### Fairchild AN-42047 Power factor correction Datasheet

```www.fairchildsemi.com
Application Note 42047
Power Factor Correction (PFC) Basics
What is Power Factor?
Power factor (pf) is defined as the ratio of the real power (P)
to apparent power (S), or the cosine (for pure sine wave for
both current and voltage) that represents the phase angle
between the current and voltage waveforms (see Figure 1).
The power factor can vary between 0 and 1, and can be either
inductive (lagging, pointing up) or capacitive (leading, pointing down). In order to reduce an inductive lag, capacitors are
added until pf equals 1. When the current and voltage waveforms are in phase, the power factor is 1 (cos (0°) = 1). The
whole purpose of making the power factor equal to one is to
make the circuit look purely resistive (apparent power equal
to real power).
Real power (watts) produces real work; this is the energy
transfer component (example electricity-to-motor rpm).
Reactive power is the power required to produce the magnetic fields (lost power) to enable the real work to be done,
where apparent power is considered the total power that the
power company supplies, as shown in Figure 1. This total
power is the power supplied through the power mains to produce the required amount of real power.
When the power factor is not equal to 1, the current waveform does not follow the voltage waveform. This results not
only in power losses, but may also cause harmonics that
travel down the neutral line and disrupt other devices connected to the line. The closer the power factor is to 1, the
closer the current harmonics will be to zero since all the
power is contained in the fundamental frequency.
Understanding Recent Regulations
In 2001, the European Union put EN61000-3-2, into effect to
establish limits on the harmonics of the ac input current up to
the 40th harmonic. Before EN61000-3-2 came into effect,
there was an amendment to it passed in October 2000 that
stated the only devices required to pass the rigorous Class D
(Figure 2) emission limits are personal computers, personal
computer monitors, and television receivers. Other devices
were only required to pass the relaxed Class A (Figure 3)
emission limits.
“Total Power”
Apparent Power
(S) = Volt Amperes = I2Z
Reactive Power
(Q) = vars = (XL – XC) | 2
θ
Figure 2. Both Current and Voltage Waveforms are in
Phase with a pF =1 (Class D)
Real Power
(P) = Watts = (I2R)
Figure 1. Power Factor Triangle (Lagging)
The previously-stated definition of power factor related to
phase angle is valid when considering ideal sinusoidal waveforms for both current and voltage; however, most power
supplies draw a non-sinusoidal current. When the current is
not sinusoidal and the voltage is sinusoidal, the power factor
consists of two factors: 1) the displacement factor related to
phase angle and 2) the distortion factor related to wave
shape. Equation 1 represents the relationship of the displacement and distortion factor as it pertains to power factor.
Irms(1)
PF =
cos θ = Kd ⋅ Kθ
Irms
Figure 3: This is What is Called Quasi-PFC Input,
Achieving a pF Around 0.9 (Class A)
(1)
Irms(1) is the current’s fundamental component and Irms is
the current’s RMS value. Therefore, the purpose of the
power factor correction circuit is to minimize the input
current distortion and make the current in phase with the
voltage.
Causes of Inefficiencies
One problem with switch mode power supplies (SMPS) is
that they do not use any form of power factor correction and
that the input capacitor CIN (shown in Figure 4) will only
charge when VIN is close to VPEAK or when VIN is greater
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AN-42047
APPLICATION NOTE
than the capacitor voltage VCIN. If CIN is designed using the
input voltage frequency, the current will look much closer to
the input waveform (load dependent); however, any little
interruption on the mainline will cause the entire system to
react negatively. In saying that, in designing a SMPS, the
hold-up time for CIN is designed to be greater than the frequency of VIN, so that if there is a glitch in VIN and a few
cycles are missed, CIN will have enough energy stored to
V
Input Voltage
(Full Rectified)
Input Current
0
–
+
Vo (to PWM)
Cin
R1
RTN
Figure 4. SMPS Input Without PFC
Figure 5 represents a theoretical result of VCIN(t) (shown in
the circuit in Figure 4) with a very light load, and hence, very
little discharge of CIN. As the load impedance increases,
there will be more droop from VCIN(t) between subsequent
peaks, but only a small percentage with respect to the overall
VIN (e.g. with the input being 120V, maybe a 3-5 volt droop.
As previously stated, CIN will only charge when VIN is
greater than its stored voltage, meaning that a non-PFC circuit will only charge CIN a small percentage of the overall
cycle time.
