STMicroelectronics AN1895 Eval6562-375w evaluation board Datasheet

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
5
Testbench results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2/16
AN1895
1
Board description
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.
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
Value
90 to 265 VAC
Minimum line frequency (fL)
47 Hz
Regulated output voltage
400 V
Rated output power
375 W
Maximum 2fL output voltage ripple
Hold-up time
20V pk-pk
17 ms
Maximum switching frequency (@ VIN = 90 VAC, POUT = 375 W)
100 kHz
Minimum estimated efficiency (@ VIN = 90 VAC, POUT = 375W)
90%
Maximum ambient temperature
50° C
3/16
Board description
Figure 1.
Electrical schematic diagram
Figure 2.
PCB layout, silk + bottom layer (top view) (150 x 81.5 mm)
4/16
AN1895
AN1895
2
Power stage design procedure
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 (kmin ÷ kmax) associated to the line voltage range is:
k min =
2.
Vin (RMS )min
2 ----------------------------------- =
Vout
90
2 ---------- = 0.318, k max =
400
Vin (RMS )max
2 ------------------------------------ =
Vout
265
2 ---------- = 0.937 .
400
The required tOFFmin is derived from the specification on the maximum switching
frequency (on the top of the line voltage sinusoid) fswmax at minimum line voltage:
k min
0.318 - = 3.18µs
t OFFmin = ------------------ = ----------------------3
f sw max
100 ⋅ 10
3.
The maximum expected input power Pin0 = Pout0/η and the maximum line peak current,
Ipkmax are:
2Pin 0
2 ⋅ 417 - = 6.56A
= ---------------------------Pin 0 = 375
---------- = 417W; Ipk max = ------------------------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, ILpkmax, is calculated:
8
8
I Lpkmax = ------------------- lpk max = ------------------------- ⋅ 6.56 = 7.39A
8 – 3Kr
8 – 3 ⋅ 0.3
7.
The maximum sense resistor Rsensemax is:
1.6 - = ---------1.6- = 0.216Ω
R sense max = -------------------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- = 10.6A
I Lpksat = ---------0.17
8.
From the formulae in [2], table 4, the MOSFET RMS current is:
Pin 0
16k min
417
16 ⋅ 0.318
- = ----------------------------- 2 – -------------------------- = 3.96A
I Q ( rms ) = ------------------------- 2 – -----------------k min Vout
0.318 ⋅ 400
3⋅π
3⋅π
;
5/16
Power stage design procedure
AN1895
The diode RMS current is:
Pin 0
16k min
417
⋅ 0.318- = 2.41A
- = ----------------------------- 16
------------------------I Q ( rms ) = ------------------------- -----------------0.318 ⋅ 400
k min Vout 3 ⋅ π
3⋅π
The dissipation on the sense resistor will be 0.17·3.962=2.7W, which justifies the use of
four resistors; the selected MOSFET is the STP12NM50, a 0.3Ω/500V MDmesh™ type
from STMicroelectronics, housed in a TO220 package; two of them will be paralleled to
handle the rated power; the selected diode is an STTH806DTI, an 8A/600V Tandem
diode, again from STMicroelectronics, housed in a TO220 package. All of them must be
dissipated to keep their temperature within safe limits.
As for the inductor, the core size will be determined by saturation since the ripple is
relatively low. Assuming a peak flux density Bmax=0.3T, the minimum required AreaProduct is:
⎛ 1 – k min Kr Pin 0 t OFF⎞
AP min ≈ 186 ⎜ ---------------------------- -------------------------⎟
B max ⎠
⎝ k min Kr
1.31
– 6 1.31
1 – 0.318 ⋅ 0.3 417 ⋅ 3.18 ⋅ 10
= 186 ⎛⎝ ------------------------------------- -------------------------------------------⎞⎠
0.3
0.318 ⋅ 0.3
4
= 2.92 [cm ]
An E42 core (AP = 3.15 cm4) has been chosen. See table 3 for the complete inductor
spec.
