SNVS856B – JUNE 2012 – REVISED MAY 2013
SWITCHING FREQUENCY SELECTION
TPS92314/14A can operate at high switching frequency in the range of 60kHz to 150kHz. In most off-line
applications, with considering of efficiency degradation and EMC requirements, the recommended switching
frequency range will be 60kHz to 80kHz. In this design example, switching frequency at 75kHz is selected.
SWITCHING ON TIME
The maximum power switch on-time, t
ON
depends on the low line condition of 85V
AC
. At 85V
AC
the switching
frequency was chosen at 75kHz. This transformer design will follow the formulae as shown below.
(8)
TRANSFORMER PRIMARY INDUCTANCE
The primary inductance, L
P
of the transformer is related to the minimum operating switching frequency f
SW
,
converter output power P
OUT
, system efficiency
η
and minimum input line voltage V
AC_RMS(MIN)
. For CRM
operation, the output power, P
OUT
can be described by the equation in below.
(9)
By re-arranging terms, the transformer primary inductance required in this design example can be calculated with
the equation follows:
(10)
The converter minimum switching frequency is 75kHz, t
ON
is 5.3µs, V
AC_RMS(MIN)
= 85V and P
OUT
= 8W, assume
the system efficiency,
η
= 85%. Then,
(11)
From the calculation in above, the inductance of the primary winding required is 0.81mH.
After the primary inductance and transformer turn ratio is determined, the current sensing resistor, R
ISNS
can be
calculated.
The resistance for R
ISNS
is governed by the output current and transformer turn ratio, the equation in below can
be used.
where
•
V
REF
is fixed to 0.14V internally.
(12)
Transformer turn ratio, N
P
: N
S
is 3.8 : 1 and I
LED
= 0.35A
(13)
In
resistor R
FILTER
is used to reduce the high frequency noise into ISNS pin. the typical value is 300 x
R
ISNS
.
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15
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