14 | SAMLEX AMERICA INC.
4.1 CHARACTERISTICS OF SWITCHED MODE POWER SUPPLIES (SMPS)
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audio and video devices, radios etc. SMPS use large capacitors in their input section for
fi ltration. When the power supply is fi rst turned on, there is a very large inrush cur-
rent drawn by the power supply as the input capacitors are charged (The capacitors act
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turn-on is several to tens of times larger than the rated RMS input current and lasts for
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given in Fig. 4.1. It will be seen that the initial input current pulse just after turn-on is >
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Further, due to the presence of high value of input fi lter capacitors, the current drawn
by an SMPS (With no Power Factor correction) is not sinusoidal but non-linear as shown
in Fig 4.2. The steady state input current of SMPS is a train of non-linear pulses instead
of a sinusoidal wave. These pulses are two to four milliseconds duration each with a very
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the NTC (Negative Temperature Coeffi cient) resistor. The NTC resistor has a high resist-
ance when cold and a low resistance when hot. The NTC resistor is placed in series with
the input to the power supply. The cold resistance limits the input current as the input
capacitors charge up. The input current heats up the NTC and the resistance drops
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on, the NTC resistor will be hot so its low resistance state will not prevent an inrush
current event.
The inverter should, therefore, be sized adequately to withstand the high inrush current
and the high Crest Factor of the current drawn by the SMPS. Normally, inverters have
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Hence, it is recommended that for purposes of sizing the inverter to accommodate
Crest Factor of 3, the Maximum Continuous Power Rating of the inverter should be > 2
times the Maximum Continuous Power Rating of the SMPS. For example, an SMPS rated
at 100 Watts should be powered from an inverter that has Maximum Continuous Power
Rating of > 200 Watts.
SECTION 4 |
Powering Direct / Embedded Switch
Mode Power Supplies (SMPS)