130
100
Vc(t)
270 360
Deg
Boost Converters the Heart of Power
Factor Correction
Boost converter topology is used to accomplish this active
power-factor correction in many discontinuous/continuous
modes. The boost converter is used because it is easy to
implement and works well. The simple circuit in Figure 7 is
a short refresher of how inductors can produce very high
voltages. Initially, the inductor is assumed to be uncharged,
so the voltage VO is equal to VIN. When the switch closes, the
current (IL) gradually increases through it linearly since:
1
VL dt .
L∫
Vin(t)
-100
-130
0
50
Time (s)
100
Voltage (VL) across it increases exponentially until it stabilizes at VIN. Notice the polarity of the voltage across the
inductor, as it is defined by the current direction (inflow side
is positive). When the switch opens causing the current to
change from Imax to zero (which is a decrease, or a negative
slope). Looking at it mathematically:
Figure 5. VIN with Charging CIN
After 90 degrees (Figure 6), the half cycle from the bridge
drops below the capacitor voltage (CIN); which back biases
the bridge, inhibiting current flow into the capacitor (via
VIN). Notice how big the input current spike of the inductor
is. All the circuitry in the supply chain (the wall wiring, the
diodes in the bridge, circuit breakers, etc) must be capable of
carrying this huge peak current. During these short periods
the CIN must be fully charged, therefore large pulses of current for a short duration are drawn from VIN. There is a way
to average this spike out so it can use the rest of the cycle to
accumulate energy, in essence smoothing out the huge peak
current, by using power factor correction.
2
180
In order to follow VIN more closely and not have these high
amplitude current pulses, CIN must charge over the entire
cycle rather than just a small portion of it. Today’s non-linear
loads make it impossible to know when a large surge of current will be required, so keeping the inrush to the capacitor
constant over the entire cycle is beneficial and allows a much
smaller CIN to be used. This method is called power factor
correction.
IL =
0
90
Figure 6. Voltage and Current Waveforms in a
Simple Rectifier Circuit
D1
V1
Charging Bulk Input
Capacitor Voltage (Vcin)
VL = L
di
∆i
≈L ,
dt
∆t
or L times the change in current per unit time, the voltage
approaches negative infinity (the inductor reverses polarity).
Because the inductor is not ideal, it contains some amount of
series resistance, which loads this “infinite” voltage to a
finite number. With the switch open, and the inductor discharging, the voltage across it reverses and becomes additive
with the source voltage VIN. If a diode and capacitor were
connected to the output of this circuit, the capacitor would
charge to this high voltage (perhaps after many switch
cycles). This is how boost converters boost voltage, as shown
in Figure 8.
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APPLICATION NOTE
rotating machinery and transformers and noise emissions in
many products, and bringing about early failure of fuses and
other safety components. They also can cause skin effect,
which creates problems in cables, transformers, and rotating
machinery. This is why power companies are concerned
with the growth of SMPS, electronic voltage regulators,
and converters that will cause THD levels to increase to
unacceptable levels. Having the boost preconverter voltage
higher than the input voltage forces the load to draw current
in phase with the ac main line voltage that, in turn, rids
harmonic emissions.
Vi
IL
Vin
AN-42047
Vi
L V
L
Vo
Vin
Imax
0
Vo
0
IL
Vin
0
VL
Modes of Operation
Figure 7. Flyback Action of an Inductor
D1
V1 –
+
Lp
D2
Vo
Cin
Iin
PWM
CONTROL
There are two modes of PFC operation; discontinuous and
continuous mode. Discontinuous mode is when the boost
converter’s MOSFET is turned on when the inductor current
reaches zero, and turned off when the inductor current meets
the desired input reference voltage as shown in Figure 9. In
this way, the input current waveform follows that of the input
voltage, therefore attaining a power factor of close to 1.
Inductor Peak Current
Q1
Inductor
Current
Inductor Average
Current
RTN
Figure 8. PFC Boost Pre-Regulator
The input to the converter is the full-rectified AC line voltage. No bulk filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges (at
twice line frequency) from zero volts to the peak value of the
AC input and back to zero. The boost converter must meet
two simultaneous conditions: 1) the output voltage of the
boost converter must be set higher than the peak value
(hence the word boost) of the line voltage (a commonly used
value is 385VDC to allow for a high line of 270VACrms),
and 2) the current drawn from the line at any given instant
must be proportional to the line voltage.
