SNVS856B – JUNE 2012 – REVISED MAY 2013
Q1
TPS92314
GATE
ISNS
PGND
DLY
R
DLY
R
ISNS
R
FILTER
Figure 20. R
ISNS
Resistor Interface
D
VCC
VCC
R
AUX1
ZCD
D
ZCD
R
AUX2
L
AUX
PGND
PGND
PGND
Figure 21. Auxiliary Winding Interface to ZCD
Auxiliary Winding Interface To ZCD
In
R
AUX1
and R
AUX2
forms a resistor divider which sets the thresholds for over voltage protection of
V
LED,
V
ZCD-OVP
, and V
ZCD-PEAK
. Before the calculation, we need to set the voltage of the auxiliary winding, V
LAUX
at
open circuit.
• For example :
– Assume the nominal forward voltage of LED stack (V
LED
) is 21V.
– To avoid false triggering ZCD
OVP
voltage threshold at normal operation, select ZCD
OVP
voltage at 1.3
times of the V
LED
is typical in most applications. In case the transformer leakage is higher, the ZCD
OVP
threshold can be set to 1.5 times of the V
LED
.
– In this design example, open circuit AUX winding OVP voltage threshold is set to 30V. Assume the current
through the AUX winding is 0.4mA typical.
– As a result, R
AUX1
is 66kΩ and R
AUX2
is 12kΩ.
Auxiliary Winding V
cc
Diode Selection
The VCC diode D
VCC
provides the supply current to the converter, low temperature coefficient , low reverse
leakage and ultra fast diode is recommended.
Compensation Capacitor And Delay Timer Resistor Selection
To achieve PFC function with a constant on time flyback converter, a low frequency response loop is required. In
most applications, a 4.7µF C
COMP
capacitor is suitable for compensation.
16
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