Output Circuit - Linear Technology DC1317A Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-G
ACTIVE RESET ISOLATED 9-36V INPUT TO 12V @8A DC/DC POWER CONVERTER
ACTIVE RESET CIRCUIT
The Active Reset circuit on DC1317A-F demo board
consists of a small P-Channel MOSFET Q13 and
reset capacitor C25. The MOSFET Q13 is used to
connect the reset capacitor across the transformer
T1 primary winding during the reset period when
Q1 MOSFET is off. The voltage across capacitor
C25 automatically adjusts with the duty cycle to
provide complete transformer reset under all oper-
ating conditions.
Also the active reset circuit shapes the reset voltage
into a square waveform that results in lower drain
voltages for Q1 and Q2 MOSFETs. The lower
MOSFET drain voltages allow lower voltage and
lower Rdson MOSFETs to be used. The MOSFETs
must be avalanche rated for the peak reset voltage.
If non-avalanche rated MOSFETs are used a proper
drain voltage derating should be used.
The main benefit of active reset circuit in the case of
DC1317A-G demo board is high efficiency (shown
in Figure 3), wide input range, high power density
and small size. To achieve such high efficiency all of
the power components were carefully selected.
Please consult LT factory for assistance if any
changes to the circuit are required.
If PCB space is limited, a smaller 1825R2-3366.006
transformer from Champs engineering can be used.
However, the efficiency will be about 1% lower. For
more information please consult LT factory.
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Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
DC1317A-G 12V Output Efficiency
95%
94%
93%
92%
91%
90%
89%
0 1 2 3
Iout [A]
Figure 3. High efficiency of DC1317A-G allows the
board to be used in thermally critical applications
18V OUTPUT CIRCUIT
The DC1317A-G demo board can easily be modified
for 18V output. Only few minor component changes
are required and are shown in the schematic in Figure
8.
The efficiency of 18V Output version of DC1317A-G is
shown in the Figure 4. The efficiency is slightly lower
than the efficiency of 12V version due to higher DC
resistance of transformer and output MOSFETs.
Also, the 18V output can be further increased to 19.5V
just by changing the voltage divider on the FB pin of
LT4430 circuit. The 19.5V version is suitable for sup-
plying laptop computer power from 12V automotive
power source.
12V input
24V input
4 5 6 7 8 9
3