SECTION 4 |
Powering Direct / Embedded Switch
Mode Power Supplies (SMPS)
14
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 various
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 40 to 60 milliseconds for 50 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
resistance 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.
