MAX5020ESA Maxim Integrated Products, MAX5020ESA Datasheet - Page 11

• Manufacturer Part Number: MAX5020ESA
• Description: Current Mode PWM Controllers w/Integrated Startup
• Manufacturer: Maxim Integrated Products

Specifications of MAX5020ESA

• Number Of Outputs: 1
• Duty Cycle (max): 50 %
• Output Current: 1000 mA
• Mounting Style: SMD/SMT
• Package / Case: SOIC-8 Narrow
• Switching Frequency: 275 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Flyback, Forward

1) 36V ≤ V
2) To set the output voltage calculate the values of
3) The turns ratio of the transformer is calculated based
Current-Mode PWM Controllers with Integrated
50mV
resistors R1 and R2 according to the following
equation:
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 MAX5020 (44%). To
enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V use the
MAX5020 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
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
IN
= 5.
REF
= Minimum Input voltage (36V).
≤ 72V, V
P
N
N
N
N
R R
V
V
P
S
P
P
S
based on core losses and DC resis-
OUT
1
REF
is the reference voltage of the shunt
is the number of primary turns).
//
≥
≥
______________________________________________________________________________________
1
V
5V + 0.5V 0.44
=
2
≅
OUT
and R
V
D
<<
V
OUT
SS_
0 44 36
REF
MAX
(
.
+
50
SHDN
(
S
= 5V, I



V
k
×
2
1
×
D1
is the number of secondary
Ω
V
+
are the resistors shown in
×
IN_MIN
×
R
R
≅
V
D
1
2
2 4
OUT
MAX



.
)
V
=
)
= 10A, V
0 330
.
S
, rounding
RIPPLE
P
= 14
≤
4) The reset winding turns ratio (N
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
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
DSMAX
IN_MAX
R
MAX
/N
P
’ = Maximum value of Maximum Duty Cycle.
= Reset winding turns ratio.
IN_MAX
R
= Maximum MOSFET drain-source voltage.
D
= Maximum input voltage.
V
DSMAX
to the nearest smallest integer.
MIN
N
N
=
is the maximum input voltage (72V).
R
R
Startup Circuit
≥



≤
≤
V
V
IN_MAX
N
1
IN_MAX
4
P
×
×
1- 0.5
1-D
V
D
0.5
×
OUT
MAX
×
MAX



1 +
N
N
=
S
P
′
14
′



R
- V
N
N
/N
P
R
D1



P
) needs to be
MIN
R
) use the
/N
P
) will
11