DESIGN IDEAS
High Isolation Converter Uses
Off-the-Shelf Magnetics
Figure 1 shows a complete schematic for a converter based on the
uncoupled SEPIC. The converter
operates from a 12V battery-backed
input supply and outputs 24V at
200mA. The key feature is that the
second coil is not a coil at all, but
rather an off-the-shelf gate drive
transformer. This component offers
3750VRMS isolation and full VDE
approval, and functions flawlessly in
SEPIC service.
Feedback is derived from the primary winding, through D3. R1 acts to
filter the leakage-inductance spike at
switch turn-off, and C4 smooths the
recovered feedback voltage. Note that
the transformer is wound 1:1; C4
peak detects a voltage roughly equal
to the output. Sizing R1 and C4 is a
trade-off between minimum load and
load regulation. As shown, a minimum load of 3600Ω is recommended.
Output regulation is shown in Figure
2. Line regulation from 10V to 20V
input at full load is 0.13%/V.
High Isolation Converter Uses
Off-the-Shelf Magnetics ............26
Mitchell Lee
Wide Input Range, Low Voltage
Flyback Regulator .................. 27
30
Kurk Mathews
28
OUTPUT VOLTAGE (V)
Isolated flyback converters usually
evoke thoughts (or bitter memories)
of custom transformers, slipped
delivery schedules and agency
approval problems. Off-the-shelf flyback transformers are available from
several vendors, but these carry
isolation ratings of only 300V–500V,
and, rarely, of up to 1kV. Flyback
transformers with isolation ratings of
3750VRMS are impossible to find, and
if an application requires this level of
isolation, an expensive, custom design is likely the only solution.
Gate-drive transformers, designed
to couple switching regulator controllers to MOSFET gates, are readily
available from stock with high isolation ratings and low cost. These are
wound on ungapped cores and have
very high inductance (500µH to 2mH),
and will quickly saturate in a normal
flyback converter circuit. It is possible to use a gate-drive transformer
in a forward converter, but an optoisolator or a tertiary winding is needed
for feedback. Another topology that
can utilize a gate drive transformer is
the uncoupled SEPIC.
by Mitchell Lee
26
How to Design High Order Filters
with Stopband Notches Using the
LTC1562 Quad Operational Filter
(Part 1) .....................................28
24
Nello Sevastopoulos
LT1610 Micropower Step-Up DC/DC
Converter Runs at 1.7MHz .......32
22
Steve Pietkiewicz
20
50
150
100
TOTAL LOAD CURRENT (mA)
0
200
DI_1068_04. EPS
Figure 2. Output regulation for Figure 1’s
circuit
An Optimized, Low Distortion,
Lower Power ADSL Line Driver
Using the LT1207 ....................33
Adolfo A. Garcia
Positive-to-Negative Converter
Powers –48V Telecom Circuits
................................................36
Mitchell Lee
D1
P6KE36A
+
T1**
C1
100µF
50V
+
L1*
100µH
+
VIN
10V–15V
D2
MUR120
C3
100µF
50V
D4
MUR120
PE63387
C5
100µF
50V
VOUT
24V/200mA
R4
3.6k
(6.7mA MIN LOAD)
D3
1N914
Bootstrapped Synchronous Boost
Converter Operates at 1.8V Input
................................................37
Tom Gross
Battery-Powered Buck-Boost
Converter Requires No Magnetics
................................................38
John Seago
R1
200Ω
* L1 = PE53829
** T1 = PE63387
VIN
R2
18k
SW
LT1172 FB
GND
VC
C2
100nF
PULSE ENGINEERING
(619) 674-8100
C4
10nF
R3
1k
for
the latest information
on LTC products,
visit
www.linear-tech.com
Figure 1. 24V/200mA bulk supply with 3750VRMS isolation
26
Linear Technology Magazine • May 1998