In a conventional power supply with switching regulators, a rectifying circuit that converts an AC input source into a DC
source for the primary circuit is used. In this rectifying circuit, a capacitor with a large capacitance is used to soften transient
response and reduce ripple so that the switching regulator is not over-stressed. However, the peak charge of the capacitor
becomes greater with greater capacitance, and this leads to non-linear bursts of peak over-current into the primary circuit.
Such peaks of current distort output voltage, create harmonic frequencies, and reduce power factor. There is now an
international standard for controlling harmonics (IEC100-3-2) and PFC is mandatory for home appliances consuming 70W
or more power in EU nations as of January, 2001. This PFC circuit is largely classified into two types: active and passive.
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PFC (Power Factor Correction)
Explanation of Technical Terms
1) Active PFC
This uses a switching regulator technology called ’boost-up’, using active elements such as IC, FET and diodes, to
create a PFC circuit. This circuit has a theoretical power factor of over 95%, accepts a full range (90VAC ~ 260VAC) of AC
input, and reduces total harmonics noticeably. However, it needs a complicated EMI filter and an input source circuit, and
is costly to build. Zalman Tech’s ATX /ATX12V power supply uses this type of PFC.
2) Passive PFC
This type of PFC uses passive elements such as a ferrite inductor on the input source to create a countering reactance.
While this can be easily applied to the existing power circuitry without much modification, the power factor is low
(60 ~ 80%), the AC input must be chosen (115VAC / 230VAC), and the harmonics produced from the difference between
the capacitance and the inductance are hard to control. Therefore, there is a possibility that significant electromagnetic
noise could result with an 115VAC input source.
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Comparing Active PFC and Passive PFC
DC output voltage regulation is mainly divided into AC line regulation (stability of DC output in relation to fluctuation in AC
input) and load regulation (stability of DC output in relation to fluctuation in load). The regulation is largely dependent on the
system design specifications, and in the case of an ATX/ATX12V power supply, it must be within 5% or 10% of the nominal
output voltage (i.e. +5V
±
5%
+4.75V ~ +5.25V). Should a voltage outside this range be supplied to the system, the
system may malfunction.
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DC Output Voltage Regulation
A ripple is normally defined as the peak-to-peak voltage (or current) caused by an imperfect rectification of an AC source. In
the case of the ATX / ATX12V switching regulator, ripple factors from the low-frequency AC input source, the high-frequency
(tens to hundreds of KHz) switching, and impulse noise contribute to the DC output ripple. This ripple and impulse noise can
be reduced to below regulation limit by inductive canceling within the rectifier circuit, but if the output ripple exceeds the limits
and is carried into the system, the logic level of active elements becomes unstable and the system can malfunction.
Normally, the listed DC output ripple and noise (measured in voltage, in scales of mVp-p) is solely of the switching regulator
and excludes that of the AC input.
▣
DC Output Ripple Voltage & Noise
*For more information on the product, please visit our website at www.zalman.co.kr.
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