INSIDE THE POWER SUPPLY

The power supply has evolved over time however the basic technology has remained the same. Most of the changes over the years were driven by many different forces.

BRIDGE RECTIFIER

It takes 4 power diodes to covert the 115V AC to about 150V DC.  The reason the voltage is higher is that DC is based in the peak voltage of AC. This type of power system was used widely with high voltage systems back during the vacuum tube era. There are no material losses when compared to a simple series diode which can only use ½ the wave, wasting the other.

Diodes can also be used is series or parallel to increase voltage or current capacity. Resisters can be used regulate voltages easily to prevent damage.

Power diodes are still widely used however the in the ATX form factor, the tight space limits options somewhat. All PC power supplies from the original 65W PSU to the latest model universally use bridge rectifiers in the first stage.

FILTER CAPACITOR

Adding a high voltage electrolyte capacitor across the output of a bridge rectifier can smooth out the jagged raw output.  Usually the bridge and filter capacitor are placed after a transformer. Adding a 400V 1000µF 105°C capacitor to the output of the bridge can reduce the ripple considerably. Higher voltage capacitors are available but they are more expensive.

POWER MOSFET

Following the bridge rectifier, the DC is next fed to the source of a power MOSFET transistor. The the drain is fed into a capacitor. That is all there is, one transistor and one capacitor.

The MOSFET has a gate which can be manipulated to work as a regulator. They can be used in parallel to increase the current capacity. The heat sinks are typical.

The processor uses CMOS which use a pair of MOSFET transistors to reduce current draw to only when switched.

VOLTAGE CONTROLLED REGULATOR

There are many ways to regulate voltage. The Zener diode uses a breakdown voltage as a reference. A differential amplifier uses 2 bipolar transistors and has 90db sensitivity. So compare the voltage in the power capacitor with the Zener. Then feed the Zener difference into an oscillator. The NE555 oscillator chip  can be configured to do pulse width modulation. The operational amplifier design uses a differential amplifier which is extremely precise.

The Schottky rectifier is the typical way modern power supplies. These are more suitable for higher current applications. These can clamp voltage to the desired value.

CAN WE DO BETTER?

The basic design is very simply as has been known for many years. Many of the changes have benefited the power company but the private citizen have benefitted too.

RF NOISE

The fist gripe is that the simple 1970s design is very hard in the grid. It feeds large amounts of RF energy back into the grid.

To remove RF from the grid the AC side uses a 500V 2.2µF capacitor across the line which bleeds off any higher frequencies quickly while lower frequency are untouched.

To block RF from returning to the grid requires an inductor (coil/resistor) which is found in all commercial power supplies. In effect an old style analog low pass filter is used. Filters have been known since the before 1920s when the first radios came to the high end consumer market.

One way to block RF feeding back to the grid is with an inductor, another way can use a simple resistor. Problem solved.

CAPACITORS

There are many types of capacitor available. Depending mostly for the needs, new surface mounted designs are increasingly popular.

BLEEDER RESISTOR

The main filter capacitor will often store up to 325 Volt long after the power cord has been removed from the wall. Not all power supplies contain a small “bleeder” resistor to slowly discharge this capacitor. Any contact with this capacitor may result in a severe electrical shock.

The primary and secondary side may be connected with a capacitor to reduce EMI and compensate for various capacitive couplings in the converter circuit. The current flowing from line or neutral through a 2000 Ω resistor to any accessible part must according to IEC 60950 be less than 250 μA for IT equipment.

To deal with these problems, before disassembling an unplugged machine. Hit the power button a couple of times to drain any residual power.

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