DN509 - Micropower Isolated Flyback Converter with Input Voltage Range from 6V to 100V

Micropower Isolated Flyback Converter with Input Voltage
Range from 6V to 100V
Design Note 509
Zhongming Ye
Introduction
Flyback converters are widely used in isolated DC/DC
applications because of their relative simplicity and
low cost compared to alternative isolated topologies. Even so, designing a traditional flyback is not
easy—the transformer requires careful design, and
loop compensation is complicated by the well known
right-half plane (RHP) zero and the propagation delay
of the opto-coupler.
Linear Technology’s no-opto flyback converters, such
as the LT ® 3573, LT3574, LT3575, LT3511 and LT3512,
simplify the design of flyback converters by incorporating a primary-side sensing scheme and running the
converter in boundary mode. The LT8300 high voltage
monolithic isolated flyback converter further simplifies
flyback design by integrating a 260mA, 150V DMOS
power switch, an internal compensation network and a
soft-start capacitor. The LT8300 operates with input supply voltages from 6V to 100V and delivers output power
of up to 2W with as few as five external components.
The LT8300 operates in boundary mode and offers low
ripple Burst Mode® operation, enabling the design of
converters that feature high efficiency, low component
count and minimal power loss in standby.
Simple and Accurate Primary-Side Voltage Sensing
The LT8300 eliminates the need for an opto-coupler
by sensing the output voltage on the primary side
when the output diode current drops to zero during the
primary switch-off period. This greatly improves the
load regulation since the voltage drop is zero across
the transformer secondary winding and any PCB traces.
This allows an LT8300-based flyback converter to produce ±1% typical load regulation at room temperature.
Figure 1 shows the schematic and Figure 2 the load
regulation curves of a flyback converter with a 5V output.
Very Small Size, Low Component Count Solution
The LT8300 integrates a 260mA, 150V DMOS power
switch along with all high voltage circuitry and control
logic into a 5-lead TSOT-23 package. The isolated
output voltage is set via a single external resistor with
compensation and soft-start circuitry integrated in the
IC. Low ripple Burst Mode operation maintains high
efficiency at light loads while minimizing the output
voltage ripple.
The converter turns on the internal switch immediately
after the secondary diode current reduces to zero, and
turns off when the switch current reaches the predefined
current limit; the diode has no reverse-recovery loss.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
5.1
7:1
2.2µF
VIN
806k
LT8300
EN/UVLO
53.6k
•
47µF
•
VOUT–
SW
365k
GND
RFB
F01
Figure 1. A Complete 5V Flyback Converter
for a 22V to 75V Input
12/12/509
VOUT
5V,
1mA TO
250mA
OUTPUT VOLTAGE (V)
VIN
22V TO
75V
VIN = 22V
5.0
+
4.9
4.8
4.7
4.6
4.5
0
0.05
0.15
0.2
0.1
OUTPUT CURRENT (A)
0.25
F02
Figure 2. Regulation of a 22V to 75V Input to
5V Flyback Converter of Figure 1
The LT8300 features an accurate minimum current limit
and very small propagation delay. At very light loads, it
further reduces the loss by running in low ripple Burst
Mode operation, where the part switches between sleep
mode and active mode. The typical quiescent current
is 70µA in sleep mode and 330µA in switching mode,
reducing the effective quiescent current.
Figure 3. Demonstration Circuit of 22V to 75V
to 5V/0.3A Converter (See Figure 1)
Furthermore, since the switch is turned on with zero
current, switching losses are minimized. The reduction in power losses allows the converter to operate
at a relatively high switching frequency, which in turn,
allows the use of a smaller transformer than would be
required at a lower operating frequency. Overall, the
LT8300 significantly reduces converter size compared
to other solutions.
Figure 3 shows the standard demo circuit DC1825A for
an isolated flyback using a small EP7 core transformer.
The six key components are the input and output capacitors (C2, C3), output diode (D1), feedback resistor
(R3), transformer (T1) and the LT8300. For the same
application, a traditional flyback circuit would require,
at minimum, eleven additional components, plus complicated start-up and bias power circuits in both the
primary and secondary sides.
Low IQ, Small Preload and High Efficiency
As the load lightens, the LT8300 reduces the switching
frequency until the minimum current limit is reached,
and the converter then runs in discontinuous mode.
1:1
•
VIN
2.2µF
EN/UVLO
VOUT+
24V
20mA
VOUT–
SW
243k
GND
The LT8300 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 a simple, low parts-count
solution for low power flyback converters.
100
90
RFB
F04
Data Sheet Download
www.linear.com/8300
Linear Technology Corporation
VIN = 6V
70
60
50
40
30
20
10
0
Figure 4. Flyback Converter Optimized for
Low Standby Power (6V–12V to 24V/20mA)
VIN = 12V
80
2.2µF
•
LT8300
Conclusion
The LT8300 is an easy-to-use flyback converter with a
rich set of unique features integrated in a small 5-lead
TSOT-23 package. It accepts a wide input voltage range,
from 6V to 100V, with very low shutdown current and
standby power consumption. Boundary mode operation
reduces switching loss, shrinks converter size, simplifies
system design and offers superior load regulation. Other
features, such as internal soft-start, accurate current
limit, undervoltage lockout and internal loop compensation further facilitate an easy flyback converter design.
EFFICIENCY (%)
VIN
6V TO
12V
The typical minimum switching frequency is about
7.5kHz, with the circuit requiring a very small preload
(typical 0.5% of full load). Therefore, LT8300 power
losses in standby mode are very low—important for
applications requiring high efficiency in always-on
applications. Figure 4 shows a solution that produces
20mA at 24V from a 12V input. Efficiency peaks at
87%, and remains high at 84% with a 20mA load, as
shown in Figure 5.
0
5
15
20
10
LOAD CURRENT (mA)
25
30
F05
Figure 5. Efficiency of the Converter in Figure 4
For applications help,
call (408) 432-1900, Ext. 3798
509 LT/AP 1212 196K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2012
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