Without using power factor correction a typical switchedmode power supply would have a power factor of around
0.6, therefore having considerable odd-order harmonic distortion (sometimes with the third harmonic as large as the
fundamental). Having a power factor of less than 1 along
with harmonics from peaky loads reduces the real power
available to run the device. In order to operate a device with
these inefficiencies, the power company must supply additional power to make up for the loss. This increase in power
causes the power companies to use heavier supply lines, otherwise self-heating can cause burnout in the neutral line conductor. The harmonic distortion can cause an increase in
operating temperature of the generation facility, which
reduces the life of equipment including rotating machines,
cables, transformers, capacitors, fuses, switching contacts,
and surge suppressors. Problems are caused by the harmonics creating additional losses and dielectric stresses in
capacitors and cables, increasing currents in windings of
REV. 0.9.0 8/19/04
Gating
Signal
Figure 9. Discontinuous mode of operation
Discontinuous mode can be used for SMPS that have power
levels of 300W or less. In comparison with continuous mode
devices, discontinuous ones use larger cores and have higher
I2R and skin effect losses due to the larger inductor current
swings. With the increased swing a larger input filter is also
required. On the positive side, since discontinuous mode
devices switch the boost MOSFET on when the inductor current is at zero, there is no reverse recovery current (IRR)
specification required on the boost diode. This means that
less expensive diodes can be used.
Continuous mode typically suits SMPS power levels greater
than 300W. This is where the boost converter’s MOSFET
does not switch on when the boost inductor is at zero current,
instead the current in the energy transfer inductor never
reaches zero during the switching cycle (Figure 10).
With this in mind, the voltage swing is less than in discontinuous mode—resulting in lower I2R losses—and the lower
ripple current results in lower inductor core losses. Less
voltage swing also reduces EMI and allows for a smaller
input filter to be used. Since the MOSFET is not being
turned on when the boost inductor’s current is at zero, a
very fast reverse recovery diode is required to keep losses to a
minimum.
3
AN-42047
Continuous Mode:
Average Current Mode
3
2.5
Inductor (Line) Current (A)
APPLICATION NOTE
2
1.5
1
0.5
0
Figure 10. Continuous Mode of Operation
Fairchild offers products for all discontinuous and continuous modes of PFC operation, including critical conduction
mode (FAN7527B), average current mode (FAN4810), and
input current shaping mode (FAN4803).
Discontinuous Mode:
Critical Conduction Mode
A Critical Conduction mode device is a voltage mode
device that works in the area between continuous and discontinuous mode. To better explain critical conduction mode
lets look at the difference between discontinuous and continuous mode in a SMPS design such as a flyback converter. In
discontinuous mode, the primary winding of the transformer
has a dead time once the switch is turned off (including is a
minimum winding reset time) and before it is energized
again (Figure 11).
Ipk
0
Figure 11. Discontinuous Mode, Flyback Power Supply Ip
(Primary Current)
In continuous mode, the primary winding has not fully
depleted all of its energy. Figure 12 shows that the primary
winding does not start energizing at zero, rather residual
current still resides in the winding.
Ipk
The heart of the PFC controller is the gain modulator. The
gain modulator has two inputs and one output. As shown in
Figure 13, the left input to the gain modulator block is called
the reference current (ISINE). The reference current is the
input current that is proportional to the input full-wave-rectified voltage. The other input, located at the bottom of the
gain modulator, is from the voltage error amplifier. The error
amplifier takes in the output voltage (using a voltage divider)
after the boost diode and compares it to a reference voltage
of 5 volts. The error amplifier will have a small bandwidth so
as not to let any abrupt changes in the output or ripple erratically affect the output of the error amplifier.
The gain modulator multiplies or is the product of the reference current and the error voltage from the error amplifier
(defined by the output voltage).
Figure 13 shows the critical blocks within the ML4821
(a stand alone PFC controller) to produce a power factor of
greater than 95 percent. These critical blocks include the current control loop, voltage control loop, PWM control, and
the gain modulator.
The purpose of the current control loop is to force the current
waveform to follow the shape of the voltage waveform. In
order for the current to follow the voltage, the internal current amplifier has to be designed with enough bandwidth1 to
capture enough of the harmonics of the output voltage. This
bandwidth is designed using external capacitors and resistors. Once the bandwidth has been designed which in most
cases is a few kHz (to not be affected by any abrupt transient), it uses information from the gain modulator to adjust
the PWM control that controls whether the power MOSFET
is switched on or off.
