Application Note

VISHAY GENERAL SEMICONDUCTOR
www.vishay.com
Rectifiers
Application Note
Power Factor Correction with Ultrafast Diodes
More and more switched mode power supplies (SMPS) are
being designed with an active power factor correction (PFC)
input stage. This is mainly due to the introduction of
regulations aimed at restricting the harmonic content of the
load current drawn from power lines. However, both the
user and the power company benefit from PFC, so it just
makes good sense.
Non-PFC power supplies use a capacitive input filter, when
powered from the AC power line. This results in rectification
of the AC line, which in turn causes high peak currents at the
crests of the AC voltage, as in Fig. 1a. These peak currents
lead to excessive voltage drops in the wiring and imbalance
problems in the three-phase power delivery system. This
means that the full energy potential of the AC line is not
utilized.
V
IAV
a. No Power Factor Correction
L
Bridge
Rectifier
Power Not Used
I
IL
D
Vin
Power Used
V
Mathematically, Power Factor (PF) is equal to Real Power /
Apparent Power.
The basic concept behind PFC is to make the input look as
much like a resistor as possible. Resistors have a power
factor of 1 (unity). This is ideal, because it allows the power
distribution system to operate at its maximum efficiency.
Lets consider a continuous conduction mode (CCM) boost
converter being used for active PFC. The boost topology
was chosen because it is the least expensive (cheapest)
solution, and cost is always a major consideration. Please
refer to Fig. 2.
I
C
Vout
MOSFET
Fig. 2 - Continuous Mode Boost Converter Circuit
IAV
b. Power Factor Corrected Input
Fig. 1 - Non-PFC vs. PFC Waveforms (Current, Voltage)
Revision: 25-Nov-15
Document Number: 88839
1
For technical questions within your region: [email protected]ishay.com, [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
Power Factor Correction (PFC) can be defined as the
reduction of the harmonic content, and / or the aligning of
the phase angle of incoming current so that it is in phase
with the line voltage. By making the current waveform look
as sinusoidal and in phase with the voltage waveform as
possible, as in Fig. 1b, the power drawn by the power supply
from the line is maximized for real power.
Real power is equal to VRMS × IRMS × cos , where  is
the phase difference between the voltage and current
waveforms. Therefore, as  approaches zero, cos 
approaches unity, which maximizes the real power (now just
VRMS × IRMS).
The input full-wave bridge rectifier converts the alternating
current (AC) to direct current (DC). The MOSFET is used as
an electronic switch, and is cycled “on” and “off” by an
external source. While the MOSFET is “on”, the inductor (L)
current increases. While the MOSFET is “off”, the inductor
delivers current to the capacitor (C) through the forward
biased output diode (D). The inductor current does not fall
to zero during each switching cycle, which is why this
is known as a “continuous conduction mode.” The MOSFET
is pulse-width-modulated so that the input impedance of the
circuit appears purely resistive, and the ratio of peak to
average current is kept low.
The most cost-effective way of reducing losses in the circuit
is by choosing a suitable diode for the application. Diodes
for use in PFC circuits typically have higher forward voltages
than conventional ultrafast epitaxial diodes, but much
shorter (faster) reverse recovery times.
Vishay recommends the use of the UH-series for PFC
applications.
Application Note
www.vishay.com
Vishay General Semiconductor
Power Factor Correction with Ultrafast Diodes
TABLE 1 - PFC ULTRAFAST RECTIFIERS - MINI SELECTOR GUIDE
VISHAY PART NUMBERS
CASE OUTLINE
DESCRIPTION
IAV (A)
VRRM (V)
trr (ns)
USB260
DO-214AA (SMB)
Plastic SMD
2
600
30
MURS260
DO-214AA (SMB)
Plastic SMD
2
600
50
31GF6
DO-201AD
Plastic Axial
3
600
30
SUF30J
P600
Plastic Axial
3
600
35
MURS360
DO-214AB (SMC)
Plastic SMD
3
600
50
MUR460
DO-201AD
Plastic Axial
4
600
50
UHF5JT
ITO-220AC
Isolated Power Pack
5
600
25
UH5JT
TO-220AC
Plastic Power Pack
5
600
25
UG5JT
TO-220AC
Plastic Power Pack
5
600
25
UGB5JT
TO-263AB
Power Pack SMD
5
600
25
UGF5JT
ITO-220AC
Isolated Power Pack
5
600
25
UH8JT
TO-220AC
Plastic Power Pack
8
600
25
UHF8JT
ITO-220AC
Isolated Power Pack
8
600
25
UG8JT
TO-220AC
Plastic Power Pack
8
600
25
UGB8JT
TO-263AB
Power Pack SMD
8
600
25
UGF8JT
ITO-220AC
Isolated Power Pack
8
600
25
UG12JT
TO-220AC
Plastic Power Pack
12
600
30
UGB12JT
TO-263AB
Power Pack SMD
12
600
30
UGF12JT
ITO-220AC
Isolated Power Pack
12
600
30
UG15JT
TO-220AC
Plastic Power Pack
15
600
35
UGB15JT
TO-263AB
Power Pack SMD
15
600
35
UGF15JT
ITO-220AC
Isolated Power Pack
15
600
35
APPLICATION NOTE
Revision: 25-Nov-15
Document Number: 88839
2
For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000