TP2379 English version FINAL2

Efficient and easy 350W PFC solution
based on LM-FOT current control
Luigi Galioto
ABSTRACT
As part of power factor corrector (PFC) boost topology circuits,
STMicroelectronics has developed the new controller L4984D
dedicated to manage circuits with 150W <P <300W power range.
The key feature of this device is linked to the new and proprietary
method of the inductor current control called "line-modulated fixed
off-time (LM-FOT)".
This method includes the main features of the two most widely
used control techniques: the simplicity typical of the transition
mode (TM) used for P <100W, and the ability to handle high
power (P> 500W) typical of fixed frequency continuous current
mode(FF-CCM).
In this article it will be showed the details of the LM-FOT
control method, the major features of the new controller L4984D
and the related evaluation tools.
INTRODUCTION
STMicroelectronics, with the introduction of the new device
L4984D, is able to offer a complete range of controllers dedicated
to the PFC regulators. According with the power to manage, the
control techniques and the ICs used are different. The traditional
control methods for boost topologies are the transition mode (TM)
and the fixed frequency continuous current mode (FF-CCM).
The TM method uses the simple peak current control, brings the
boost inductance to work always at the boundary between
continuous and discontinuous mode and uses a variable switching
frequency but with ton constant (Fig. 1).
This method is easily implemented through low cost controllers
and making use of a small number of external components; all this
makes the circuit a less expensive solution. However, the TM
cannot be used for power higher than 100 W otherwise there would
be too high peak currents, which means having high current ripple,
high RMS current value and heavy EMI filter. The ST controllers
that implements this method are L6562A, L6563* and L6564*.
Fig.1 – Line and inductor current waveforms in TM control
500W. The ST controllers that implement this method are L4981A
and L4981B.
Fig.2 – Line and inductor current waveforms in CCM control
Considering the PFC boost topology with 150W <P <300W
power range, it is necessary to have the best compromise cost /
performance of the two methods discussed before. For this
purpose, STMicroelectronics has developed a new control method
called "line modulated fixed off time" (LM-FOT). This method has
been implemented in the new controller L4984D we discuss in the
next sections.
THE LM-FOT CONTROL METHOD
The innovative LM-FOT control method uses the conventional
peak current control in which, assuming that the inductance works
in CCM, the ton is determined by the achievement of the
programmed peak value, and the toff is modulated by the input
voltage value so that the switching period is constant (see
formula).
∙
1 ∙
→ ∙
∙
∙
∙
The LM-FOT method allows fixed frequency (FF) switching
operations as long as the converter operates in continuous current
mode CCM (i.e. inductor current IL is greater than zero); this
behavior in a semi period is not always guaranteed but depends on
the Vin and Iout values (Fig. 3).
Fig.3 – Boost inductor current envelope with LM-FOT control
The FF-CCM method uses the average current control, brings
the boost inductance to work always in continuous mode (i.e. the IL
never goes to zero), and uses a constant switching frequency (Fig.
2). This method ensures low input current ripple and a high
performance converter in terms of power factor, THD and
efficiency. The FF-CCM is quite complex so the related controllers
are more expensive and require a greater number of external
components compared with the TM ones. Considering these
aspects, the FF-CCM is recommended for power higher than
The line voltage increasing and / or the load decreasing, in the
regions close to zero, drive the boost inductor in DCM
(discontinuous current mode) working mode, and produce a
switching frequency increase; this means fsw is no longer constant
in the line voltage period (Fig. 4). In any case, this frequency
increase is considerably lower than what would occur in TM.
More details on the LM-FOT method are reported in the
application note AN4149.
Fig.4 – Frequency change in LM-FOT control
voltage mode error amplifier and an accurate (1%@Tj=25°C)
internal voltage reference.
To meet the latest regulations on energy saving (Blue Angel,
Energy Star, Energy 2000, as examples), the device is optimized
for low power consumption and includes a disable function for IC
remote on/off. Thanks to this function, in conditions of minimum
load, the downstream DC-DC converter is able to turn off the
L4984D, saving the entire energy system consumption.
Regarding the line current (IAC) harmonic content, the multiplier
present in the device includes a special circuit that reduces the
current distortion resulting from the Vin zero crossing; in this way a
low THD value is obtained over a Vin wide range and even over a
large load range.
