Power integrations InnoSwitch3, Примечание к применению, Страница: 9 / 36

Rev. A 10/18
9
Application Note AN-72
www.power.com
K K
I
I
P RP
P
R
/ =
Above a value of 1, indicating discontinuous conduction mode, K
P
is
the ratio of primary MOSFET off time to the secondary SR_FET
conduction time.
K K
t
D T
V V D
V D
1
1
P DP
MIN DS MAX
OR MAX
#
#
#
/ =
-
-
-
=
^
^
^
h
h
h
The value of K
P
should be in the range of 0.5 < K
P
< 6. Guidance is
given in the comments cell if the value of K
P
is outside this range.
Experience has shown that a K
P
value between 0.8 and 1 will result in
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.
Primary Inductance Tolerance, LPRIMARY_TOL (%)
This parameter is the assumed primary inductance tolerance. A value
of 7% is used by default, however if specific information is provided
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
R
) and EMI filter (L
F
),
bridge rectifiers (B
RD
), output rectifiers (SR
FET
) and capacitors (C
OUT
),
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).