolerance
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Output
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Ripple (mV p-p)
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+3.3V
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50
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+5V
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50
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+12V1
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120
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+12V2
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120
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-12V
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120
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+5VSB
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50
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Obviously when measuring AC noise and ripple on the DC outputs the cleaner (less recorded) means we have a better end result. We measured this AC signal amplitude to see how closely the unit complied with the ATX standard.
| AC Ripple (mV p-p) | ||||
| DC Load | +3.3V | +5V | +12V | 5VSB |
| 75W | 10 | 15 | 15 | 10 |
| 150W | 25 | 20 | 25 | 15 |
| 250W | 25 | 25 | 35 | 15 |
| 300W | 35 | 30 | 50 | 25 |
A decent set of results, +3.3V, and +5V both peak at 35 mV and 30 mV respectively. +12V peaks at 50mV at full load.
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Efficiency (%)
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75W
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87.34
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150W
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92.12
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250W
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88.34
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300W
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87.42
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Again, very good results from the 80 Plus Gold rated Power supply, peaking just over 92% efficiency at 50 percent load.
We take the issue of noise very seriously at KitGuru and this is why we have built a special home brew system as a reference point when we test noise levels of various components. Why do this? Well this means we can eliminate secondary noise pollution in the test room and concentrate on components we are testing. It also brings us slightly closer to industry standards, such as DIN 45635.
Today to test the Power Supply we have taken it into our acoustics room environment and have set our SkyTronic DSL 2 Digital Sound Level Meter (6-130dBa) one meter away from the unit. We have no other fans running so we can effectively measure just the noise from the unit itself.
As this can be a little confusing for people, here are various dBa ratings in with real world situations to help describe the various levels.
KitGuru noise guide
10dBA – Normal Breathing/Rustling Leaves
20-25dBA – Whisper
30dBA – High Quality Computer fan
40dBA – A Bubbling Brook, or a Refrigerator
50dBA – Normal Conversation
60dBA – Laughter
70dBA – Vacuum Cleaner or Hairdryer
80dBA – City Traffic or a Garbage Disposal
90dBA – Motorcycle or Lawnmower
100dBA – MP3 Player at maximum output
110dBA – Orchestra
120dBA – Front row rock concert/Jet Engine
130dBA – Threshold of Pain
140dBA – Military Jet takeoff/Gunshot (close range)
160dBA – Instant Perforation of eardrum
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Noise (dBA)
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75W
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32.1
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150W
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33.2
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250W
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35.5
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300W
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36.9
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The small fan doesn't spin that hard but the noise levels are noticeable due to the small size. It is quiet until about 200 watts are demanded. At 300 watts the fan spins up quite a bit and it becomes clearly audible. I actually found it a little intrusive.
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Temperature (c)
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||
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Intake
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Exhaust
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75W
|
36
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41
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150W
|
38
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45
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250W
|
40
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49
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300W
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43
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53
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The small fans copes admirably under load, with the exhaust temperature rising to a maximum of 53c at 100 percent load.
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Maximum load
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Efficiency
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333W
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86.67
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Pushing the power supply above its rated limits generates an efficiency level of around 86.7%. This is not a viable ‘real world’ situation, but its interesting nonetheless.
