Power-factor-corrected (PFC) inrush question based on power supply specs

[Anonymous user]

As I understand it, when calculating the power requirement for a power-factor-corrected (PFC) power supply, you consider the maximum rating of the power supply as briefly being required from the UPS during power on.

For example, I have an Antec Earthwatts EA-380. Its rated power is 380 W, and it is PFC. That means it could require 380 W / (power factor) / (efficiency) from the UPS. The efficiency specification I read on the box is "Up to 85%", and there is an "80 Plus Certified" label on the box. So let's say the efficiency is 80%. Power factor is claimed "Up to 99%". So let's say the power factor is 96%. Therefore the calculation would be:
380 W / 0.96 / 0.8 = 495 W.

However, among the specifications on the box are these items:
Inrush current: 60 A @ 115 V, 120 A @ 230 V
Input current: 6 A @ 115 V, 3 A @ 230 V

This is confusing to me. I would be using this in a 115 V AC environment, so we can skip the 230 V specs. The "Input current" results in 6 A * 115 V = 690 VA. That seems quite different from the 495 W calculated above. Furthermore, the "Inrush current" results in ten times that amount, or 6900 VA -- and I was thinking of the 495 W as representing the brief inrush upon power-on.

Given these specs and calculations, what is the best way to approach this for UPS sizing?

[BillP]

Your original calculation should be fine. That 60A inrush doesn't make much sense, considering your 120v outlet will have a 15A (20A tops) breaker.

[Anonymous user]

There is one electronic component inside computer PSUs that is called thermistor. The purpose of a thermistor is to limit the inrush current of the power supply when it is energized. A thermistor works by limiting the current when it is cold and rapidly heats up allowing current to pass through it freelly, it is a temperature variable resistor so to speak. Without the function of a thermistor the inrush current could reach those 60 to 120A figures we can usually see on the psu data sheet and it can acctually reach those figure for a very short time when there is a brief power cut, let's say, 60 miliseconds. When the power returns the thermistor didn't have the time to cool itself down for a moment, so as to return to its initial current limiting state that only kicks in when the PSU is de-energized for a few seconds to one minute. The good news is an UPS will never allow the PSU to be more than 10ms without power, so the inrush current will never be above the maximum input current, which in the given exemple is 6A @ 115V.

Current surges is one of the causes of death of power supplies. When the power goes out for let's say 60 miliseconds and comes back on after that, a huge current surge can happen and blow the fuse inside a PSU or damage it completly and can also cause a circuit breaker in the braker panel to trip. So it is unlikely you'll ever see a 60A inrush current when your computer PSU is plugged to an UPS. To me that's the main protection one UPS can give to a PSU since the thermistor inside all of them is not capable of limiting the current when the PSU not allowed to cool down for a few seconds before the it is re-energized.

All ATX PSU is required to be self sufficient for 16.6ms without power running @ full load. So let's say it is running @ half load, the 16.6ms figure will now be something like 33ms. So, the longer the brief powercuts are, the bigger the inrush curren can be. There is a limit for that, since a 20 seconds power cut can be enough for the thermistor to cool itself down a bit, enough to limit the initial current to a safe value. The "danger" zone would be the "milisecond zone" without power.

Message was edited by: rau

[Anonymous user]

So you are saying that the duration of power cuts that represents the maximum transfer times of typical UPS units (around 10 mS) would result in only the input current (6 A) demanded by the PSU, but not some higher amount up to the inrush current (60 A)?. If that is the case, then how does one know the time frame involved for which the actual inrush current applies? In other words, if I understand you correctly:

- if there is no power outage, the PSU's demand current is just its typical demand, namely the typical load plus some overhead for the PSU

- if there is a short power outage to the PSU, < 10 mS due to the presence of a UPS, you say the PSU will demand something less than its specified input current, 6 A in this case.

- if there is a longer power outage to the PSU, 60 mS to a few seconds, the PSU will demand current up to its specified inrush current, 60 A in this case.

