POWERING DIRECT / EMBEDDED SWITCHED
MODE POWER SUPPLY (SMPS)
Non-linear nature of current drawn by Switched Mode Power Supplies.
Power supplies are used to convert AC voltages like 230 VAC to various DC voltages like
3.3 V, 5 V, 12 V, 24 V, 48 V etc. Majority of modern day electronic devises use embedded
general purpose
S
witch
M
ode type of
P
ower
S
upplies (SMPS) to drive the electronic
circuitry. General purpose
S
wich
M
ode
P
ower
S
upplies (SMPS)
(exepting those that
have power factor correction)
have one major disadvange- the current drawn by them
from the AC power source has a
non linear waveform
(the wave form is not sinusoidal as
the input voltage wave form but is in the form of short, larger value pulses around the area
of + Vpeak and - Vpeak). This is due to the charging of the input filter capacitor(s) mostly
around the positive and negative peak portions of the sinusoidal input voltage.
The degree of non-linearity is measured by the
“Crest Factor”
:
Crest Factor = Peak Current / RMS Current
In a linear load, the Crest Factor is 1.414. However, in a general purpose SMPS, dueto its
non linear nature, this factor will be much higher - in the region of up to 4. This will mean
that for a particular rated RMS current (applicable for a linear load), the general purpose
SMPS will draw much larger peak currents - approx.up to 4 times more than its rated
current. Inverters are protected against over current (also called overloading) by either
clipping the peaks of the output voltage ( this will result in a sine wave becoming a square
wave, reduction in the RMS value of the output voltage and generation of harmonics and
electrical noise) or by shutting down the output voltage of the inverter completely. Thus,
if an inverter / generator is used to power a general purpose SMPS, it will be force to
deliver higher peak currents resulting in premature triggering of the inverter’s / generator’s
over current protection circuits. Thus, for safe operation, the continuous RMS current
rating of the inverter / genrator should be at least 2.8 times the continuous RMS current
rating of the general purpose SMPS it is required to power:
Peak current of inverter = Peak current of SMPS
or
RMS current of inverter x 1.414 = RMS current of SMPS x 4
or
RMS current of inverter = 4/1.414xRMS current of SMPS
or
RMS current of inverter = 2.8 x RMS current of SMPS
Alternatively, the continuous power rating of the inverter / generator in Watts / VA
should be at least 2.8 times the continuous power rating of the SMPS in Watts / VA
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