DN1011 - Simple High Efficiency Isolated Flyback Supply Without Optoisolator

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Simple High Efficiency Isolated Flyback Supply Without
Optoisolator – Design Note 1011
Ryan Huff
Introduction
While simplicity and high efficiency (for cool running) are
no longer optional features in isolated power supplies, it
is traditionally difficult to achieve both. Achieving high
efficiency often requires the use of advanced topologies
and home-brewed secondary synchronous rectification
schemes once reserved only for higher power applications. This only adds to the parts count and to the design
complexity associated with the reference and optocoupler
circuits typically used to maintain isolation. Fortunately, a
breakthrough IC makes it possible to achieve both high efficiency and simplicity in a synchronous flyback topology.
The LT® 3825 simplifies and improves the performance of
low voltage, high current flyback supplies by providing
precise synchronous rectifier timing and eliminating the
need for optocoupler feedback while maintaining excellent
regulation and superior loop response.
current of 12A compares favorably to that of a higher
parts count forward converter design. This is primarily
the result of a simple, well-controlled implementation of
synchronous rectification. Furthermore, due to its high
efficiency operation, the greatest temperature rise of any
component is only 60°C above the ambient temperature
with a paltry 100LFM of airflow.
The use of the feedback winding to regulate the output
voltage vs an optocoupler and secondary-side reference
yields great results. The regulation curve shown in Figure
3 shows that ±1.25% is easily attainable. Figure 4 shows
the supply’s transient response for a 9A to 12A load step
at a 10A/µs slew rate with a 48V input. With this 25%
load step, the output voltage remains within ±3% of its
set point.
LT3825 Operation
Instead of using a parts intensive secondary-side voltage
reference and error amplifier to drive an optocoupler, the
primary bias winding on the flyback transformer (T1) is
High Performance 48V to 3.3V, 12A Supply
The circuit in Figure 1 shows an isolated, no-opto, synchronous flyback, 48V to 3.3V at 12A supply. Figure 2
shows its efficiency. The converter’s efficiency of over
88% at the nominal input voltage of 48V and an output
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
L1
0.1µH
VIN
36V TO 72V
T1
28.7k
T1
•
3.01k
C6
47µF
402k
FB
2.2µF
Si4490DY
SG
SG
PG
LT3825EFE
100k 750Ω
C4
470µF
47Ω
Si7336ADP
Q2
SG
1µF
0.02Ω
150k
47pF
330Ω
SENSE–
VC
PGDLY
SGND
tON SYNC ROCMP ENDLY OSCAP PGND SFST
12k
Q1
SENSE+
UVLO
15k
•
20Ω
+
VCC
•
100pF
250V
D1
BAS21
R1
R2
47k
20Ω
C1 TO C3
47µF ×3
CCMP
0.22µF
0.1µF
15Ω
2.2nF
0.1µF
10nF
10k
•
PA0184
•
10k
BAT54
1nF
DN1011 F01
C1 TO C3: TDK C3225X5R0J476M
C4: SANYO 6TPD470M
L1: VISHAY IHLP2525CZERR10M
Q1: ZETEX FMMT618
Q2: ZETEX FMMT718
T1: PULSE PA1477NL
Figure 1. Simple, High Efficiency 36VIN-72VIN to 3.3VOUT at 12A Synchronous Flyback
12/05/1011
VOUT
3.3V
12A
90
36VIN
EFFICIENCY (%)
88
IOUT
2A/DIV
48VIN
86
72VIN
84
82
VOUT
100mV/DIV
80
78
2
3
4
9
5
6 7 8
LOAD CURRENT (A)
10 11 12
100µs/DIV
DN1011 F04
DN1011 F02
Figure 2. Efficiency of Circuit in Figure 1
OUTPUT VOLTAGE REGULATION (%)
4
Figure 4. 9A to 12A to 9A Load Current Step (Top Trace)
and Output Voltage (Bottom Trace) Response for Circuit
in Figure 1
complicated, discrete timing circuits allows the designer
to set optimum dead-times since this timing is well controlled within the LT3825. The LT3825 also precludes
the need for a secondary-side synchronous controller
IC and its associated circuitry.
VOUT = 3.3V
3
2
36VIN
1
48VIN
0
72VIN
–1
–2
–3
–4
4
2
6
8
CURRENT (A)
10
12
DN1011 F03
Figure 3. Output Voltage Regulation of Circuit in Figure 1
used (see Figure 1). Proprietary feedback circuitry inside
the LT3825 reads the reflected output voltage information
on this winding during the flyback pulse. This voltage
is then compared to a precision internal reference and
an error signal is obtained. The error signal is used to
modulate the on-time of Q1 in such a way as to regulate
the output voltage. An important benefit of this technique
is that output voltage information arrives at the controller instantly after the switching cycle is terminated. In a
conventional optocoupler-based design, delays of tens to
hundreds of microseconds occur in the optocoupler alone,
severely limiting the converter’s transient response.
The synchronous rectifier output (SG pin) of the LT3825
makes driving the synchronous rectifier MOSFET
(Q2) simple while maintaining a low parts count. Setting the dead-time of Q2 relative to Q1 only requires
one resistor to program. Avoiding traditional, more
Additional Features
An optional, resistor programmable, input undervoltage
lockout is available. The slew rate of the output voltage
during start-up can be controlled with an optional softstart capacitor; this also limits the inrush current of
the input power supply. Since the LT3825 incorporates
current mode control, both short-circuit behavior and
ease of loop compensation are improved over voltage
control schemes. Switching frequency is selectable
from 50kHz to 250kHz optimizing the solution size vs
efficiency. The switching frequency can be synchronized
to an external system clock for further flexibility.
Related Controller
For lower input voltages (5V to 18V) and an even simpler
design, the LT3837 is available to compliment the LT3825.
The LT3837 starts up and runs with the input voltage
connected directly to the VCC pin, so no bias supply is
needed, eliminating components D1, C6, R1, and R2 from
the schematic in Figure 1.
Conclusion
The LT3825 allows a designer to improve the performance
of isolated flyback circuits while lowering parts count,
simplifying implementation and eliminating the need for
an optoisolator.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2593
www.linear.com
Linear Technology Corporation
dn1011f 1205 • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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