The gain modulator and the voltage control loop2 work
together to sample the input current and output voltage,3
respectively. These two measurements are taken and than
compared against each other to determine if a gain should be
applied to the input of the current control. This decision is
than compared against a sample of the output current to
determine the duty cycle of the PWM.
The PWM control uses trailing-edge modulation as shown in
Figure 14.
0
Figure 12. Continuous Mode, Flyback Power Supply IP
(Primary Current)
In critical conduction mode there are no dead-time gaps
between cycles and the inductor current is always at zero
before the switch is turned on. In Figure 9, the ac line current
is shown as a continuous waveform where the peak switch
current is twice the average input current. In this mode, the
operation frequency varies with constant on time.
4
1The
bandwidth is set by Fswitching/6
voltage control loop also needs to be bandwidth limited,
Again, this is designed using external passive components.
3The output voltage of a continuous inductor current boost
regulator has to be set above the maximum peak of the input
voltage in order to function correctly as a PFC. The output
should be 1.414 times the maximum input voltage.
2The
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APPLICATION NOTE
AN-42047
I
DC IN
+
IL
Q
+
AC
IN
RL
IL
IGM
RC
Current Control Loop
IA–
DC
OUT
IC
–
IA OUT
ID
C
IPR
DBR
ZCF
D
–
RS
Voltage Control Loop
RCL
2
3
–
–
R
+
IA+
+
4
S
Q
14
IGM
ISINE
OUT
GAIN
MODULATOR
5
Clock
ZF
EA OUT
6
OSC
INV
ZI
–
7
VREF
E/A
Ramp
+
Figure 13. Example of an Average Current Mode PFC Control (ML4821)
+
–
Current
Reference
Input
Inductor Current
TON
Output
TS
Figure 14. Trailing-Edge Modulation4
4
Figure 15. Typical Average Current Mode Waveform
Trailing edge modulation is when the output switches on when the output of the comparator passes through the trailing edge
of the sawtooth wave created.
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5
AN-42047
APPLICATION NOTE
The line that goes through the saw tooth waveform is the output of the differential amplifier within the current loop control. The output of the differential amplifier (located on the
top of Figure 13) goes into an R-S flip flop that controls the
power MOSFET. The average current mode waveform is
shown in Figure 14. Figure 15 shows the waveform of what a
typical average current PFC device looks like.
Using the continuous mode characteristic, the following
equations show that the inductor current is proportional to
the sinusoidal waveform at the turn-on time. Therefore the
inductor current minimum value during one switching cycle
follows the sinusoidal current reference as shown in Figure
18. However, the inductor current peak value during one
switching cycle is not controlled to follow the sinusoidal reference. Therefore the average inductor current might not be
sinusoidal. To make the average inductor current close to the
sinusoidal reference, the inductance has to be high enough to
make the current ripple small.
Continuous Mode:
Input Current Shaping
Fairchild’s FAN4803 features input current shaping, another
control method of the continuous current mode PFC. Figure
16 shows the internal PFC block of the FAN4803. Unlike the
conventional/typical average current mode PFC controller,
the FAN4803 does not need input voltage information and a
multiplier. It changes the slope of an internal ramp according
to the error amplifier output voltage, while the current sense
information and the ramp signal are used to determine the
turn-on time. As shown in Figure 17a, the switch is turned on
when the current sense voltage meets the internal ramp signal and the switch is turned off by the internal clock signal.
To control the output voltage, the slope of the internal ramp
signal is adjusted. By comparing Figure 17a and Figure 17b,
one can see that the average current increases if the slope
increases and decreases if the slope decreases.