The output capacitor is determined by the hold-up time requirement. Assuming a
minimum voltage of 300V after the line drop, a minimum of 180 µF is needed and a
220µF/450V capacitor will be used.
9.
The peak multiplier bias voltage VMULT @90V mains must meet the condition:
I Lpkmax R sense
V in ( RMS )min
----------------------------------------- ≤ V MULTpk ≤ 3 ---------------------------------1.65
V in ( RMS )max
,
where 1.65 is the minimum slope of the multiplier characteristic associated to the error
amplifier saturated high (see Figure 9 in [1]). With the selected value for Rsense
(0.17 Ω):
907.39 ⋅ 0.17 = 0.761 ≤ V
----------------------------------MULTpk ≤ 3 ⋅ 265 = 1.02
1.65
Choosing the ratio of the resistor divider that biases the multiplier input (pin 3, MULT)
KP = 8·10-3 lets the peak voltage on the multiplier pin will go from 1V to 3V, thus
meeting the above condition.
The high-side resistor of the output divider that sets the output voltage is chosen on the
basis of the maximum allowed overvoltage on the output. Considering 40V overvoltage,
the high-side resistor must be 1MΩ (see [3] for details, in this respect the L6561 and
the L6562 are exactly equal). The low-side resistor will then be 1MΩ · (400/2.5 - 1)
= 6.29kΩ; the 6.34kΩ standard value will be used.
Based on the model given in [2], a compensation network made with an RC series
(R = 6.8 kΩ, C = 1 µF) guarantees a minimum of 25° phase margin (with 9 Hz bandwidth) at
minimum line and a bandwidth not exceeding 20 Hz (with 50° phase margin) at maximum
line.
6/16
AN1895
3
Setting up FOT control with the L6562
Setting up FOT control with the L6562
FOT control is implemented with the L6562 using the circuit shown in Figure 3, which shows
some significant waveforms as well. Before starting the design, the desired value of tOFF at
the maximum line voltage must be specified. Application note AN1792 shows that to reduce
high-voltage distortion, tOFF must be greater than 7µs, hence we choose tOFF = 8µs.
Figure 3.
Circuit implementing FOT control with the L6562 and relevant timing waveforms.
Following the design procedure given in AN1792, with the aid of the diagram of Figure 4:
1.
The ratio of the maximum tOFF value to the minimum tOFF value is:
–6
8 ⋅ 10 - = 2.52
ρ = --------------------------–6
3.18 ⋅ 10
2.
Consider the value of VMULTpk at minimum line voltage (VMULTpk = 1V), in Figure 4 draw
a horizontal line located at ρ = 2.52 (on the left vertical axis) as long as it intercepts the
ρ curve relevant to the value VMULTpk=1V in P1. The abscissa of P1 gives the value
K1=0.891.
3.
From P1 draw a vertical line as long as it intercepts the K2 curve relevant to VMULTpk=1
in P2. The ordinate of P2 (on the right vertical axis) gives the value K2=4.17.
4.
The required time constant is:
–6
t OFFmin
–6
3.18 ⋅ 10
τ = --------------------= --------------------------- = 0.76 ⋅ 10 s
K2
4.17
5.
.
A capacitor C = 560 pF is selected, then the associated resistance value will be:
–6
τ- = ---------------------------0.76 ⋅ 10 - = 1357Ω
R' = --– 12
C
560 ⋅ 10
6.
.
R1 and R2 will be respectively:
R' - = ----------------------1357 - = 12450Ω
R1 = ---------------1 – K1
1 – 0.891
R'- = -------------1357- = 1523Ω
R2 = -----K1
0.891
the standard values R1 = 12kΩ and R2 = 1.5kΩ will be chosen.
7/16
Setting up FOT control with the L6562
7.
