of the control IC. The actual compensation values will need to be determined by taking
the prototype supply into the laboratory and making measurements and adjustments.
The selection of current sense components and under-voltage components are the same as
the MAX5052 example in the fl yback converter section.
6.9 Non-isolated Forward Converter Example
The current levels in the fl yback example for automotive use were quite high. The input
current consists of very large, short pulses. The output also consists of very large, short
pulses. A forward converter can reduce both the output ripple and the input ripple by
allowing the duty cycle to be larger than 50%. Our next example, Figure 6.17 , shows how
to implement such a supply.
The duty cycle in an off-line forward converter is limited to 50% by the voltage required
to reset the fl ux in the transformer and the switch breakdown voltage. At 50% duty cycle,
the reverse voltage can be equal to the input voltage. In our automotive application, we
can use a high voltage switch to advantage. The high reverse voltage will allow the fl ux in
the transformer to reset in a very short period of time.
We start from the same set of requirements as the fl yback example and use the same
control IC. We choose the same 167-kHz operating frequency for a cycle time of 6 s.
We can set the maximum duty cycle to 75% at 11.0 V input. The data sheet shows that
the maximum duty cycle will vary from IC to IC, from about 70% to about 78%, when
we set the nominal value to 75%. Our calculations will need to allow for 80% duty cycle
as the worst case. The volt-seconds during switch on-time will need to equal the volt-
seconds when the switch is off. The ratio of on-time to off-time is 80/20, so the reverse
voltage on the transformer primary during off-time will be four times the input voltage.
This sets the turns ratio for the clamp winding to 4:1. The switch withstand voltage will
be fi ve times the input voltage (4 for the clamp plus 1 for the input supply) at the
highest input voltage. This gives a minimum value of 15.0 V 5 75 V. A check of the
International Rectifi er website shows either 100 V or 150 V MOSFETs. It probably
makes the most sense to choose a 150 V device to ensure margin in the presence of
transients. The IRF3415 is a TO-220 package that has 150 V V
DSS
, 42 m on resistance,
and 43 A I
DSS
. The IRF3315 is a similar and less expensive part, but it only has 15 A I
DSS
at 100 C.
A 150 V Schottky diode is a reasonable fi rst try at the output rectifi er. The turns ratio
of the transformer is likely to be very close to 1.5:1 for primary to secondary, since our
goal is to reduce the input ripple and output ripple. However, we are allowing the reverse
voltage during transformer reset to be four times the input voltage. This means the reverse
180
www.newnespress.com
Chapter 6
13.6
GND1
V
in
1
GND2
U1
LT1680
SL/ADJ
Ct
Iave
SS
Vc
SGND
FB
Sense
Sense
Run/SHDN
PGND
GATE
12V
in
5V
ref
M1
IRF3415
R1
0.0094
R2
10.0 K
R3
1020
150µF
C2
670nF
C3
1.5nF
1µF
220 pF
C6
220nF
R4
1K
100K
18V
10
R7
5.0K
D4
R8
10
T1
D1
30CPQ150
L1
10.8µH
C1
82µF/16 WV
150µF
1:4:6.96
Figure 6.17 : Non-isolated forward converter with 13.6 V output
www.newnespress.com

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