SNVS856B – JUNE 2012 – REVISED MAY 2013
APPLICATION INFORMATION
The TPS92314/14A is an off-line controller specifically designed to drive LEDs. This device operates in Critical
Conduction Mode (CRM) with adaptive Constant ON-Time control, so that high power factor can be achieved
naturally. The TPS92314/14A can be configured as an isolated or non-isolated off-line converter. Refer to
TPS92314/14A typical schematic, on the front page, in the following discussion. The TPS92314/14A flyback
converter consists of a transformer which includes three windings L
P
, L
S
and L
AUX
. An external MOSFET Q
1
and
inductor current sensing resistor R
ISNS
. Secondary side components are secondary side transformer winding L
S
,
output diode D
OUT
, and output capacitor C
OUT
. An auxiliary winding is required, and serves two functions.
Auxiliary power is developed from the winding to power the TPS92314/14A after start-up, and detect the zero
crossing point due to the end of a complete switching cycle. During the on-period, Q
1
is turned on, and current
flows through L
P
, Q
1
and R
ISNS
to ground, input energy is stored in the primary inductor L
P
. Simultaneously, the
I
SNS
pin of the device monitors the voltage of the current sensing resistor R
ISNS
to perform the cycle-by-cycle
inductor current limit function. During the time MOSFET Q
1
is off, current flow in L
P
ceases and the energy stored
during the on cycle is released to output and auxiliary circuits. During Q
1
off-time current in the secondary
winding L
S
charges the output capacitor C
OUT
through D
OUT
and supplies the LED load. During Q
1
on-time, C
OUT
is responsible to supply load current to LED load during subsequent on-period. Also during Q
1
off-time current is
delivered to the auxiliary winding through D
VCC
and powers the TPS92314/14A. The voltage across L
AUX
, V
LAUX
is fed back to the ZCD pin through a resistor divider network formed by R
AUX1
and R
AUX2
to perform zero crossing
detection of V
LAUX
, which determines the end of the off-period of a switching cycle. The next on period of a new
cycle will be initiated after an inserted delay of 2 x t
DLY
. The t
DLY
is programmable by a single resistor connecting
the DLY pin and ground. The setting of the delay time, t
DLY
will be described in a separate paragraph. The driver
signal t
ON
time width is generated by comparing an internal generated saw-tooth waveform with the voltage on
the COMP pin (V
COMP
). Since V
COMP
is slow varying, t
ON
is nearly constant within an AC line cycle. The duration
of the off-period (t
OFF
) is determined by the rate of discharging of the secondary current through the transformer.
Also,
where
•
n is the turn ratio of L
P
and L
S
.
(1)
shows the typical waveforms in normal operation.
8
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