AN1895
Assuming that the VCC voltage never falls below 14-15V, the limiting resistor Rs can be
selected according to:
V GDx – V ZCDclamp – V F
Rs > -------------------------------------------------------------------------------------------------------------------------------------------------------------------------V ZCDclamp R2 + ( V ZCDclamp – V MULTpkmax – V BE )R1
I ZCDx + ---------------------------------------------------------------------------------------------------------------------------------------------------R1R2
where VGDx = 15V is the maximum clamp value of the gate drive voltage, VZCDclamp ≈
5.7V is the clamp value of the ZCD pin voltage, VF ≈ 0.5V the forward drop on the
diode, IZCDx = 10mA the maximum ZCD clamp current D and VBE ≈ 0.55V the emitterto-base forward drop of T. Substituting:
15 – 5.7 – 0.5
Rs > ---------------------------------------------------------------------------------------------------------------------------- = 739Ω
– 3 5.7 ⋅ 1500 + ( 5.7 – 3 – 0.55 ) ⋅ 12000
10 ⋅ 10 + ---------------------------------------------------------------------------------------------1500 ⋅ 12000
in this case a 1.5kΩ resistor will be chosen
8.
Cs will be selected according to the relationship:
V ZCDclamp
– 12
5.7
---------------------------------- = 363pF
Cs < C ---------------------------------------------------------------- = 560 ⋅ 10
V GDx – V ZCDclamp – V F
15 – 5.7 – 0.5
;
a standard value Cs=330 pF will be used.
Figure 4.
8/16
Diagrams for the design of the circuit of Figure 3 (valid for wide-range
mains operation).
AN1895
4
Getting started with the evaluation board
Getting started with the evaluation board
The AC voltage, generated by an AC source ranging from 90 to 265 VAC, will be applied to
the input connector, located close to the bottom left-hand corner.
The 400 VDC output is located close to the bottom right-hand corner and will be connected
to the load. If an electronic load is going to be used pay attention to the right polarity: the (+)
output terminal is that located closer to the corner.
Warning:
Like in any offline circuit, extreme caution must be used
when working with the application board because it contains
dangerous and lethal potentials.
The application must be tested with an isolation transformer
connected between the AC mains and the input of the board
to avoid any risk of electrical shock.
4.1
Testbench results and significant waveforms
The following diagrams summarize the results of certain testbench evaluations. A number of
waveforms under different load and line conditions are shown for user's reference.
Figure 5.
Evaluation data
9/16
Getting started with the evaluation board
Figure 6.
Compliance with JEIDA-MITI & EN61000-3-2 standards
Figure 7.
Harmonic emissions at light load (70W)
Figure 8.
Line current waveforms @ POUT = 375W
10/16
AN1895
AN1895
Getting started with the evaluation board
Figure 9.
Line current waveforms @ POUT = 180W
Figure 10. Line current waveforms @ POUT = 70W
Note:
1
Note that input LC filter is provided only to clean the line current waveform enough to
prevent the measurement system from being misled by an excessive noise level. The filter is
not designed nor tested for EMI compliance.
2
The board, as is, is able to handle properly an output load as low as 15 W. With lower load
levels, the system will not start up correctly at low line because the OVP generated at startup lasts so long that the VCC voltage drops below the UVLO of the L6562 (e.g. with 4W load
the system would stop for 600 ms @ VIN = 90VAC). Load transients from the maximum load
to levels below 15W may cause the VCC to be lost as well. If supplying the L6562 with an
external source, the minimum load that can be handled properly drops to virtually zero.
11/16
References
5
AN1895
References
[1] "L6562 Power Factor Corrector" Datasheet.
[2] "Design of Fixed-Off-Time-Controlled PFC Pre-regulators with the L6562", AN1792.
[3] “L6561, Enhanced Transition-Mode Power Factor Corrector”, AN966.
12/16
AN1895
Bill of materials
Appendix A
Table 2.
