| Power
supply unit (PSU) is a device that
transfers electric energy from a source to a load using electronic
circuits. A typical application of power supplies is to convert
utility's AC input voltage into regulated DC voltage(s) required for
electronic equipment. Depending on the mode of operation, PSU can be
linear or switching (SMPS). What is SMPS? SMPS stands for switch mode power supply. In such a device power handling electronic components are continuously switching on and off with high frequency in order to provide the transfer of electric power via energy storage components (inductors and capacitors). By varying duty cycle, frequency or a relative phase of these transitions average value of output voltage or current is controlled. The frequency range of an SMPS varies from 20 kHz to several MHz. Below is the block diagram of a typical off-line switching power supply. The AC input voltage first passes through fuses and a line filter and is rectified by a full-wave bridge rectifier. The rectified input voltage is next applied to PFC (power factor correction) pre-regulator followed by output DC-DC converter(s). F1 and F2 shown on the left of the circuit diagram are input fuses. Fuse is a safety device designed to physically open the circuit when the current being drawn through it exceeds its rating for a certain period of time. The fusing time depends on the degree of overload. |
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| Due
to
this time
delay fuses will not always protect a PSU circuit from a
catastrophic failure caused by some abnormal conditions. Their main
purpose
is to protect input line from overloading and overheating, prevent
tripping of an external circuit
breaker and prevent fire inside the PSU that may be triggered by failed
components. Lowpass EMI filter is designed to reduce high frequency currents getting from PSU into the AC line to an acceptable level. This is necessary to prevent the PSU from causing interference on the other devices connected to the input wiring. There is a number of standards (such as EN55022 for Information Technology equipment) that govern the maximum level of EMI caused by PSU. The filter is followed by the bridge rectifier- the circuit that converts bipolar AC voltage to unipolar pulsating voltage. It uses four diodes in a bridge arrangement to provide the same polarity of output voltage for both polarities of input voltage. |
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The rectified input voltage
is next applied to
PFC
pre-regulator. PFC front end increases power factor (PF) and in the
process usually boosts voltage to 370-400 VDC. There are also
topologies where boost voltage follows peak of input AC voltage as well
as where buck is used instead of boost. Power factor in general is
the ratio between real
power (or watts) and volt-amps. PFC pre-regulator
controls input AC current so that it is in phase with mains AC
voltage and its waveform repeats the input voltage waveform.
Without this, the input current would be delivered to the SMPS in short
high peak pulses, which have a high harmonic content. The current
harmonics do not deliver any power to the load, but cause additional
heating in the wiring and distribution equipment. They also reduce the
maximum power that can be taken from a standard wall outlet, since
circuit breakers are rated by current rather then by watts. There are
various regulations that limit the input current harmonic content, such
as EN61000-3-2 (for equipment connected to public low voltage
distribution systems) or DO-160 (for airborne equipment). To meet these
harmonics requirements you can use PF correction techniques:
a PSU with high PF draws nearly
sinusoidal current from input (assuming sinusoidal input voltage),
which results in low harmonic content.
Currently there are no mandatory international standards that
specifically regulate the power factor of electronic equipment, but
there are various national and industry standards as well as voluntary
incentive programs. For example, 80 PLUS® and Energy Star®
programs
require computer PSU demonstrate PF>0.9 at rated load. Power
factor as well as current
harmonics can be measured with commercially available power
meters or special AC voltage
sources. There are two main types of power factor correction circuits- active and passive. Below is the block-diagram of a typical active PFC converter.  For PSU design notes see Unitrode seminar manuals. Efficiency of PSU is the ratio between the values of output and input power: Efficiency=Pout/Pin. To measure Pin you would need a true wattmeter: since any power supply has PF <1, you can't just multiply input volts and amps. A typical commercially available power meter usually displays both Pin and power factor. To measure Pout you will need a volt meter and ampmeter. Finally, housekeeping supply provides bias for all control circuitry and may also provide a separate stand-by voltage (SBV) which remains active even when the PSU is shut down for any reason. In today's computer power supplies a 5VDC SBV is a standard feature. Also see: What power supply do I need SMPS glossary Custom Power Electronics Search Engine Power Supply Design Jobs (updated daily) |
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