question about PC power supplies

They are not rated to draw maximum rated current from all the voltage outputs at the same time. And ... who knows how accurate the ratings are in the first place?

Ed

If the power supply "brick" is fixed voltage (non-adjustable), then the maximum current specification is at that voltage. A 12V 10A power supply will deliver 12V at 10A. However, you'll need to read the fine print for how long the power supply can deliver that 120 watts. It may have a 20% duty cycle, or perhaps 10 minutes on, 50 minutes off, or som similar spec. Unless the power supply has plenty of heat sinking and a big fan, it's not likely to run continuously at the maximum rated output.

If there is more than one fixed output (not variable), the power supply should deliver power on both outputs simultaneously at the rated load (and duty cycle). Often, the maximum power output and duty cycle are rated by the total output, where you can trade power between outputs, as long as the total output is within the maximum rated output power. The limitation is usually not the circuitry, but rather the cooling system.

Power supplies with adjustable outputs usually include a graph showing how much output current is considered safe at various voltage output setting. Of course, nobody looks at the graph until after they blow up the power supply.

Good luck and try not to get too close to the maximum ratings.

For UL or CSA safety rated power supplies, the combined output powers shouldn't exceed the continuous output power rating of the product, as written on the label carrying the certification marking.

The unit is safety tested with any/all combination of continuous output channel powers that can occur below the total power rating for the unit.

Individual outputs are good for their labelled C x V rating. If there is a special pulsed limit for an output, it should be documented on it's spec sheet, but this need not show up on labelling. A minimum load condition may be specified to maintain the expected regulated output voltage.

You'll get longer life out of the unit if you keep a derating on average loads, keep design air flow levels unrestricted and minimize the number of times the input power is cycled.

RL

Yep.

Yes, but not any/all combination, but rather whatever the manufacturer declares to be safe. For example, if the power supply shares the +5V and +3.3V circuitry, the maximum total power will be whatever the circuit can handle. Example of such a nightmare specification: The +5V and +3.3V share quite a bit of circuitry. The labels says that the total simultaneous dissipation for the two sections is 130 watts. Yet, if I calculate the maximum for each section, I get: 3.3V * 22A = 72.6W 5V * 16A = 80.0W ============== Total = 153W which is more than the rated 130 watts. Same with the other sections on the chart. If I add together all the outputs: 3.3V * 22A = 72.6W 5V * 16A = 80.0W 12V1 * 15A = 180W 12V2 * 16A = 192W -12V * 0.8A = 9.6W 5VSB * 2.5A = 12.5W =============== Total = 547W which is more than the rated total output of 500 watts.

Where the label specifically gives grouped power ratings, they should be followed.

The same goes with any other grouped rating, including the rating for the full power supply, 500W.

Users are seldom provided with such detailed ratings info on the actual label, suggesting that this unit is marketed as a stand-alone product for evaluation/use by lower-volume end-user/developers.

Safety testing will have evaluated those ratings as listed on the label, under any worse-case shift of load. It can be time consuming to do so, but may reduce the actual part cost to the mfr, in the long run.

Too much information often indicates a not-very-rugged design, but in cost-critical markets, this is seldom a deciding factor.

RL

There doesn't seem to be much of a defacto standard in computah power supply labeling or specs. I picked a suitable label from what Google Image Search found: Looking at the various labels, some are overly complexicated, like the one I offered as an example, while others are sparse. The sparsely labeled power supplies are a problem because they will give the voltage and currents, the total power rating, and nothing else. As I demonstrated, just adding up the power for each section results in a total power that is far greater than the total power rating. This leaves the decision for how much power to draw from each section up to the user, with not guidance from the ratings. That's probably not a problem for PC applications. However, users that like to convert ATX power supplies into bench power supplies may have a problem. To make things worse, few of the specs and none of the labels bother to mention the rated duty cycle and operating temperature limits.

Duty cycle or temperature limits are usually present on the device spec sheet - for commercial product the defacto temp before deratimgs is 40C. Duty continuous unless stated otherwise.

As previously mentioned, safety certified units are stressed with any possible combination of outputs that do not exceed the combined power rating of the unit. For a any test not specifically aimed at the effect of output ratings, the outputs will be loaded to a derating of Prated / Psum, on each output, as a defacto standard 'rated' load.

ATX power supplies are designed to meet the ATX application, which requires only the minimum labeling requirements for certification and is often a captive or single supply chain spec as far as outputs, total ratings and environment/mtbf are concerned.

Refer to the Intel ATX developers guidelines for the copyright owner's standard spec. This is largely a mechanical and interconnection requirement, not an electrical one..

These products are normally limited by the temperature index of their isolation components, fan life and electrolytic cap life. None of those items are on the label, but will be on the mfr's spec sheet. Only the isolation components are safety evaluated. Anything else is subject to single-fault abnormal testing alone.

The only outputs normally expected to handle pulsed loads are those powering motors - fan and disc drive loads typically on the +12.

RL