Powering Direct / Embedded Switch Mode Power Supplies (Smps) - SamplexPower PST-300-12 Manuel Du Propriétaire

seCtIOn 4 | Powering Direct / embedded switch
Mode Power supplies (sMPs)
Switch Mode Power Supplies (SMPS) are extensively used to convert the incoming AC
power into various voltages like 3.3V, 5V, 12V, 24V etc. that are used to power various
devices and circuits used in electronic equipment like battery chargers, computers, audio
and video devices, radios etc. These power supplies use large capacitors in their input
section for filtration. When the power supply is first turned on, there is a very large
inrush current drawn by the power supply as the input capacitors are charged (The ca-
pacitors act almost like a short circuit at the instant the power is turned on). The inrush
current at turn-on is several to tens of times larger than the rated rMS input current
and lasts for a few milliseconds. An example of the input voltage versus input current
waveforms is given in fig. 4.1. It will be seen that the initial input current pulse just after
turn-on is > 15 times larger than the steady state RMS current. The inrush dissipates in
around 2 or 3 cycles i.e. in around 33 to 50 milliseconds for 60 Hz sine wave.
further, due to the presence of high value of input filter capacitors, the current drawn
by an SMPS (With no Power factor correction) is not sinusoidal but non-linear as shown
in fig 4.2 above. 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
when on 50 Hz power, with a very high Crest factor corresponding to peak values
around 3 times the rMS value of the input current:
(Crest factor = Peak value ÷ RMS value).
Many SMPS units incorporate "Inrush Current Limiting". The most common method is
the NTC (Negative Temperature Coefficient) 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
during normal operation. However, if the power supply is quickly turned off and back
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. Hence, it is recommended
that for purposes of sizing the inverter, the continuous power of the inverter should be
> 3 times the continuous rated power of the SMPS. For example, an SMPS rated at
100 Watts should be powered from an inverter that has continuous power of
> 300 Watts.
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