Samlexpower Evolution EVO-2212 Owner's Manual page 15

SECTIOn 1 |
• Shield the DC side wires with metal sheathing / copper foil / braiding.
• use coaxial shielded cable for all antenna inputs (instead of 300 ohm twin leads).
• use high quality shielded cables to attach audio and video devices to one another.
• Limit operation of other high power loads when operating audio / video equipment.
1.3.5 characteristics of Switch 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. These power supplies 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. 1.2. 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 1.3.
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 is defined by the following equation: 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 higher 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 recom-
mended 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 maxi-
mum continuous rated Power 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.
Safety Instructions & general Information
SAMLEX AMErICA INC. | 15