SNVS856B – JUNE 2012 – REVISED MAY 2013
Design Example
The following design example illustrates the procedures to calculate the external component values for the
TPS92314/14A isolated single stage fly-back LED driver with PFC.
•
Design Specifications:
– Input voltage range, V
AC_RMS
= 85VAC – 132VAC
– Nominal input voltage, V
AC_RMS(NOM)
= 110VAC
– Number of LED in serial =7
– LED current, I
LED
= 350mA
– Forward voltage drop of single LED = 3.0V
– Forward voltage of LED stack, V
LED
= 21V
•
Key operating Parameters:
– Converter minimum switching frequency, f
SW
= 75kHz
– Output rectifier maximum reverse voltage, V
DOUT(MAX)
= 100V
– Power MOSFET rating, V
Q1(MAX)
=800V
– Power MOSFET Output Capacitance, C
DS
= 37pF (estimated)
– Nominal output power, P
OUT
= 8W
START UP BIAS RESISTOR
During start up, the V
CC
will be powered by the rectified line voltage through external resistor, R
1
. The V
CC
start
up current, I
VCC(SU)
must set in the range I
VCC(MIN)
> I
VCC(SU)
> I
STARTUP(MAX)
to ensure proper restart operation
during OVP fault at maximum voltage input. In this example, a value of 0.88mA is suggested. The resistance of
R
1
can be calculated by dividing the nominal input voltage in RMS by the start up current suggested.
So, R
AC
= 132V / 0.88mA = 150KΩ is recommended.
TRANSFORMER TURN RATIO
The transformer winding turn ratio, n is governed by the MOSFET Q1 maximum rated voltage, (V
Q1(MAX)
), highest
line input peak voltage (V
AC-PEAK
) and output diode maximum reverse voltage rating (V
OUT(MAX)
). The output diode
rating limits the lower bound of the turn ratio and the power MOSFET rating provide the upper bound of the turn
ratio. The transformer turn ratio must be selected in between the bounds. If the maximum reverse voltage of
D
OUT
(V
DOUT(MAX)
) is 100V. the minimum transformer turn ratio can be calculated with the equation in below.
(5)
In operation, the voltage at the switching node, V
SW
must be small than the 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.
(6)
where
•
V
OS
is the maximum switching node overshoot voltage allowed, in this example, 50V is assumed.
(7)
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.
14
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