ST AN1895 Application note
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 Transitionmode 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 Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Power stage design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Setting up FOT control with the L6562 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 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) preregulator 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.
Ta bl e 1 summarizes the electrical specification of the application and Ta ble 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 Value
Line voltage range 90 to 265 V
Minimum line frequency (fL)47 Hz
Regulated output voltage 400 V
Rated output power 375 W
Maximum 2f
Hold-up time 17 ms
Maximum switching frequency (@ V
Minimum estimated efficiency (@ V
Maximum ambient temperature 50° C
output voltage ripple 20V pk-pk
L
= 90 VAC, P
IN
= 90 VAC, P
IN
= 375 W) 100 kHz
OUT
= 375W) 90%
OUT
AC
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Board description AN1895
Figure 1. Electrical schematic diagram
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
Vin
k
min
---------------------------------- -
2
÷ k
min
(RMS)min
Vout
) associated to the line voltage range is:
max
2
90
--------- -
400
0.318, k
max
Vin
(RMS)max
------------------------------------
2
Vout
2
265
--------- -
400
0.937======
.
2. The required t
frequency (on the top of the line voltage sinusoid) f
3. The maximum expected input power Pin
Ipk
are:
max
Pin
is derived from the specification on the maximum switching
OFFmin
t
OFFmin
375
--------- - 417W; Ipk
0
0.9
k
min
-------------------
f
sw max
max
0.318
----------------------- - 3.18µs== =
⋅
100 10
= Pout0/η and the maximum line peak current,
0
2Pin
0
-------------------------
k
Vout
min
at minimum line voltage:
swmax
3
2417⋅
----------------------------- 6.56A== = = =
0.318 400⋅
.
4. The ripple amplitude on the top of the sinusoid at minimum line voltage, assuming it is
75% of the maximum specified, will be:
Lpk
------------------ -
83Kr–
Ipk
max
I
∆
6Kr
60.3⋅
-------------------------
830.3⋅–
6.56⋅ 1.66A== =
5. The required inductance L of the boost inductor is:
L1k
–()
min
Vout
------------- -
I
∆
Lpk
t
OFFmin
400
1 0.318–()
-----------
⋅⋅⋅ 523µH== =
1.66
3.18 10
6–
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
8
I
Lpkmax
------------------ -
83Kr–
lpk
max
, is calculated:
Lpkmax
-------------------------
830.3⋅–
8
6.56⋅ 7.39A== =
7. The maximum sense resistor R
R
sense max
sensemax
is:
1.6
-------------------- -
I
Lpkmax
1.6
----------- 0.216Ω===
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:
I
Lpksat
1.8
----------- 10.6A==
0.17
8. From the formulae in [2], table 4, the MOSFET RMS current is:
I
Qrms()
Pin
0
-------------------------
k
Vout
min
16k
min
2
------------------ -–
3 π⋅
417
-----------------------------
0.318 400⋅
16 0.318⋅
2
------------------------- -– 3.96A== =
3 π⋅
5/16
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