LNK454/456-458/460
IC Supply and BYPASS Pin
Overload Protection
The internal 5.85 V regulator charges the bypass capacitor
connected to the BYPASS pin to 5.85 V by drawing current
from the voltage on the DRAIN pin whenever the power MOSFET
is off. The BYPASS pin is the internal supply voltage node.
When the power MOSFET is on, the device operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows LinkSwitch-PL to
operate continuously from current it takes from the DRAIN pin.
A bypass capacitor value of 1 µF is sufficient for both high
frequency decoupling and energy storage. Dimming
applications may require a higher bypass capacitor value.
In case of overload, the system will increase the operating
frequency and on-time each AC half-cycle until the maximum
frequency and maximum on-time are reached. When this state
is reached, the controller enters auto-restart protection, thus
inhibiting the gate of the power MOSFET for approximately
1.28 s if the main line frequency is 50 Hz, 1.02 s if it is 60 Hz.
After this auto-restart off-time expires, the power MOSFET is
re-enabled and a normal start-up is initiated, i.e. at fMIN and
tON(MIN), stepping up until regulation is achieved again. In case of
a persistent overload condition, the auto-restart duty cycle DCAR
is ~33%.
During phase angle dimming when the conduction angle is
small the AC input voltage is present for only short periods of
time. In that case the IC should not rely on the integrated
high-voltage current source, but instead external bias circuitry
should be used to supply the IC from the output (DES and RES in
Figure 4). If the output voltage is less than 7 V, external bias
circuitry should be implemented. This is accomplished by
adding an auxiliary winding on the transformer, which is then
rectified and filtered via a diode (ultrafast) and capacitor. The
winding voltage (turns) should be selected such that the maximum
IC consumption can be supported at the lowest operating
output current.
Overload protection is inhibited during phase dimming when the
TRIAC conduction duty cycle is less than 60%.
Output Overvoltage Protection
If a no-load condition is present on the output of the supply, the
output overvoltage Zener (DZOV in Figure 4) will conduct once its
threshold is reached. A voltage VOV in excess of VFB(AR) = 2 V will
appear across the FEEDBACK pin and the IC will enter auto-
restart.
Output Short-Circuit
If the output of the supply (i.e. the LED load) is short-circuited,
then a large amount of energy will be delivered to the sense
resistor, generating a high-voltage at the FEEDBACK pin. If this
condition develops more than 2 V on the FEEDBACK pin, then
the IC will interpret this event as an output short-circuit and will
enter auto-restart.
Start-up, Switching Frequency and On-time Range
At start-up the controller uses an initial switching frequency fMIN
and minimum on-time tON(MIN). The charging of the output
capacitor together with the energy delivery to the output LEDs
determines a step-by-step increase of the power MOSFET
switching frequency and on-time updated every half-cycle of the
AC input voltage.
Safe Operating Area (SOA) Protection
If 3 consecutive cycles of the power MOSFET are prematurely
terminated due to the power MOSFET current exceeding the
current limit after the leading edge blanking time, SOA protection
mode is triggered and the IC will enter auto-restart.
The steady-state switching frequency and on-time are
determined by the line voltage, voltage drop across the LEDs
and converter efficiency.
Hysteretic Thermal Shutdown
The thermal shutdown circuitry senses the die junction
temperature. The thermal shutdown threshold is set to 142 °C
typical with a 75 °C hysteresis. When the die temperature rises
above this threshold (142 °C) the power MOSFET is disabled
and remains disabled until the die temperature falls by 75 °C, at
which point the power MOSFET is re-enabled.
At light load when the device reaches the minimum frequency
fMIN and on-time tON(MIN), the controller regulates by skipping
cycles. In this mode of operation the input current is not power
factor corrected and the average output current is not guaranteed
to fall within the normal range. A properly designed supply will
not operate in this mode under normal load conditions.
A power supply designed correctly will operate within the
switching frequency range [fMIN … fMAX], with an on-time falling
between tON(MIN) and tON(MAX) when connected to a normal load.
5
www.powerint.com
Rev. C 10/11
POWERINT [ Power Integrations ]
