16 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 17
SECTION 4 |
Powering Direct / Embedded Switch
Mode Power Supplies (SMPS)
4.1 CHARACTERISTICS OF SWITCHED 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 vari-
ous devices and circuits used in electronic equipment like battery chargers, computers,
audio and video devices, radios etc. SMPS 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 capacitors 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. 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
high Crest Factor of around 3 (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. Normally, inverters have
short duration Surge Power Rating of 2 times their Maximum Continuous Power Rating.
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.
