MAX5019 Datasheet, PDF (11/14 Pages) Maxim Integrated Products – Current-Mode PWM Controllers with Integrated Startup Circuit

Current-Mode PWM Controllers with Integrated
Startup Circuit
1) 36V ≤ V_IN ≤ 72V, V_OUT = 5V, I_OUT = 10A, V_RIPPLE ≤
50mV
2) To set the output voltage calculate the values of
resistors R1 and R2 according to the following
equation:
V_OUT
≅
V_REF

¹⁺
R₁
R₂


R₁ //R₂ << 50kΩ
V_REF = V_{SS_SHDN} ≅ 2.4V
where V_REF is the reference voltage of the shunt
regulator, and R₁ and R₂ are the resistors shown in
Figures 2 and 3.
3) The turns ratio of the transformer is calculated based
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:
( ) N_{S ≥} ^V_OUT ^{+ V}_D1 ^{× D}_MAX
N_P
D_MAX × V_{IN_MIN}
where:
N_S/N_P = Turns ratio (N_S is the number of secondary
turns and N_P is the number of primary turns).
V_OUT = Output voltage (5V).
V_D1 = Voltage drop across D1 (typically 0.5V for
power Schottky diodes).
D_MAX = Minimum value of maximum operating duty
cycle (44%).
V_{IN_MIN} = Minimum Input voltage (36V).
In this example:
N_{S ≥} ^{5V +}(^{0.5V × 0.44}) _{= 0.330}
N_P
0.44 × 36V
Choose N_P based on core losses and DC resis-
tance. Use the turns ratio to calculate N_S, rounding
up to the nearest integer. In this example N_P = 14
and N_S = 5.
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
all of the leakage energy to be dissipated across a
resistor.
To calculate the minimum duty cycle (D_MIN) use the
following equation:
D_MIN
=
V_OUT


V_{IN_MAX}
×
N_S
N_P


-
V_D1
where V_{IN_MAX} is the maximum input voltage (72V).
4) The reset winding turns ratio (N_R/N_P) needs to be
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:
N_R
≤
N_P
×
1-D_MAX ′
D_MAX ′
where:
N_R/N_P = Reset winding turns ratio.
D_MAX’ = Maximum value of Maximum Duty Cycle.
N_R
≤
14
×
1- 0.5
0.5
=
14
Round N_R to the nearest smallest integer.
The turns ratio of the reset winding (N_R/N_P) will
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_DSMAX ≥ V_{IN_MAX} × _1 +
N_P
N_R


V_DSMAX = Maximum MOSFET drain-source voltage.
V_{IN_MAX} = Maximum input voltage.
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