AN1895
Application note
EVAL6562-375W Evaluation Board
L6562-based 375W FOT-controlled PFC Pre-regulator
Introduction
This application note describes a 375W evaluation board based on the L6562 Transition-
mode Power Factor Correction (PFC) controller (order code: EVAL6562-375W).
The board implements a 375W, wide-range mains input, PFC pre-regulator that is suitable
for a 300/350W ATX12V power supply unit (PSU).
To enable the use of a low-cost device like the L6562 at a power level that is usually
prohibitive for this device, the chip operates with a Fixed-Off-Time (FOT) control system.
This allows Continuous Conduction Mode operation, normally achievable with more
expensive control chips and more complex control architectures.
EVAL6562-375W evaluation board
August 2006
Rev 3
1/16
www.st.com
Contents
AN1895
Contents
1
2
3
4
Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Power stage design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Setting up FOT control with the L6562 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Getting started with the evaluation board . . . . . . . . . . . . . . . . . . . . . . . 9
4.1
Testbench results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . . 9
5
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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AN1895
Board description
1
Board description
The EVAL6562-375W evaluation board includes a Power Factor Correction (PFC) pre-
regulator for a 300W ATX 12V power supply unit (PSU). It is able to deliver 375W continuous
power on a regulated 400V rail from a wide range of mains voltage. This rail will be the input
for the cascaded isolated DC-DC converter (typically a forward converter) that will provide
the output rails of the silver box. Although the ATX specification envisages air cooling,
typically realized with a fan capable of an airflow in the range of 25-35 CFM, this is not
allowed for in the design of this evaluation board. Enough heat sinking will be provided to
allow full-load operation in still air. With an appropriate airflow and without any change in the
circuit, the evaluation board can easily deliver up to 400-420W.
The L6562 controller chip is designed for Transition-Mode (TM) operation where the boost
inductor works next to the boundary between Continuous (CCM) and Discontinuous
Conduction Mode (DCM). However, with a slightly different usage, the chip can operate so
that the boost inductor works in CCM, hence surpassing the limitations of TM operation in
terms of power handling capability. The gate-drive capability of the L6562 (±0.8A min.) is
also adequate to drive the MOSFETs used at higher power levels.
This approach, which couples the simplicity and cost-effectiveness of TM operation with the
high-current capability of CCM operation, is the Fixed-Off-Time (FOT) control. The control
modulates the ON-time of the power switch, while its OFF-time is kept constant. More
precisely, it will be used the Line-Modulated FOT (LM-FOT) where the OFF-time of the
power switch is not rigorously constant but is modulated by the instantaneous mains
voltage. Please refer to [2] for a detailed description of this technique.
Table 1
summarizes the electrical specification of the application and
Table 3
lists
transformer specifications.
The electrical schematic is shown in
Figure 1
and the PCB layout in
Figure 2.
Appendix A
lists the bill of materials.
Table 1.
Electrical specifications
Parameter
Line voltage range
Minimum line frequency (f
L
)
Regulated output voltage
Rated output power
Maximum 2f
L
output voltage ripple
Hold-up time
Maximum switching frequency (@ V
IN
= 90 V
AC
, P
OUT
= 375 W)
Minimum estimated efficiency (@ V
IN
= 90 V
AC
, P
OUT
= 375W)
Maximum ambient temperature
Value
90 to 265 V
AC
47 Hz
400 V
375 W
20V pk-pk
17 ms
100 kHz
90%
50° C
3/16
Board description
Figure 1.
Electrical schematic diagram
AN1895
Figure 2.
PCB layout, silk + bottom layer (top view) (150 x 81.5 mm)
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AN1895
Power stage design procedure
2
Power stage design procedure
The step-by-step procedure of an LM-FOT controlled PFC pre-regulator outlined in [2] will
be followed. The design will be done on the basis of a ripple factor (the ratio of the maximum
current ripple amplitude to the inductor peak current at minimum line voltage) Kr=0.3.
1.
The range of k (k
min
÷
k
max
) associated to the line voltage range is:
k
min
=
Vin
(RMS
)min
-
2
---------------------------------- =
Vout
90
-
2
--------- =
0.318, k
max
=
400
Vin
(RMS
)max
-
2
----------------------------------- =
Vout
265
-
2
--------- =
0.937
.
400
2.
The required t
OFFmin
is derived from the specification on the maximum switching
frequency (on the top of the line voltage sinusoid) f
swmax
at minimum line voltage:
k
min
0.318
-
t
OFFmin
= ------------------ = ----------------------- =
3.18µs
-
3
f
sw max
100
⋅
10
3.
The maximum expected input power Pin
0
= Pout
0
/η and the maximum line peak current,
Ipk
max
are:
2Pin
0
2
⋅
417
-
Pin
0
=
375
=
417W; Ipk
max
= ------------------------- = ---------------------------- =
6.56A
---------
-
k
min
Vout
0.318
⋅
400
0.9
.
4.
The ripple amplitude on the top of the sinusoid at minimum line voltage, assuming it is
75% of the maximum specified, will be:
6Kr
-
6
⋅
0.3
-
∆
I
Lpk
= ------------------
Ipk
max
= ------------------------
⋅
6.56
=
1.66A
8
–
3Kr
8
–
3
⋅
0.3
5.
The required inductance L of the boost inductor is:
–
6
Vout
400
-
-
L
=
(
1
–
k
min
)
-------------
t
OFFmin
=
(
1
–
0.318
) ⋅
----------
⋅
3.18
⋅
10
=
523µH
∆
I
Lpk
1.66
This value will be rounded up to 550 µH; the resulting value of Kr will be slightly smaller
than 0.3, but we will go on using the target value, this will give some additional margin.
6.
The maximum inductor peak current, I
Lpkmax
, is calculated:
8
-
8
-
I
Lpkmax
= ------------------
lpk
max
= ------------------------
⋅
6.56
=
7.39A
8
–
3Kr
8
–
3
⋅
0.3
7.
The maximum sense resistor R
sensemax
is:
1.6
-
1.6
-
R
sense max
= -------------------- = ---------- =
0.216Ω
I
Lpkmax
7.39
(1.6V is the minimum value of the pulse-by-pulse current limiting threshold on the
current sense pin of the L6562). It will be realized with four 0.68Ω, 1W-rated paralleled
resistors, for a total resistance of 0.17Ω. This provides some extra power capability. The
inductor peak current that the inductor must be able to carry without saturating will be:
1.8
-
I
Lpksat
= ---------- =
10.6A
0.17
8.
From the formulae in [2], table 4, the MOSFET RMS current is:
Pin
0
16k
min
417
-
16
⋅
0.318
-
-
I
Q
(
rms
)
= -------------------------
2
– ------------------ = ----------------------------
2
– ------------------------- =
3.96A
k
min
Vout
0.318
⋅
400
3
⋅π
3
⋅π
;
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