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Switching Power Supply:
As we all know, the vehicle's charging system produces less than 15
volts (generally between 13.5 and 14 volts when the engine is running).
This is simply not enough for many uses. For amplifiers and high output
preamplifiers, you need more voltage. To boost the voltage, we use a
switching power supply.
A switching power supply (also known as a switch mode power supply or
SMPS) pulses a DC current
through the primary windings of a transformer at very high frequency
(~20,000 - 50,000 pulses per second). This creates an alternating magnetic
field around the primary winding and the core of the transformer. Since
the secondary winding is also wrapped around the core and is in close
proximity to the primary windings, a voltage is induced in the secondary
windings. The secondary winding may or may not be electrically connected
to any point of the source of the DC power. The output of the secondary is
an AC
voltage. In most power supplies, the AC output of the transformer is
rectified by using diodes and therefore provides DC voltage at its
outputs. This rectified output voltage is commonly reffered to as 'rail
voltage' in amplifiers.
The diagram above shows a simplified version of the type of
transformer that is commonly used in car audio equipment. It consists of a
primary winding, a secondary winding and a toroidal core. This type of
transformer is lightweight, compact and very efficient. The transformers
can be much smaller than those used in home audio equipment becaust the
operating frequency is much higher. Most home equipment simply has its
power transformer connected to the AC mains supply. The mains in the US
runs at 60hz (60 cycles per second). On the other side of the pond, the
mains operate at 50 hz. At 50 or 60 hz, you need a much larger
transformer. With a switching power supply, the engineer can pick any
frequency he or she desires (after carefully considering all of the
different variables).
The diagram above shows a simplified version of a switching power
supply. In a typical design, Point "b" is connected to a source of power,
such as the vehicle's battery. Points "a" and "c" will be alternately
connected to ground at a high frequency (sorta like an electronic see-saw)
through high speed MOSFET
switching transistors. Point "x" would be the ground for the secondary of
the transformer and can be completely isolated from the vehicle's ground. Point
"y" would be the positive output rail voltage (referenced to the secondary
ground). Point Z is the negative rail voltage. The diodes are used to
rectify the AC output
from the transformer. The capacitors
are used to reduce the ripple on the output of the power
supply.
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