max5015esat Maxim Integrated Products, Inc., max5015esat Datasheet - Page 11
max5015esat
Manufacturer Part Number
max5015esat
Description
Current-mode Pwm Controllers With Integrated Startup Circuit For Isolated Power Supplies
Manufacturer
Maxim Integrated Products, Inc.
Datasheet
1.MAX5015ESAT.pdf
(14 pages)
3) The turns ratio of the transformer is calculated based
Current-Mode PWM Controllers with Integrated
where V
regulator, and R
Figures 2 and 3.
on the minimum input voltage and the lower limit of
the maximum duty cycle for the MAX5015 (44%). To
enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V use the
MAX5015 with the 50% maximum duty cycle.
Calculate the turns ratio according to the following
equation:
Choose N
tance. Use the turns ratio to calculate N
up to the nearest integer. In this example N
and N
For a forward converter choose a transformer with a
magnetizing inductance in the neighborhood of
200µH. Energy stored in the magnetizing inductance
of a forward converter is not delivered to the load
and must be returned back to the input; this is
accomplished with the reset winding.
The transformer primary to secondary leakage
inductance should be less than 1µH. Note that all
leakage energy will be dissipated across the MOS-
FET. Snubber circuits may be used to direct some or
D
cycle (44%).
V
In this example:
where:
N
turns and N
V
V
power Schottky diodes).
IN_MIN
OUT
MAX
D1
S
/N
= Voltage drop across D1 (typically 0.5V for
P
S
= Output voltage (5V).
= Minimum value of maximum operating duty
= Turns ratio (N
= 5.
= Minimum Input voltage (36V).
Startup Circuit for Isolated Power Supplies
REF
P
N
N
N
N
P
S
P
based on core losses and DC resis-
is the reference voltage of the shunt
S
P
is the number of primary turns).
≥
______________________________________________________________________________________
≥
1
V
5V + 0.5V 0.44
V
V
OUT
and R
OUT
REF
D
MAX
0 44 36
(
.
+
S
=
(
2
is the number of secondary
V
×
×
R
D1
are the resistors shown in
1
V
×
R
IN_MIN
+
×
2
V
R
D
2
MAX
)
=
)
0 330
.
S
, rounding
P
= 14
=
4) The reset winding turns ratio (N
5) Choose the tertiary winding turns ratio (N
all of the leakage energy to be dissipated across a
resistor.
To calculate the minimum duty cycle (D
following equation:
where V
low enough to guarantee that the entire energy in
the transformer is returned to V+ within the off cycle
at the maximum duty cycle. Use the following equa-
tion to determine the reset winding turns ratio:
where:
N
D
Round N
The turns ratio of the reset winding (N
determine the peak voltage across the N-channel
MOSFET.
Use the following equation to determine the maxi-
mum drain-source voltage across the N-channel
MOSFET:
V
V
Choose MOSFETs with appropriate avalanche
power ratings to absorb any leakage energy.
that the minimum input voltage provides the mini-
mum operating voltage at V
DSMAX
IN_MAX
R
MAX
/N
P
’ = Maximum value of Maximum Duty Cycle.
= Reset winding turns ratio.
IN_MAX
D
V
= Maximum MOSFET drain-source voltage.
R
= Maximum input voltage.
V
DSMAX
MIN
DSMAX
to the nearest smallest integer.
=
N
N
R
V
is the maximum input voltage (72V).
R
IN_MAX
≥
≤
≤
≥
7
N
1
V
2
4
P
IN_MAX
V
×
V
×
×
OUT
×
1- 0.5
1-D
0.5
D
N
N
1 +
MAX
P
S
MAX
DD
×
- V
=
′
14
14
D1
1 +
14
′
(13V). Use the follow-
=
R
= 19 8 .
/N
N
N
144
P
R
P
) needs to be
V
MIN
R
) use the
/N
T
/N
P
P
) will
) so
11