VL = VIN = L
di L
t on
: During on-time
VL = (VIN − VOUT ) = L
di L
t off
VIN • t on = (VOUT − VIN ) • t off ,
VCS = Vramp = Veao
Rs • i L (t O + t off ) =
t off
TS
: During off-time
t off
=
TS
= Veao
V IN
: CCM condition
VOUT
VIN
VOUT
: Switch off to on
instant
Veao
VIN • sin(ω t)
VOUT
∴ I L (min) = i L (t O + t off ) ∝ sin(ω t )
VOUT = 400V
RP
VC1
VEAO
4
–
Gate
Output
COMP
C1
30pF
RCOMP
35µA
CCOMP
+
5V
R1
CZERO
ISENSE
3
–4
VI SENSE
Figure 16. Example of the Input Current Shaping PFC Controller (FAN4803
6
REV. 0.9.0 8/19/04
APPLICATION NOTE
AN-42047
Vramp = Veao (toff / TS)
Vcs
Average Current
Vcs = Rs • iL
Vramp
to
to + toff
toff
t o +TS
TS
PFC OUT
Clock
Figure 17a. Typical Input Current Shaping PFC Waveform
Vramp = Veao (toff / TS)
Vcs
Average Current
Vcs = Rs • iL
Vramp
to
t off
to + toff
t o +TS
TS
PFC OUT
Clock
Figure 17b. Typical Input Current Shaping PFC Waveform
Inductor
Current
Current
Reference
Figure 18. Input Current Shaping PFC Waveform
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7
AN-42047
Modulation (LEM/TEM) versus Trailing
Edge Modulation/Trailing Edge Modulation
(TEM/TEM)
Leading edge/trailing edge modulation is a patented Fairchild technique to synchronize the PFC controller to the
PWM controller. Typically TEM/TEM is used in PFC/PWM
controllers which results in an additional step as well as a
larger PFC bulk capacitor (as shown below).
Trailing Edge Modulation/Trailing Edge
Modulation (TEM/TEM)
Figure 19a shows the PFC inductor being energized.
APPLICATION NOTE
Trailing Edge Modulation (LEM/TEM)
Technique
In LET/TEM the PFC and PWM switches are tied together,
but opening and closing 180 degrees out of phase, so when
the PFC switch is open the PWM switch is closed and vice
versa. Initially when the PFC switch is closed, the PFC
inductor is energized, once the PWM switch is closed, both
the output and the PFC bulk capacitor are energized. Figures
20a and 20b show that upon repetition of this cycle, the PFC
bulk capacitor does not have to be that large because it is not
powering the output all by itself, the PFC inductor is helping
out as well.
Figure 19b shows the energy from the inductor being transferred into the PFC bulk capacitor.
When the PWM switch is closed, as shown in Figure 19c, the
energy stored within the PFC bulk capacitor is used to drive
the load. Every time this cycle is repeated, the PFC bulk
capacitor has to be fully charged since it is fully discharged
when the PWM switch is closed.
8
REV. 0.9.0 8/19/04
APPLICATION NOTE
AN-42047
PWM Section
PFC Section
Open
Vout
Signal
AC
Current
Output
Cap
Closed
PFC Bulk
Cap
GND
Figure 19a. Energizing the PFC Inductor
PFC Section
PWM Section
Open
Vout
Current
Signal
AC
Output
Cap
Open
PFC Bulk
Cap
GND
Figure 19b. Charging the PFC Bulk Capacitor
PFC Section
PWM Section
Closed
Vout
Current
Signal
AC
Output
Cap
Open
PFC Bulk
Cap
GND
Figure 19c. Powering the Output
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9
AN-42047
APPLICATION NOTE
PFC Section
PWM Section
Open
Vout
Signal
AC
Current
Closed
Output
Cap
PFC Bulk
Cap
GND
Figure 20a. Energizing the PFC Inductor
PFC Section
PWM Section
Current
Vout
Closed
Signal
AC
Output
Cap
Open
PFC Bulk
Cap
GND
Figure 20b. Charging the PFC Bulk Capacitor and Powering the Output
Conclusion
Power companies do not get excited over low power factor
driven devices, plus the extra cost of unused or wasted power
can be quite large. This is why PFC on the device side has
become an important part of the final power system design
for so many products. There are many standards in place
(example, EN 61000-3-2) to drive power consumption to a
power factor of 1 and keep total harmonic distortion to a
minimum. Depending on the output power and the designer’s
needs, a SMPS can be designed with either a discontinuous
or continuous mode stand alone PFC controller, or a continuous PFC/PWM mode device can be used. PFC controllers
are forecasted to grow to \$175 million in 2006, and standards are reducing the minimum power limits on systems
that require PFC, more and more PFC controllers will be
used.
10
REV. 0.9.0 8/19/04
AN-42047
APPLICATION NOTE
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY
PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY
LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER
DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, or (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
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```
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