Power integrations InnoSwitch3 Application Note page 9

AN-72
K
/
P
Above a value of 1, indicating discontinuous conduction mode, K
the ratio of primary MOSFET off time to the secondary SR_FET
conduction time.
K / K
P DP
V
^
OR
=
V
MIN
The value of K should be in the range of 0.5 < K
P
given in the comments cell if the value of K
Experience has shown that a K
P
higher efficiency by ensuring DCM or critical mode operation (CRM)
which is desirable for most charger designs.
The spreadsheet will calculate the values of peak primary current,
primary RMS current, primary ripple current, primary average current,
and the maximum duty cycle for the design based on the selection of
the these parameters.
Typical Primary Inductance, LPRIMARY_TYP (µH)
This is the typical transformer primary inductance target.
www.power.com
I
K = R
RP I
P
^ h
1 - D # T
=
t
^ h
1 - D
#
h
MAX
- V D
#
DS MAX
< 6. Guidance is
P
is outside this range.
P
value between 0.8 and 1 will result in
Primary Inductance Tolerance, LPRIMARY_TOL (%)
This parameter is the assumed primary inductance tolerance. A value
is of 7% is used by default, however if specific information is provided
P
from the transformer vendor, then this may be entered in the grey
override cell. A value of 7% helps to reduce unit-to-unit variation and
is easy to meet for most magnetics vendors. A value of 3% will help
improve production tolerance further but will be more challenging to
vendors.
The other important electrical parameters are automatically calculated
by the spreadsheet. These can used to appropriatley select the other
components in the circuit, such as input fuse (F
bridge rectifiers (B ), output rectifiers (SR
RD
as described in Figure 1.
PRIMARY CURRENT
IPEAK_PRIMARY − Peak primary current
IPEDESTAL_PRIMARY − Primary MOSFET current pedestal in CCM mode
IAVG_PRIMARY − Primary MOSFET average current
IRIPPLE_PRIMARY − Primary MOSFET ripple current
IRMS_PRIMARY − Primary MOSFET RMS current
SECONDARY CURRENT
IPEAK_SECONDARY − Peak secondary current
IPEDESTAL_SECONDARY − Secondary winding current pedestal
IRMS_SECONDARY − Secondary winding RMS current
Minimum Rectified Input Voltage, VMIN
Valley of the rectified minimum AC input voltage at full power is
calculated based on input capacitance (CAP_INPUT).
Application Note
) and EMI filter (L
R
) and capacitors (C
FET OUT
Rev. A 10/18
),
F
),
9