Samlex America PST-15S-24A - Manual (Page 13)

Page 12
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 120 VAC to various DC voltages
like 3.3 V, 5 V, 12 V, 24 V, 48 V etc. Majority of modern day electronic devices use
embedded general purpose Switch Mode type of Power Supplies (SMPS) to drive the
electronic circuitry. General purpose Switch Mode Power Supplies (SMPS) ( excepting
those that have power factor correction ) have one major disadvantage – the current
drawn by them from the AC power source has a non linear waveform ( the waveform is
not sinusoidal as the input voltage waveform but is in the for
m 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, due to
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 RMS 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 forced 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 / generator 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 = 4/1.414 X RMS 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