AN4149
Application note
Designing a CCM PFC pre-regulator based on the L4984D
By Hiroshi Andrea Fusillo
Introduction
Two methods of controlling power factor corrector (PFC) pre-regulators based on boost
topology are currently in use: the fixed-frequency (FF) PWM and the transition mode (TM)
PWM (fixed on-time, variable frequency).
With the first method the boost inductor works in a continuous conduction mode (CCM) and
employs average current-mode control, a relatively complex technique requiring
sophisticated controller ICs (e.g. the L4981A/B from STMicroelectronics) and a
considerable component count.
The second one uses the more simple peak current-mode control and makes the inductor
work on the boundary between continuous and discontinuous mode, which is implemented
with cheaper controller ICs (e.g. the L6562A, L6563x and L6564x from STMicroelectronics),
and much fewer external parts, making it far more cost efficient. For a given power
throughput, TM operation involves higher peak currents compared to FF-CCM (see the
figures below).
Figure 1. Line and inductor currents in
CCM PFC
Figure 2. Line and inductor currents in TM
PFC
AM13300v1
AM13301v1
This demonstration, consistent with the above-mentioned cost considerations, suggests the
use of TM in a lower power range, while FF-CCM is recommended for higher power levels.
In the power range of around 150-300 W, assessing which approach gives the better cost/
performance trade-off needs to be done on a case-by-case basis, considering the cost and
stress of both power semiconductors and magnetic components, but also of the EMI filter.
At the same power level, the switching frequency component to be filtered out in a TM
system is twice the line current, whereas it is typically 1/3 or 1/4 in a CCM system. In this
document the CCM using a fixed-off-time (FOT) control mode, fully integrated in the
controller, is proposed, coupling simplicity and low port count similar to a TM control. The
design procedure is explained too.
June 2013
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Contents
AN4149
Contents
1
2
CCM PFC using FOT control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Designing a CCM FOT-controlled PFC . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1
2.2
2.3
Input specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Power section design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
2.3.6
Bridge rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Boost inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Power MOSFET selection and power dissipation calculation . . . . . . . . 18
Boost diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3
L4984D biasing circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1
3.2
3.3
3.4
Feedback and OVP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Current sense resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Mult divider and VFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Gate driver (GD) and VCC pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4
5
6
7
8
FOT PFC boost control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Layout hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Design example using the L4984D-CCM PFC excel spreadsheet . . . 38
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
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AN4149
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Line and inductor currents in CCM PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Line and inductor currents in TM PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Block diagram of an FOT-controlled PFC preregulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Line-modulated FOT modulator internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Line-modulated FOT modulator key waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Boundary between DCM and CCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Typical frequency change along a line half-cycle in a boost PFC operated in LM FOT . . . . 8
Typical frequency change along a line half-cycle in a boost PFC operated in TM . . . . . . . . 8
LM FOT controlled boost PFC: current waveforms (line current) . . . . . . . . . . . . . . . . . . . . . 8
LM FOT controlled boost PFC: current waveforms (boost inductor current envelope). . . . . 8
Line-modulated, FOT controlled boost PFC: input current harmonic contents . . . . . . . . . . . 9
Total MOSFETs losses in the 350 W FOT PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Multiplier characteristics family for V
FF
= 1 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Multiplier characteristics family for V
FF
= 3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Small signal model of a PFC stage with a constant-power load (DC-DC converter) . . . . . 30
Bode plots of the control-to-output transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Schematic diagram of a type II compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Bode plots of a type II amplifier's transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Bode plots of the closed-loop transfer function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
115 V
ac
step load (50 % to 100 %): improper compensation . . . . . . . . . . . . . . . . . . . . . . . 36
115 V
ac
step load (50 % to 100 %): good compensation . . . . . . . . . . . . . . . . . . . . . . . . . . 36
EVL4984-350W PCB layout (SMT side view). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Excel spreadsheet design specification input table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Other design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
CCM PFC schematic based on the L4984D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Excel spreadsheet BOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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CCM PFC using FOT control
AN4149
1
CCM PFC using FOT control
Fixed-frequency PWM is not the only alternative when CCM operation is desired. An
additional approach that couples the simplicity and affordability of TM operation with the
high-current capability of CCM operation can be a solution to the problem.
Fixed-frequency PWM modulates both switch ON and OFF times (their sum is constant by
definition), and a given converter operates in either CCM or DCM depending on the input
voltage and the loading conditions. Exactly the same result can be achieved with FOT
approach: a conventional "peak" current mode control, where the ON-time T
ON
of the
external power switch is determined by the peak inductor current reaching the programmed
value and the OFF-time T
OFF
is determined by a special Fixed-off-time (FOT) modulator in
such a way that the resulting switching period is constant as long as the boost converter is
operated in CCM (i.e. the current in the boost inductor remains greater than zero in a
switching cycle).
In
Figure 3
a block diagram of an FOT-controlled CCM PFC pre-regulator is shown. An error
amplifier (VA) compares a portion of the boosted output voltage V
out
with a reference VREF
and generates an error signal V
C
proportional to their difference, a DC voltage by
hypothesis, which is fed into an input of the multiplier block and multiplied by a portion of the
rectified input voltage V
MULT
.
The multiplier output (VCSREF) is a rectified sine wave whose amplitude is proportional to
that of V
MULT
and to V
C
, and is used as a reference for PWM modulation. The multiplier
output is fed into the inverting input of a PWM comparator that, on the non-inverting input,
receives the voltage VCS from the sense resistor Rsense, proportional to the current flowing
through the switch M (typically a MOSFET) and the inductor L during the ON-time of M.
When the two voltages are equal, the comparator resets the PWM latch and M is turned off.
As a result, the multiplier output determines the peak current through the switch and the
inductor, and as, it is a rectified sinusoid, the inductor peak current is also enveloped by a
rectified sinusoid.
When VCSREF and VCS are equal the PWM latch output Q going high activates the timer
that, after a predetermined time in which T
OFF
has elapsed, sets the PWM latch, therefore
turning the switch on and starting another switching cycle.
If T
OFF
is such that the inductor current does not fall to zero, the system operates in CCM.
For the CCM PFC controller, please refer to
Figure 4.
To understand how T
OFF
needs to be modulated to achieve a fixed switching frequency
independent of the instantaneous line voltage and the load, it is useful to consider the V·s
balance equation for the boost inductor under the assumption of CCM operation:
Equation 1
T
ON
Vpk sin
θ =
T
OFF
(
Vout
−
Vpk sin
θ
)
where Vpk is the peak line voltage, V
out
is the regulated output voltage, and
θ
is the
instantaneous phase angle of the line voltage. Solving for T
ON
we get:
Equation 2
Vout
T
ON
=
−
1
T
OFF
Vpk sin
θ
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AN4149
then, the switching period T
SW
will be:
Equation 3
CCM PFC using FOT control
Vout
Vout
T
sw
=
T
ON
+
T
OFF
=
−
1
T
OFF
+
T
OFF
=
T
OFF
Vpk sin
θ
Vpk sin
θ
In the end, if T
OFF
is changed proportionally to the instantaneous line voltage, i.e. if:
Equation 4
T
OFF
=
K
t
Vpk sin
θ
then T
SW
will be equal to Kt·V
out
and, since V
out
is regulated by the voltage loop, also T
SW
(and f
SW
=1/T
SW
) will be fixed. This result is based on the sole assumption that the
instantaneous line voltage and the output load are such that the boost inductor operates in
CCM.
Figure 3. Block diagram of an FOT-controlled PFC preregulator
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