DN410 - Flyback Controller Simplifies Design of Low Input Voltage DC/DC Converters

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Flyback Controller Simplifies Design of Low Input Voltage DC/DC
Converters – Design Note 1016
David Burgoon
Introduction
Small, high efficiency DC/DC converters are critical to
the design of leading-edge electronics. Achieving high
accuracy and efficiency traditionally means adding extra
components, complexity, and size. Not so with the LT3837.
This flyback controller serves 10W to 60W isolated applications with high performance, simplicity, small size,
and a minimum component count.
blanking yields a high performance loop that is easy to
compensate.
High-Efficiency Controller Capabilities
The LT® 3837 operates from a 4.5V to 20V input, but the
converter input range can be extended upwards by using
a VCC regulator and/or a bias winding on the transformer.
It also provides a synchronous rectifier output with
adjustable timing to optimize efficiency and enhance
cross-regulation in multiple-output supplies.
3.3V, 10A Converter Operates from a 9V to18V
Source
The circuit shown in Figure 1 is a flyback design for a 3.3V,
10A output from a 9V to 18V input with a minimum of
external components. The LT3837 samples the voltage on
the primary winding during the flyback interval to provide
superb regulation. Figure 2 shows ruler-flat regulation at
9V input, and a tight regulation window of ±0.7% over
line and load. Synchronous rectification with adjustable
The LT3837 eliminates the need for the traditional secondary-side reference, error amplifier, and optoisolator
circuits by sampling the flyback voltage on a primary-side
winding. Accuracy is enhanced with output resistance
compensation. Current mode control with leading edge
VIN
9V TO 18V
The operating frequency is adjustable from 50kHz to
250kHz or can be synchronized to an external clock.
Soft-start provides well controlled start-up with limited
inrush current. Protection features include current limit
with soft-start cycling for severe overloads, undervoltage
lockout, and thermal shutdown.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
IHLP2525CZERR10M
0.1µH
T1
2.2nF
22µF
25V
•
47µF × 3
BAS16
22.1k
1%
10
•
1nF
2N3906
VOUT
3.3V AT 10A
220µF
6TPE220MI
B0540W 10
Si7852DP
1µF
1µF
SG
86.6k
1%
4.7
SG
VCC
FB
UVLO
15k
1%
PG SENSE+
SENSE–
U1
LT3837EFE
12k
100k
1.37k
1%
SG
330
VC
PGDLY
tON SYNC ROCMP ENDLY OSCAP SGND/PGND SFST CCMP
3.01k
1%
Si7336ADP
0.008
0.5W
15
0.1µF
2.2nF
3.3nF
•
150k
20k
47pF
0.1µF
PA0184
1nF
•
10k
BAT54
DN1016 F01
Figure 1. Low Parts Count, 9V to 18V Input to 3.3V/10A Output Isolated Flyback Converter with ±0.7% Regulation
12/06/1016
timing yields excellent efficiency—88% over a wide range
of operating conditions—as shown in Figure 3.
3.3V, 10A Converter Operates from a 9V to 36V Source
Figure 4 shows an enhanced circuit that extends the input
operating range of the LT3837 to 9V to 36V. Operation is
converted to hysteretic start-up for efficient wide-range
operation. Q1 provides a low-drop current source for
start-up, and Q2 creates a suitable undervoltage circuit
for VCC. These circuits, together with the VCC winding
on the transformer, result in low VCC power at higher
input voltages and low dissipation cycling when operating into a short circuit. This circuit is implemented in a
1.5in2 footprint. This circuit exhibits excellent regulation
of ±1.2% over line and load, and efficiency of 88% over
much of its operating range.
Conclusion
The LT3837 is part of a new class of flyback controllers
developed by Linear Technology to satisfy the demand
for economical, high performance power converters. It
provides synchronous rectifier drive and eliminates the
need for secondary regulation circuits and optoisolators.
The LT3837 makes it easy to implement high performance Flyback designs that are cost effective, small
and efficient.
90
3.6
9VIN
88
3.5
86
EFFICIENCY (%)
OUTPUT (V)
3.4
9VIN
3.3
18VIN
3.2
84
18VIN
82
80
3.1
78
76
3.0
2
3
4
8
7
6
5
LOAD CURRENT (A)
2
10
9
4
3
5
6
7
8
10
9
LOAD CURRENT (A)
DN1016 F03
DN1016 FO2
Figure 2. Regulation of the Converter in Figure 1
Figure 3. Efficiency of the Converter in Figure 1
4.7nF
250V
VIN
9V TO 36V
1µH
+
100µF
50V
–VIN
9V TO 36V
0.1µH
3.3µF × 3
50V
BAT43WS
100k
•
316
470pF
Q1
MBT3906DW1T1
VCC
BAS21
121
100k
33
EFD20
LP = 5µH
(N = 6 : 2)
PA1835NL
•
1nF
SG
VCC
100k
24.9k
0.1µF
HAT2173H
4.64k
SG
100k
21.5k
UVLO
MBT3904DW1
BZT52C6V2S
Q2
17.4k
100k
470pF
47k
•
PA0184
1.07k
•
BAT54WS
10k
SENSE–
VC
PGDLY
tON SYNC ROCMP ENDLY OSCAP SGND/PGND SFST CCMP
3.01k
12.1k
220k
U1
LT3837EFE
15
4.7nF
PG SENSE+
7m
1µF
FMMT718
0.1µF
SG
VCC
FB
10
FMMT618
Si7336ADP
4.7
120
33µF
16V
BAT54WS
CMPSH1-4
VOUT
3.3V AT 10A
220µF
6.3V
–VOUT
3.3V AT 10A
•
VCC
+
+
47µF × 3
6.3V
147k
1.5nF
2.15k
47pF
0.1µF
220pF
DN1016 F04
Figure 4. Wide Range, 9V to 36V to 3.3V/10A Isolated Flyback Converter with ±1.2% Regulation
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2134
www.linear.com
Linear Technology Corporation
dn1016f LT 1206 • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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