Datasheet
MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
19Maxim Integrated
Duty-Cycle Calculation
The accurate value of the duty cycle (D
NEW
) for the
selected primary inductance L
PRI
can be calculated
using the following equation:
( )
PRI OUTF D OUTF SW
NEW
INMIN
2.5 L V V I f
D
V
× × +× ×
=
Turns Ratio Calculation (Ns/Np)
Transformer turns ratio (K = Ns/Np) can be calculated as:
( )
+ ×−
=
×
OUTF D NEW
INMIN NEW
V V (1 D )
K
VD
Peak/RMS Current Calculation
RMS current values in the primary and secondary are
needed by the transformer manufacturer to design the
wire diameter for the different windings. Peak current
calculations are useful in setting the current limit. Use the
following equations to calculate the primary and second-
ary peak and RMS currents:
Maximum primary peak current:
INMIN NEW
PRIPEAK
PRI SW
VD
I
Lf
×
=
×
Maximum primary RMS current:
NEW
PRIRMS PRIPEAK
D
II
3
= ×
Maximum secondary RMS current:
PRIPEAK
SECPEAK
I
I
K
=
Maximum secondary peak current:
××
=
×
OUT PRIPEAK
SECRMS
2I I
I
3K
For current-limit setting, the peak current can be calcu-
lated as:
LIMF PRIPEAK
I I 1.2= ×
Primary Snubber Selection
Ideally, the external nMOSFET experiences a drain-
source-voltage stress equal to the sum of the input volt-
age and the reflected voltage across the primary wind-
ing during the off period of the nMOSFET. In practice,
parasitic inductances and capacitors in the circuit, such
as leakage inductance of the flyback transformer, cause
voltage overshoot and ringing. Snubber circuits are used
to limit the voltage overshoots to safe levels within the
voltage rating of the external nMOSFET. The snubber
capacitor can be calculated using the following equation:
22
LK PRIPEAK
SNUB
2
OUTF
2L I K
C
V
×× ×
=
where L
LK
is the leakage inductance obtained from the
transformer specifications (usually 1% to 2% of the pri-
mary inductance).
The power dissipated in the snubber resistor is calcu-
lated using the following equation:
2
SNUB LK PRIPEAK SW
P 0.833 L I f= ×× ×
The snubber resistor can be calculated based on the
following equation:
2
OUTF
SNUB
2
SNUB
6.25 V
R
PK
×
=
×
The voltage rating of the snubber diode is:
OUTF
DSNUB INMAX
V
V V 2.5
K
= +×
Output-Capacitor Selection
X7R ceramic output capacitors are preferred in industrial
applications due to their stability over temperature. The
output capacitor is usually sized to support a step load
of 50% of the maximum output current in the application
so that the output-voltage deviation is contained to 3% of
the output-voltage change. The output capacitance can
be calculated as:
STEP RESPONSE
OUTF
OUTF
It
C
V
×
=
∆
RESPONSE
C SW
0.33 1
t ()
ff
≅+
where I
STEP
is the load step, t
RESPONSE
is the response
time of the controller, DV
OUTF
is the allowable output volt-
age deviation, and f
C
is the target closed-loop crossover
frequency. f
C
is chosen to be 1/10 of the switching fre-
quency (f
SW
). For the flyback converter, the output
capacitor supplies the load current when the main switch