AN3112
Application note
Solution for designing a fixed off-time controlled PFC pre-regulator
with the L6564
Introduction
In this document we propose a third approach to the operation of PFC pre-regulators. In
addition to the transition mode (TM) and the fixed-frequency continuous conduction mode
(FF-CCM) operation of PFC pre-regulators, an alternative approach is offered that couples
the simplicity and affordability of TM operation with the high-current capability of FF-CCM
operation. This solution is a peak current-mode control with fixed-off-time (FOT). Design
equations are given and a practical design for a 400 W board is illustrated and evaluated.
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). The first method 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 uses
the more simple peak current-mode control, which is implemented with cheaper controller
ICs (e.g. the L6561, L6562, L6562A and L6564 from STMicroelectronics), and much fewer
external parts making it far more cost efficient. In the first method the boost inductor works
in a continuous conduction mode (CCM), while TM makes the inductor work on the
boundary between continuous and discontinuous mode. For a given power throughput, TM
operation involves higher peak currents compared to FF-CCM (Figure
1
and
Figure 2).
Figure 1.
Line, inductor, switch and diode
currents in FF-CCM PFC
Figure 2.
Line, inductor, switch and diode
currents in TM PFC
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.
This criterion, though always true, is sometimes difficult to apply, especially for a mid-range
power level 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 the stress of both power semiconductors and magnetics, 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.
February 2011
Doc ID 16820 Rev 3
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www.st.com
Contents
AN3112
Contents
1
2
3
4
Introduction to FOT control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Operation of an FOT- controlled PFC pre-regulator . . . . . . . . . . . . . . . . 5
Implementing the line-modulated fixed-off-time . . . . . . . . . . . . . . . . . . 6
Designing a fixed-off-time PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1
4.2
4.3
Input specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Operating condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power section design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.1
4.3.2
4.3.3
4.3.4
4.3.5
4.3.6
4.3.7
Bridge rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Boost inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Power MOSFET selection and power dissipation calculation . . . . . . . . 16
Boost diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
L6564 biasing circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5
6
7
Design example using the L6564-FOT PFC Excel
®
spreadsheet . . . . 31
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2/36
Doc ID 16820 Rev 3
AN3112
List of figures
List of figures
Figure 1.
Line, inductor, switch and diode currents in FF-CCM PFC. . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2.
Line, inductor, switch and diode currents in TM PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 3.
Basic waveforms for fixed frequency PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 4.
Basic waveforms for fixed-off-time PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 5.
Block diagram of an FOT-controlled PFC pre-regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 6.
Circuit implementing FOT control with the L6564 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 7.
ZCD pin signal with the fixed off-time generator circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 8.
Switching frequency fixing the line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 9.
The effect of fixing off-time - boundary between DCM and CCM . . . . . . . . . . . . . . . . . . . . 16
Figure 10. Conduction losses and total losses in the STP12NM50FP MOSFET couples for the 400W
FOT PFC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 11. L6564 internal schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 12. Open loop transfer function-bode plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 13. Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 14. Multiplier characteristics family for VFF =1 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 15. Multiplier characteristics family for VFF=3 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 16. Switching frequency function on the peak of the sinusoid input voltage waveform and the cor-
responding off- time value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Figure 17. Off-time vs. input mains voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Figure 18. Switching frequency vs. input mains voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Figure 19. Excel spreadsheet design specification input table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Figure 20. Other design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Figure 21. Excel spreadsheet FOT PFC schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Figure 22. Excel spreadsheet BOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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Introduction to FOT control
AN3112
1
Introduction to FOT control
In the output power range already mentioned, where the TM/CCM usability boundary is
uncertain, a third approach that couples the simplicity and affordability of TM operation with
the high-current capability of CCM operation may offer a solution to the problem. Generally
speaking, FF PWM is not the only alternative when CCM operation is desired. FF 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 if just the on-time is modulated and the
off-time is kept constant, in which case, however, the switching frequency is no longer fixed
(Figure
3
and
Figure 4).
This is referred to as fixed-off-time (FOT) control. Peak-current-
mode control can still be used.
Figure 3.
Basic waveforms for fixed
frequency PWM
Figure 4.
Basic waveforms for fixed-off-time
PWM
It is worth noting that FOT control does not need a specialized control IC. A simple
modification of a standard TM PFC controller operation, requiring just a few additional
passive parts and no significant extra cost, is all that is needed.
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AN3112
Operation of an FOT- controlled PFC pre-regulator
2
Operation of an FOT- controlled PFC pre-regulator
In
Figure 5
a block diagram of an FOT-controlled PFC pre-regulator is shown. An error
amplifier (VA) compares a portion of the pre-regulator's output voltage Vout with a reference
Vref and generates an error signal V
C
proportional to their difference. V
C
, a DC voltage by
hypothesis, is fed into an input of the multiplier block and multiplied by a portion of the
rectified input voltage V
MULT
. At the output of the multiplier, there is a rectified sinusoid,
V
CSREF
, whose amplitude is proportional to that of V
MULT
and to V
C
, which represents the
sinusoidal reference for PWM modulation. V
CSREF
is fed into the inverting input of a
comparator that, on the non-inverting input, receives the voltage V
CS
on the sense resistor
Rsense, proportional to the current flowing through the M switch (typically a MOSFET) and
the L inductor during the on-time of M. When the two voltages are equal, the comparator
resets the PWM latch, and M, supposedly already on, is switched off.
Figure 5.
Block diagram of an FOT-controlled PFC pre-regulator
As a result, V
CSREF
determines the peak current through M and the L inductor. As V
CSREF
is
a rectified sinusoid, the inductor peak current is also enveloped by a rectified sinusoid. The
line current Iin is the average inductor current that is the low-frequency component of the
inductor current resulting from the low-pass filtering operated by the EMI filter. 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 M 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. It is apparent that FOT control requires nearly the same architecture as TM control,
the only change is the way the off-time of M is determined. It is not a difficult task to modify
externally the operation of the standard TM PFC controller so that the off-time of M is fixed.
For the controller, we refer to the L6564 [4]. For a more detailed and complex description of
the fixed off-time technique and in particular the line modulated FOT, please refer to [5].
Doc ID 16820 Rev 3
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