STMICROELECTRONICS’ SYSTEM SOLUTION
STMicroelectronics has developed, to better appreciate the
L4984D features, some system evaluation tools: a system
evaluation board (hardware) and a simulation tool (software).
Concerning the evaluation board, a 350W PFC solution based
on L4984D (order code EVL4984-350W) was developed (Fig. 6).
L4984D, THE LM-FOT CONTROLLER
The L4984D controller (Fig. 5), which is available in SSOP10, implements the LM-FOT control. The device is able to drive
Power MOSFETs and IGBTs up to 600mA and 800mA power
switch-off, and includes a wide range of functions, most
importantly those mentioned below, that improve the performance
and safety of the PFC converter.
Fig.6 – Evaluation board EVL4984-350W
Fig.5- L4984D block diagram
The main electrical system specifications of EVL4984-350W
are summarized in Table 1. It is a conventional boost converter
(Fig. 7) with excellent performance based on the use of the
following ST devices. The L4984D is the controller. The main
switch is formed by paralleling two MOSFETs: the STF21N65M5
(650V/21A, MDmeshTM V Power MOSFET) and the output diode
STTH8S06FP (600V/8A, Turbo 2 ultrafast high voltage rectifier).
Regarding protection, the device is able to manage the
transient output overvoltage and those due to accidental feedback
loop failures. For overcurrent, the L4984D includes protection
against the inductor saturation, the lowering of input voltage
(brownout), and the usual consequences of the start-up phase
converter (soft start).
The loop stability is optimized by the voltage feedforward
function that compensates for the variation of the power stage gain
with the square of the input voltage
. The missed
compensation would lead, for certain Vin values, to a slow control
loop.
The output voltage regulation is controlled by means of a
Tab.1: EVL4984-350W, main electrical specifications
90VacVin , input voltage range
265Vac
Max output power
350W
Regulated output voltage
Switching frequency
Min efficiency (at Vin=90Vac, Pout=350W)
Max 2 fL output voltage ripple (peak to peak)
400V
70 kHz
94%
12.5V
The most significant test results of the board are shown in the
following figures.
Fig.7 – EVL4984-350W schematic
The power factor (PF), in full and half load conditions, is
greater than 0.9 over the entire range of Vin (Fig. 10), while for
70W load condition, the PF value decreases with the Vin increase.
Fig.10 – Power factor (PF) diagram
The test inductor current waveform reflects the requirements
of the LM-FOT method theory. At Vin = 115Vac and in full load
condition (Fig. 8), the boundary condition between CCM and
DCM operation takes place close to the zero crossing, and
therefore for most of the semi-sinusoid, the inductor works in
CCM mode.
The THD value is below 20% up to 230Vac and increases
with higher Vin (Fig. 11).
Fig.11 – THD diagram
Fig.8 – Inductor current envelope @ Vin=115Vac - full load
At Vin = 230Vac and in full load condition, instead, the
before mentioned boundary condition moves toward the peak of
the semi-sinusoid and so the CCM working mode happens only in
the related central area (Fig. 9)
Fig.9 – Inductor current envelope @ Vin=230Vac - full load
The converter efficiency meets the ES-2 regulations for all
load conditions and for all Vin values (Fig. 12).
Fig.12 – Efficiency diagram
The output voltage is very stable throughout the full Vin
range and for any load condition (Fig. 13).
More technical details about EVL4984-350W board can be
found in ST’s AN4163 application note.
Fig.13 – Output voltage regulation
As previously stated, it is possible to evaluate the L4984D
performance by software tools as well.
eDesignSuite, the on-line simulation tool, is primarily
oriented to the power management applications: LED lighting
(AC-DC and DC-DC), Power Supply (AC-DC and DC-DC),
Photovoltaic, Battery Charger, Filter and Antenna design.
The tool supports a lot of topologies based on the entire ST
product portfolio (controllers, MOSFETs, diodes, fuses, regulators,
etc.) included in the new L4984D. By inserting the main
specifications (Vin, Vout, Iout, etc.) of the converter and choosing
the controller that you want to adopt, eDesignSuite, in mere
seconds, is able to develop a complete design in all its parts: a fully
annotated and interactive schematic, a complete and interactive
Bill of Materials, a set of analysis diagrams (e.g the main current
and voltage simulations, the efficiency, the Bode stability, and the
power losses) and the transformer design.
By connecting to ST web site www.st.com/edesignsuite, after
simple online registering, it is possible to access the software tool
pictured below.

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