General PSU Discussion and FAQ contributions

[PeteS in CA]

Using ATX and similar power supplies (P/Ss) as a pattern, P/Ss can be described as having seven sections: AC Filter; Rectifier & Filter; Inverter; Pulse Width Modulator; Output Rectifier & Filter; Voltage Sense Amplifier and Protection; Standby Regulator.

AC Filter
The AC filter is a network of inductors and capacitors on the “primary” side of the P/S intended to reduce the amount of noise conducted back into the AC power lines (can't be interfering with those TV and radio commercials). Capacitors connected from Line to Neutral (X capacitors) and single-winding inductors reduce noise that is on Line with respect to Neutral. Capacitors connected from Line and Neutral to chassis ground (Y capacitors) and two-winding inductors reduce noise that is common to Line and Neutral with respect to chassis ground. X and Y capacitors are special types that have approval marks from various countries' safety agencies. These should not be replaced with general purpose types. Y capacitors are always in pairs, and if replaced, the same value should be used.

Rectifier & Filter
The rectifier and filter convert the AC input voltage to DC. The input voltage selector switch is part of this circuit. In the “115V” position, the rectifier circuit is configured as a voltage doubler. In the “230V” position, the rectifier circuit is configured as a full wave bridge. In either case, the unregulated high voltage bus is about 300VDC. If the switch is in the “115V” position in Europe, the metal oxide varistors (MOVs) in parallel with the two input electrolytic capacitors will fire, blowing the fuse. If the MOVs fail (usually blowing up) before the fuse blows, the two input electrolytic capacitors, usually rated 250V, will vent. If the P/S is operated in the US with the switch in the “230V” position, the P/S may seem to operate OK in a system that isn't fully loaded. In a fully loaded system the P/S may not operate, or it may perform erratically. Either way, the P/S will run hotter and its life may be reduced.

Inverter
This is the heart of a switching power supply. The inverter converts the 300VDC into a square wave of varying duty cycle . The positive part of the square wave across the transformer primary (and the secondary windings, except for the -12V and -5V secondary winding) corresponds to the time when the switch device (now usually a MOSFET) is on. The transformer steps the 300Vp-p square wave down to the appropriate voltage levels.

Pulse Width Modulator
The output voltage of a switching power supply, after the output rectifier and filter, approximately equals the duty cycle times the peak voltage of the square wave. Since the 300VDC bus is unregulated, that peak voltage varies with the AC input voltage. Regulation of the output voltage is achieved by controlling the duty cycle of the square wave. This is done by the pulse width modulator, which compares the output voltage (indirectly) to a sawtooth voltage to determine the width of each positive output pulse. In some designs, the sawtooth voltage is a sample of the transformer primary current, which is converted to a voltage by a low value resistor between the switch MOSFET Source and primary return.

Output Rectifier & Filter
Though the output rectifiers are usually two rectifiers in one package, in the most common topology, the two devices have different functions. One of the devices conducts during the “on” part of the square wave from the transformer, charging the inductor. The other rectifier conducts during the “off” time, providing a current path for the discharge of the inductor. The output inductor and output capacitors form a filter that convert the square wave to a DC voltage. The current through the inductor has two components - the current to the load, and the charging-discharging current super-imposed. The current through the filter capacitor(s) is the inductor charging-discharging current plus some transient currents due to large changes in the load current. Since the ripple current through the capacitor(s) can be several amps rms, low impedance capacitors are absolutely essential. This side of the P/S is commonly referred to as the “secondary” side.

Voltage Sense Amplifier and Protection
Here is the control center of the power supply. The main feedback circuit senses the +5V output and often the +12V output as well. The +3.3V output is usually mag-amp (magnetic amplifier) post-regulated. The -12V and -5V outputs are usually post-regulated with three-terminal linear regulators (Mac doesn't use a -5V output). P/Ss also typically have circuits that monitor the +5V, +3.3V, and +12V outputs going out of regulation high. If this occurs, the circuit will latch the P/S off (cycling AC power will clear the latch condition, but if the problem cause is still present, the P/S will latch off again). There may be circuits that sense the output currents for the +5V, +3.3V, and +12V outputs that will limit the current on the output(s) and reduce the output voltages of those outputs. Many P/Ss have a temperature sensor located on the heatsink for the output rectifiers (this is usually expected to be the hottest spot). This sensor connects to a circuit that will turn off or latch off the P/S if the temperature exceeds a preset threshold. In some P/Ss, the output voltage sense amplifier and protection circuits are integrated into a custom IC.

Standby Regulator
The Standby Regulator is a separate, low power, P/S. Unlike the main P/S, it is active almost immediately after plugging in the power cord. Depending on the current rating, this circuit may be a self-oscillating inverter with a 3-terminal linear post-regulator or a higher current switching regulator. This circuit powers power management circuitry on the computer motherboard (including the on/off function), as well as secondary-side protection circuits (e.g. latch-off protection functions).