- if a power outage to the PSU is longer than a few seconds, the thermistor will limit its input current, so when power returns to the PSU its current demand will be low and increase gradually.

It seems that my choices are to find either a UPS that can handle the full input current at 6 A * 115 V = 690 VA (and which does not necessarily have overload protection for inrush currents), or choose something like a SmartUPS that is smaller (than the seemingly oversized 690 VA for just a 380 W power supply) -- but large enough to handle average power loads -- because the SmartUPS would have some form of overload protection to deal with the large input current. Is that correct?

Message was edited by: Theorem

Message was edited by: Theorem

[Anonymous user]

The maximum inrush current depends on many things. It depends on the technical project of the PSU, the characteristics of its current limiting system, it depends on the discharging characteristics of the PSU bulk capacitors, the load on the PSU and even the returning point of the sinewave. If for exemple the returning point of the sinewave entering the PSU capacitors is exactly the peak of the sinewave, then the possibility for a big inrush current to happen is high. The 60A maximum inrush current of your PSU is what the manufacturer has found its PSU is capable of drawing very briefly at the worst case scenario, which will never happen if you have a UPS since 10ms of transfer time, which is usually lower than that, is not enough for fully discharging the PSU bulk capacitor.
How these numbers and times are defined I can't tell you for sure, but I have some documents here that show a few charts of inrush current X time caracteristcs of electronics like TV, monitors and computer PSUs and 10ms is not enough time for causing big inrush currents.

@ www.innovolt.com you can find current surge and voltage surge protectors that limit the current surges no matter what at about 10A maximum in the event of very brief power cuts. And no, I have nothing to do with them, I just found their product when I was searching on google about inrush current a couple of years ago.

Message was edited by: rau

[Anonymous user]

Inrush current is the peak current drawn by the power supply when it is plugged in and internal capacitors are charged.

When you try to cold start from a UPS, it may cause the UPS to trip, or the UPS might just provide less than the power supply will draw during the start up surge and work just fine. If the source can continue to provide lower current without tripping, the power supply should work fine.

Motor loads on other hand may not start if the source can't provide enough inrush current. For example, an air conditioner may run at 6A, but require 30A inrush. The compressor probably couldn't start up against the refrigerant pressure if the source can only provide 10A.

Message was edited by: KVAr

[Anonymous user]

The goal is to select a UPS that is not over-sized for a mere 380 W power supply in a computer, and which does not shut down due to inrush current demand from the power supply. As you state, "...UPS might just provide less than the power supply will draw during the start up surge and work just fine" -- which is what most people who are running computer systems would find useful.

[Anonymous user · Accepted solution]

As I understand it, when calculating the power requirement for a power-factor-corrected (PFC) power supply, you consider the maximum rating of the power supply as briefly being required from the UPS during power on.

For example, I have an Antec Earthwatts EA-380. Its rated power is 380 W, and it is PFC. That means it could require 380 W / (power factor) / (efficiency) from the UPS. The efficiency specification I read on the box is "Up to 85%", and there is an "80 Plus Certified" label on the box. So let's say the efficiency is 80%. Power factor is claimed "Up to 99%". So let's say the power factor is 96%. Therefore the calculation would be:
380 W / 0.96 / 0.8 = 495 W.

However, among the specifications on the box are these items:
Inrush current: 60 A @ 115 V, 120 A @ 230 V
Input current: 6 A @ 115 V, 3 A @ 230 V

This is confusing to me. I would be using this in a 115 V AC environment, so we can skip the 230 V specs. The "Input current" results in 6 A * 115 V = 690 VA. That seems quite different from the 495 W calculated above. Furthermore, the "Inrush current" results in ten times that amount, or 6900 VA -- and I was thinking of the 495 W as representing the brief inrush upon power-on.

Given these specs and calculations, what is the best way to approach this for UPS sizing?