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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 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 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.
Input voltage
Inrush current
Peak inrush
current
Rated steady state
input RMS current
NOTE: Voltage
and current scales
are different
TIME
Peak Current
RMS Current
Non-linear
Input Current
Pulse
Input Sine
Wave Voltage
Crest Factor = Peak Current = 3
RMS Current
Voltage (+)
Voltage (–)
Cur
ren
t (+)
Cur
ren
t (–)
NOTE: Voltage
and current scales
are different
Fig 4.1: Inrush current in an SMPS
Fig. 4.2: High Crest Factor of current drawn by SMPS
SECTION 4 |
Powering Direct / Embedded Switch Mode
Power Supplies (SMPS)
