NCL30082SMRTGEVB
www.onsemi.com
2
THEORY OF OPERATION
Power Stage
The power stage for the demo board is a non-isolated
coupled SEPIC converter. The controller has a built in
control algorithm that is specific to the flyback transfer
function. Specifically:
VOUT
VIN
+
Duty
(1 *Duty) (eq. 1)
This is applicable to flyback, buck-boost, and SEPIC
converters. The controller has a built in hardware algorithm
that relates the output current to a reference on the primary
side.
IOUT +
VREF @NPS
2@RSENSE
(eq. 2)
NPS +
NPRI
NSEC
(eq. 3)
Where NPRI = Primary Turns and NSEC = Secondary Turns.
We can now find RSENSE for a given output current.
RSENSE +
VREF @NPS
2@IOUT
(eq. 4)
Line Feedforward
The controller is designed to precisely regulate output
current but variation input line voltage do have an impact.
RLFF sets the line feedforward and compensates for power
stage delay times by reducing the current threshold as the
line voltage increases. RLFF is also used by the shorted pin
detection. At start up the controller puts out a current to
check for a shorted pin. If RLFF is zero, the current sense
resistor is too low a value and the controller will not start
because it will detect a shorted pin. So RLFF is required to
make the controller operate properly. In practice, RLFF
should be greater than 250 W.
Voltage Sense
The voltage sense pin has several functions sets the brown
level and line range selection.
The amplitude of VIN is important for the range detection.
Generally, the voltage on VIN should be 3.5 V peak at the
highest input voltage of interest. Voltage on VIN must not be
greater than 4 V under any operating condition. The voltage
on VIN determines which valley the power stage will operate
in. At low line and maximum load, the power stage operates
in the first valley (standard CrM operation). At the higher
line range, the power stage moves to the second valley to
lower the switching frequency while retaining the advantage
of CrM soft switching.
Auxiliary Winding
The auxiliary winding has 3 functions:
1. CrM Timing
2. VCC Power
3. Output Voltage Sense
CrM Timing
In the off time, the voltage on the transformer/inductor
forward biases DOUT and D9. When the current in the
magnetic has reached zero, the voltage collapses to zero.
This voltage collapse triggers a comparator on the ZCD pin
to start a new switching cycle. The ZCD pin also counts rings
on the auxiliary winding for higher order valley operation.
A failure of the ZCD pin to reach a certain threshold also
indicates a shorted output condition.
VCC Power
The auxiliary winding forward biases D9 to provide
power for the controller. This arrangement is called
a “bootstrap”. Initially the CVCC, is charged through
RSTART and RSTART1. When the voltage on CVCC
reaches the startup threshold, the controller starts switching
and providing power to the output circuit and the CVCC.
CVCC discharges as the controller draws current. As the
output voltage rises, the auxiliary winding starts to provide
all the power to the controller. Ideally, this happens before
CVCC discharges to the under voltage threshold where the
controller stops operating to allow CVCC to recharge once
again. The size of the output capacitor will have a large
effect on the rise of the output voltage. Since the LED driver
is a current source, the rise of output voltage is directly
dependent on the size of the output capacitor.
There are tradeoffs in the selection of COUT and CVCC.
A low output ripple will require a large COUT value. This
requires that CVCC be large enough to support VCC power to
the controller while COUT is charging up. A large value of
CVCC requires that RSTART and RSTART1 be lower in
value to allow a fast enough startup time. Smaller values of
RSTART and RSTART1 have higher static power
dissipation which lowers efficiency of the driver.
Output Voltage Sense
The auxiliary winding voltage is proportional to the
output voltage by the turns ratio of the output winding and
the auxiliary winding. The controller has an overvoltage
limit on the VCC pin at about 26 V minimum. Above that
threshold, the controller will stop operation and enter
overvoltage fault mode such as when an open LED string
occurs.
In cases where the output has a lot of ripple current and the
LED has high dynamic resistance, the peak output voltage
can be much higher than the average output voltage.
The auxiliary winding will charge the CVCC to the peak of
the output voltage which may trigger the OVP sooner than
expected so in this case the peak voltage of the LED string
is critical.
SD Pin
The SD pin is a multi-function protection input.
1. Thermal Foldback Protection
2. Programmable OVP
