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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. The most common type of switching power supply uses a transformer to boost voltage. A less common method uses diodes and capacitors.
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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