Input voltage waveform
Approximately 5 ms
Input current waveform
Effective value
Peak value
Effective value
Peak value
Crest factor =
Figure Info-1 Input voltage and current waveforms
Crest Factor=Peak value of the current supplied to the device / Effective value of the current
When selecting a power supply unit, you need to estimate the power supply output capacity,
based on the crest factor of the input current supplied to every connected device. Estimate
the device crest factors by using the following approximate values when selecting a power
supply unit:
•
100 V system: Crest factor about 3
•
220 V system: Crest factor about 6
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How to Decide the Power Supply Unit
The commonly used methods for determining the power supply unit capacity with considera-
tion of the crest factor are shown below. Note that the power supply unit capacity should be
finally decided after consulting with the power supply unit supplier:
•
If the power unit crest factor is larger than the device crest factor, the power unit can be
used to its full rated capacity. However, factors such as device inrush current, backup
time, and reserve capacity must be taken into consideration separately.
•
If the power unit crest factor is smaller than the device crest factor, the power unit capaci-
ty must be calculated using the following formula. Factors such as device inrush current,
backup time, and reserve capacity must be taken into consideration separately.
Power unit output capacity = Total device power consumption × Capacity coeff
icient
Capacity coefficient = Device crest factor/Power unit crest factor
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Inrush Current
When a device is turned on, the large inrush current flows in because of the momentary
charging current for capacitors. Therefore, the minimum requirement is that the inrush current
produced by turning on some devices should not cause any voltage fluctuations that may af-
fect other devices. For turning on all devices together, a soft start or switching to uninterrupti-
ble backup power should be required.
When the power is turned on, the inrush current can trip the overload protector, and then may
switch to the backup power or commercial power supply. You should select a power supply
unit that can automatically and seamlessly recover the normal power supply when overload-
ing ends.
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IM 32Q06C10-31E
4th Edition : Jan.30,2015-00