Bill of materials
Bill of material for EVAL6562-375W evaluation board
Symbol
Value
R1A and R1B
120 kΩ
R2A and R2B
620 kΩ
R3
10 kΩ
R4
47 Ω
R5
6.8 kΩ
R6 and R13
6.8 Ω
R7A, R7B, R7C and R7D
0.68 Ω
R8 and R12
1.5 kΩ
R9
12 kΩ
R10A and R10B
499 kΩ
R11
6.34 kΩ
R14
330 Ω
CX1 and CX2
330 nF
C1
1 µF
C2 and C4
10 nF
C3
47 µF
C5
1 µF
C6 and C10
330 pF
C7
560 pF
C8
470 nF
Note
Metal film, 1W
EPCOS B81131 or equivalent
400V, EPCOS B32653 or equivalent
25V electrolytic
630V, EPCOS B32653 or equivalent
C9
220 µF
450V, Electrolytic Nichicon LS or equivalent
L1
TOR73
3.9 mH / 6A, supplied by ITACOIL s.r.l.
T1
E4218
Boost inductor. See spec on table 3. Supplied by ITACOIL s.r.l.
BRIDGE
KBU8M
8A / 1000V, GI or equivalent
D1, D2, D3 and D4
1N4148
0.3A / 75V, glass case, Vishay or equivalent
D5
1N5406
3A / 600V, D0201, ON Semiconductor or equivalent
D6
STTH806DTI
DZ1
1N5248B
U1
M1 and M2
Note:
L6562
8A / 600V, Tandem Hyperfast, TO220, ST
18V / 500mW Zener, ON Semiconductor or equivalent
TM PFC controller, DIP8, ST
STP12NM50FP 0.3 Ω / 500V, MDmesh TM, TO220FP, ST
TR1
BC557
PNP, 0.1A / 45V, TO92, On Semiconductor or equivalent
NTC1
BS237
NTC 2.5 Ω, EPCOS or equivalent
F1
---
8A, 250V
PCB
---
FR-4, Cu single layer 35µm, 150 x 81.5 mm
Heat sink
OS512
2.12 °C/W, Extrusion Profile, Aavid Thermalloy
If not otherwise specified, all resistors are 1%, ¼ W, all capacitors are ceramic or plastic
film.
The Bridge, M1, M2 and D6 all share the same heat sink.
13/16
Bill of materials
Table 3.
Core
AN1895
EVAL6562-375W: boost inductor specification (part# E4218, made by ITACOIL s.r.l.)
E42/21/7, N67 Material or equivalent
Bobbin
Horizontal mounting, 10 pins
Air gap
≈ 1.9 mm for an inductance 1-10 of 550 µH
Windings
Pin
Start/End
Winding
Wire
Spec &
Build
1/10
Main
20xAWG32
6/5
Aux
AWG32
14/16
Turns
Notes
58 (N1) Pins 1 & 2, pins 10 & 9 are shorted on the PCB
6 (N2)
Evenly spaced
AN1895
Revision history
Revision history
Table 4.
Document revision history
Date
Revision
Changes
20-May-2004
2
Initial release.
28-Aug-2006
3
Corrected Equation 1. on page 5. Minor editing changes.
15/16
AN1895
Please Read Carefully:
Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the
right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any
time, without notice.
All ST products are sold pursuant to ST’s terms and conditions of sale.
Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no
liability whatsoever relating to the choice, selection or use of the ST products and services described herein.
No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this
document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products
or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such
third party products or services or any intellectual property contained therein.
UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED
WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS
OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT.
UNLESS EXPRESSLY APPROVED IN WRITING BY AN AUTHORIZED ST REPRESENTATIVE, ST PRODUCTS ARE NOT
RECOMMENDED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING
APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY,
DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. ST PRODUCTS WHICH ARE NOT SPECIFIED AS "AUTOMOTIVE
GRADE" MAY ONLY BE USED IN AUTOMOTIVE APPLICATIONS AT USER’S OWN RISK.
Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void
any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any
liability of ST.
ST and the ST logo are trademarks or registered trademarks of ST in various countries.
Information in this document supersedes and replaces all information previously supplied.
The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners.
© 2006 STMicroelectronics - All rights reserved
STMicroelectronics group of companies
Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America
www.st.com
16/16