DN143 - Single IC, Power Factor Corrected, Off-Line Supply

advertisement
Single IC, Power Factor Corrected, Off-Line Supply
Design Note 143
Kurk Mathews
An ever increasing number of off-line power supplies now
include power factor correction (PFC) in order to reduce
input current and meet future regulatory requirements.
Switching power supplies that incorporate a bridge rectifier followed by bulk capacitance create harmonic currents. These harmonics only increase the supply’s RMS
input current, while contributing nothing to real power.
The typical solution to this problem has been to add a PFC
preregulator and a separate controller to an existing
design.
The LT®1508 (voltage mode) and LT1509 (current mode)
eliminate the need for separate controllers by combining
the PFC and a pulse width modulator (PWM) function in a
single 20-pin IC. PFC is achieved by programming the
input current of a boost regulator to follow the input line
voltage. This results in a near-unity power factor compared
to 0.5 to 0.7 for a typical capacitive input switcher. Linear’s
unique architecture maintains 0.99 power factor over a
20:1 load range. Figure 1 shows input current with output
powers of 30W, 150W and 300W.
A number of issues associated with a 2-IC approach are
addressed within the LT1508 and LT1509, resulting in a
2A/DIV
5ms/DIV
DN143 F01
Figure 1. Input Current at 30W, 150W and 300W
12/96/143
lower part count and improved PC board layout. Start-up is
controlled by separate PFC and PWM soft start pins. The
PWM Soft Start pin is held low, disabling the PWM output
until the PFC stage is in regulation. The PWM will remain
enabled as long as the PFC output voltage stays above 73%
of its preset value (typically 280V out of 383V for universal
input). A separate overvoltage protection pin can be connected to the output through an independent resistor
divider. This ensures overvoltage protection during safety
agency abnormal testing conditions, such as opening the
main feedback path. The two stages are synchronized and
the PWM turn-on is delayed for 50% of the oscillator cycle.
This minimizes noise and conducted emission problems.
2A peak current gate drivers and a 1.2V optoisolator offset
on the VC pin further simplify the design.
A universal input, 24VDC, 300W converter using the LT1508
is shown in Figure 2. The circuit’s user benefits include low
cost, customizable footprint and off-the-shelf magnetics
designed to meet UL and EC safety standards. Following
the PFC boost preregulator is a 2-transistor forward converter that features low voltage (500VDC) switches, low
peak currents and automatic nondissipative core reset.
Under worst case conditions (low line, full power), the PFC
and PWM stages have efficiencies of 90% and 92% respectively. See the graph in Figure 2 for typical overall efficiency
versus input line and output power. The LT1508’s low
start-up current (250µA) minimizes start-up resistor power
dissipation. An overwinding on T1 provides the bootstrapped
chip supply. The intermediate bus voltage of 382V is well
controlled, simplifying the post regulator and increasing
capacitor holdup time compared to a typical off-line converter. This results in lower transformer primary current
and simplified magnetics design. Different or additional
outputs are easily accommodated with this topology by
modifying T2 and L1.
, LTC and LT are registered trademarks of Linear Technology Corporation.
499k
1%
70
75
80
20k
1%
0.047µF
EMI
FILTER
9
11
132
3
2
180
VRMS
0.001µF
30W
100W
264
1µF
FILM
LT1508
13
SS2
4.02k
1%
R1
0.15Ω
5W
15V
+
16
SS1
200µF
17
VCC
1µF
FILM
–
+
+
+
20
VREF
2k
18
VC
ILIM 19
GTDR2
+
330µF
35V
FUJI
ERA82-004
0.6A/40VR
0.01µF
CNY17-3
100pF
1N5819
20k
15V
1N5819
20Ω
2.2µF
50V
2.2µF
50V
6
1
CAOUT GTDR1
ERA82-004
ERA82-004
T1
20k
0.047µF
7
ISENSE
0.001µF
300pF
4.02k
1%
91k
2W
T1
GND-F COMP
6
2
1
COLL
LT1431
2.2k
+
C1
1µF
400V
33Ω 0.068µF
1k
470Ω
IRFP450
MURH860CT
(DUAL)
–
+
GND-S
5
3
V+
RTOP
10Ω
7
REF 8
RMID
4
20Ω
10:15
TURNS
0.1µF
10Ω
1µF FILM
T3
470µF
450V
382VBUS
Figure 2. 24V, 300W Off-Line PFC Supply
NOTE: UNLESS OTHERWISE SPECIFIED
1. ALL RESISTORS 1/4W, 5%
2. ALL CAPACITANCE VALUES IN MICROFARADS
RT1 = KETEMA SG57 SURGE GARD
T1 = COILTRONICS CTX02-13141-X2, (407) 241-7876
T2 = COILTRONICS CTX02-13063-X3
T3 = BI TECHNOLOGY HM41-11510, (714) 447-2345
C1 = ELECTRONIC CONCEPTS 5MP12J105K
R1 = JW MILLER/FUKUSHIMA MPC71
BR1 = GENERAL INSTRUMENTS KBU6J
L1 = COILTRONICS CTX02-13140-X2
15k
15
RSET
5 8
PKLIM MOUT
10k
12
VREF
200W/300W
4
CSET
10
0.1µF
1.8k
BR1
VREF
0.001µF
VIN
RT1
VAOUT
330k
Efficiency
GND1
GND2
IAC
OVP
14 VSENSE
0.47µF
0.0047µF
90
85
90
20k
1%
499k
1%
499k
1%
382VBUS
499k
1%
VIN
EFFICIENCY (%)
90VAC TO
264VAC
+
6A
FAST
Danger!! Lethal Voltages Present
In This Section
+
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
–
Linear Technology Corporation
–
For literature on our Power Products,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2360
LT/GP 1296 185K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
 LINEAR TECHNOLOGY CORPORATION 1996
3.4k
1%
30.1k
1%
470µF, 50V
NICHICON
PL12,5X25
(× 3)
+
4700pF
"Y"
L1
MUR150
IRF840
OUTPUT COM
1µF
63V
FILM
+
+
20k
2200pF
"Y"
20k
15V
1N965
(× 2)
2000pF
10Ω,
2W
0.51Ω, 2W
RG ALLEN
RPS2
(× 2)
IRF840
MUR150
G1
FEP
30DP
(DUAL)
1000pF
10Ω
1W
24VOUT
12.5A
T2
Similar pages