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Overclocking Elite
Join Date: Aug 2007
Location: Colorado Springs, CO
Posts: 1,062
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How a PSU works
The PSU (power supply unit) is arguably the most important part in a PC. It essentially provides power to the entire system, allowing it to operate. The PSU is not a simple device by any means, providing a wide range of voltages to different components. There are two basic types of power supplies; linear and switching. Linear power supplies are typically found in large applications, such as car battery charges. However, linear power supplies require very large components, and would take up too much space in a PC. This is where the switching power supply comes in. This design allows for much smaller components, albeit more complex, and is universal to almost every PC design out there. This is the design we will be looking at. There are several stages that the power must go through before it is used by the PC:
Stage 1: Filtering
Filtering smooths out the current flowing into a transformer, and also reduces the amount of harmonic coming out of the power supply. This is better known as Power Factor Correction or PFC. There are three types of PFC:
-No PFC, seen on most low-end models
-Passive PFC, seen on low and mid range models
-Active PFC, seen on mid and high end models
Passive PFC works by using capacitors and induction coils to control power factor (PF). Ideally, you want the PF to be as close to 1 as possible. Passive PFC is quite effective in large power systems like substations, but this requires very large components. Due to size limitations in an ATX PSU, passive PFC is only able to bring the PF to around 0.70-0.80. This is still an improvement over not having PFC at all, in which case the PF can go as low as 0.50 or less - in other words, lots of wasted electricity.
Active PFC uses electronically controlled semiconductor switches to regulate PF. Since it is actively regulated, the PF can reach as high as 0.99. This allows filtering components to be much smaller, and yet still be very effective. Another advantage is that active PFC can accommodate a wide variation in input voltages.
Stage 2: Rectification
When power enters a PSU, it is in the form of alternating current (AC), which alternates between positive and negative in the form of a sine wave. A PC uses direct current (DC). In order to get from AC to DC, this sine wave must be flattened out, in other words, rectified. This is done using a set of diodes, a component that only allows current to flow in one direction. A computer PSU uses full-wave rectification, which requires a set of four diodes. After the current is rectified, is is still not completely smoothed out, but forms a choppy wave. This is smoothed out using a large capacitor. The capacitor absorbs spikes of current and releases absorbed current when there are voltage drops.
If the PSU does not have active PFC, two capacitors are used. PSUs are designed to work at around 240V. However, countries like the US use a 120V grid. In order to get the voltage up to the desired 240V, a voltage multiplier circtut is used. Using the two capacitors in conjunction with the rectifier circuit, the voltage is increased from 120V AC to 240V DC. Everyone should be familiar with the little red voltage selection switch on the back of cheaper PSUs. When set to 120V, it enables the voltage multiplier. When it is set to 240V, it bridges the two capacitors together and they act as a single filtering capacitor, allowing 240V to pass through the circuit unchanged. It is important to note that setting the switch to 120V on a 240V supply will result in 480V flowing into the system, usually resulting in a catastrophic failure.
Stage 3: Switching
Due to a way a transformer works, it cannot operate on a steady supply of DC current. It must run on either AC current or pulsating DC current. The reason why the transformer does not run directly off of AC current is because 50/60Hz is too low of a frequency and would require a transformer too large to be practical, especially for systems requiring large amounts of power. That is why high frequency switched DC or AC current is used instead of low frequency AC. The current can be switched at a very high frequency, allowing a smaller transformer to be used. As a basic rule of thumb, the higher the frequency, the smaller the transformer and filtering components can be; however, voltage and the amount of current demanded also determines the size of components.
The rapid switching of DC/AC current involves a PWM (pulse-width modulation) circuit. The PWM chip puts out a set clock frequency, which operates a set of specialized transistors called MOSFETs (metal-oxide-semiconductor field-effect transistors). The clock signal switches the MOSFET on and off very rapidly (several thousand times a second). This creates the switched AC (or pulsed DC) current needed to operate the transformer. The PWM can also vary the duty cycle (amount of time the clock signal is ON) to adjust for demand. MOSFETs produce a fair amount of heat and are seen attached to the smaller heatsink in a typical PSU. The PWM circuit can produce either pulsed DC or AC current, depending on the transistor configuration. Most larger power supplies use an AC configuration for greater efficiency.
Stage 4: Transformation
The transformer is the heart of every PSU. Its purpose is to convert an input voltage into a voltage that is usable. In this case, usable by the computer. A transformer consists of a magnetic core and two sets of coils (windings). The current flows into the transformer and energizes the primary coil. This produces a magnetic field that is picked up by the secondary coil and converted back into electricity. The voltage on the secondary coil is determined by the ratio of windings on the primary coil to the number of windings on the secondary coil. For example, a transformer with a 2:1 winding ratio will have a 60V output if the input voltage is 120V. The secondary winding can also be tapped at different points to give different voltages - the closer to the beginning of the winding, the lower the voltage. So the 3.3V rail will tap the transformer much closer to neutral than the 12V rail will. Some transformers may even have separate secondary windings, having windings for the 3.3V/5V rail and the 12V rail - particularly in higher end units.
Stage 5: Rectification and filtering (again)
Even if the current is already DC (most high end units use AC), something is needed to make sure it only flows in one direction through the transformer and also enforce a point of neutral (zero) voltage. This is done by using an additional set of rectifiers on each rail. The rectifiers are Schottky diodes made to handle large amounts of current with very little resistance (current is inversely proportional to voltage, so there will be more current with a lower voltage). Even so, the large amount of current flowing through the diodes produces a quite a bit of heat. This is why you'll see the secondary rectifiers on the larger heatsink in a standard PSU design.
Then finally, the output current is filtered to remove as much ripple (noise) as possible. This is done using sets of capacitors and induction coils. Once this is done, the power is ready to be used by the computer.
Last edited by Goat : 02-25-2008 at 05:35 PM.
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