DN525 - Monolithic No-Opto Isolated Flyback Converter Provides Versatile Solution

Monolithic No-Opto Isolated Flyback Converter Provides
Versatile Solution
Design Note 525
George Qian
Introduction
Traditionally, isolated power supplies use an optocoupler in the feedback loop to transfer regulation
information across the isolation barrier. Unfortunately,
an opto-coupler’s gain characteristic varies widely over
temperature and lifetime, increasing the difficulty of
power supply loop compensation.
Linear Technology’s line of no-opto flyback converters,
such as the LT®3573, LT3574, LT3575, LT3511, LT3512
and LT8300, simplify flyback design by incorporating
a primary-side sensing scheme, thereby eliminating
the need for an opto-isolator.
The LT8302 monolithic no-opto flyback converter
has an integrated 65V/3.6A DMOS power switch with
internal loop compensation and soft-start. The LT8302
can operate from an input voltage as low as 2.8V to
42V, and deliver output power up to 18W. It offers
low ripple Burst Mode ® operation at light loads,
providing low standby power loss, and features a low
component count.
Performance and Ease of Use
The LT8302 simplifies the design of an isolated
flyback converter by sampling the isolated output
voltage directly from the primary-side flyback
waveform. This solution requires no third winding
or opto-coupler for regulation. The output voltage
is programmed with two external resistors and an
optional third temperature-compensation resistor.
By integrating loop compensation and soft-start,
the device minimizes the number of required external components, as shown in Figure 1. Boundary
mode operation allows the use of a modestly sized
transformer, while maintaining excellent load regulation. Low ripple Burst Mode operation yields high
efficiency at light load while minimizing the output
voltage ripple.
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
100
+*
39Ω 9µH
VIN
SW
EN/UVLO
10µF
LT8302
GND
•
1µH
•
VOUT–
INTVCC
115k
TC
10k
* 10mA TO 1.1A (VIN = 5V)
10mA TO 2.0A (VIN = 12V)
10mA TO 2.9A (VIN = 24V)
F01
Figure 1. Complete 5V Isolated Flyback Converter
for a 2.8V~32V Input
04/14/525
220µF
154k
RFB
RREF
1µF
VOUT
5V
3:1
470pF
95
90
EFFICIENCY (%)
VIN
2.8V TO
32V
VIN = 12V
85
80
VIN = 5V
75
70
65
60
55
50
10
100
1k
LOAD CURRENT (mA)
Figure 2. Efficiency of the Flyback
Converter in Figure 1
10k
F02
L1
12µH
Z1
RFB
C3
47µF
LT8302
INTVCC
C2
1µF
VIN
–4V TO
–42V
R4
118k
SW
EN/UVLO
C3
47µF
VOUT *
VIN
C1
10µF
D1
D1
RREF
R5
10k
GND
F01
* 12V/0.45A (VIN = –5V)
12V/0.8A (VIN = –12V)
12V/1.1A (VIN = –24V)
12V/1.3A (VIN = –42V)
D1: DIODES PMEG6030EP
L1: WÜRTH 744770112
Z1: CENTRAL CMHZ5243B
R1
806k
C1
10µF
R2
232k
L1
12µH
VIN
EN/UVLO
SW
Z1
VOUT
–12V
1.8A
R4
118k
LT8302
EN/UVLO RFB
VIN
–18V TO
–42V
C2
1µF
INTVCC
RREF
R5
10k
F04
D1: DIODES PMEG6030EP
L1: WÜRTH 744770112
Z1: CENTRAL CMHZ5243B
Figure 3. Negative Input to Positive Output
Buck-Boost Converter
Figure 4. Negative Input to Negative Output
Buck Converter
Figure 1 shows a complete LT8302 flyback schematic.
This converter has up to 85% efficiency, as shown in
Figure 2, and sustains 82% efficiency at a 10mA load
with a 5V input, thanks to its low quiescent current.
level-shift circuitry. LT8302’s unique feedback sensing
scheme can easily develop a regulated output voltage
directly through switch node voltage waveform, eliminating the level-shift circuitry that would otherwise
be required.
Low IQ, Low Preload and High Efficiency
At very light loads, the LT8302 reduces the switching
frequency while keeping the minimum current limit.
In these conditions, it enters low ripple Burst Mode
operation, where the part switches between sleep mode
and switching mode. The typical quiescent current is
106µA in sleep mode and 380µA in switching mode,
reducing the effective quiescent current.
The typical minimum switching frequency is about
12kHz, with the circuit requiring a very small preload
(typical 0.5% of full load). Therefore, the LT8302
power losses in standby mode are very low—a priority
for applications requiring high efficiency in always-on
systems.
Negative Input Power Supplies
In a typical negative input power supply, the IC’s
ground pin is connected to the negative input rail,
which is a varying voltage. As a result, its output
voltage changes with input voltage, assuming no
Data Sheet Download
www.linear.com/LT8302
Linear Technology Corporation
Figure 3 shows a simple negative-to-positive buckboost converter, and Figure 4 shows a simple negativeto-negative buck converter.
Conclusion
The LT8302 operates over an input voltage range of 2.8V
to 42V and delivers up to 18W of isolated output power,
requiring no opto-coupler or third winding. It integrates
a number of features to minimize component count,
including low ripple Burst Mode operation, internal
soft-start, undervoltage lockout, temperature compensation and internal feedback loop compensation.
The LT8302 is ideal for a broad range of applications,
from battery-powered systems to automotive, industrial, medical, telecommunications power supplies,
and isolated auxiliary/housekeeping power supplies.
The high level of integration yields an easy-to-use,
low component count, high efficiency and versatile
solution for isolated power delivery.
For applications help,
call (408) 432-1900, Ext. 3223
525 LT/AP 0414 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2014
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