SamplexPower PST-1500-12 Mode D'emploi page 13

SECTION 3 |
- Ensure that the inverter is firmly grounded to the ground system of the building or
the vehicle
- Locate the inverter as far away from the EMI receptors like radio, audio and video
devices as possible
- Keep the DC side wires between the battery and the inverter as short as possible.
- Do NOT keep the battery wires far apart. Keep them taped together to reduce their
inductance and induced voltages. This reduces ripple in the battery wires and im-
proves performance and efficiency.
- 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
SECTION 4 |
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 vari-
ous 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 cur-
rent 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 resist-
ance 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.
Limiting Electro-Magnetic
Interference (EMI)
Powering Direct / Embedded Switch
Mode Power Supplies (SMPS)
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