Datasheet

TPS92311
SNVS811B –MAY 2012–REVISED MAY 2013 www.ti.com
In operation, the voltage at the switching node, V
SW
must be small than the internal MOSFET maximum rated
voltage V
Q1(MAX)
, For reason of safety, 10% safety margin is recommended. Hence, 90% of V
Q1(MAX)
is used in
the following equation.
(7)
where
• V
OS
is the maximum switching node overshoot voltage allowed, in this example, 50V is assumed (8)
As a rule of thumb, lower turn ratio of transformer can provide a better line regulation and lower secondly side
peak current. In here, turn ratio n = 3.8 is recommended.
SWITCHING FREQUENCY SELECTION
TPS92311 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.
(9)
(10)
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.
(11)
By re-arranging terms, the transformer primary inductance required in this design example can be calculated with
the equation follows:
(12)
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,
(13)
From the calculation in above, the inductance of the primary winding required is 0.81mH.
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