LNK64x4-64x8
Auto-Restart and Open-Loop Protection
LinkSwitch-3 Functional Description
In the event of a fault condition such as an output short or an
open-loop condition the LinkSwitch-3 enters into an appropriate
protection mode as described below.
The LinkSwitch-3 combines a high-voltage power MOSFET switch with
a power supply controller in one device. It uses an ON/OFF control to
regulate the output voltage. In addition, the switching frequency is
modulated to regulate the output current to provide a constant current
characteristic. The LinkSwitch-3 controller consists of an oscillator,
feedback (sense and logic) circuit, 6 V regulator, over-temperature
protection, frequency jittering, current limit circuit, leading-edge
blanking, inductance correction circuitry, frequency control for
constant current regulation and ON/OFF state machine for CV control.
In the event the FEEDBACK pin voltage during the flyback period falls
below 0.7 V before the FEEDBACK pin sampling delay (~2.5 ms) for a
duration in excess of ~300 ms (auto-restart on-time (tAR-ON) the
converter enters into auto-restart, wherein the power MOSFET is
disabled for 1500 ms. The auto-restart alternately enables and
disables the switching of the power MOSFET until the fault condition
is removed.
Inductance Correction Circuitry
In addition to the conditions for auto-restart described above,
if the sensed FEEDBACK pin current during the forward period of the
conduction cycle (switch “on” time) falls below 120 mA, the converter
annunciates this as an open-loop condition (top resistor in potential
divider is open or missing) and reduces the auto-restart time from
300 ms to approximately 6 clock cycles (90 ms), whilst keeping the
disable period of 2 seconds.
If the primary magnetizing inductance is either too high or low the
converter will automatically compensate for this by adjusting the
oscillator frequency. Since this controller is designed to operate in
discontinuous-conduction mode the output power is directly
proportional to the set primary inductance and its tolerance can be
completely compensated with adjustments to the switching
frequency.
Over-Temperature Protection
Constant Current (CC) Operation
The thermal shutdown circuitry senses the die temperature. The
threshold is set at 142 °C typical with a 60 °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 60 °C, at which point the MOSFET is re-enabled.
As the output voltage and therefore the flyback voltage across the
bias winding ramps up, the FEEDBACK pin voltage increases. The
switching frequency is adjusted as the FEEDBACK pin voltage increases
to provide a constant output current regulation. The constant current
circuit and the inductance correction circuit are designed to operate
concurrently in the CC region.
Current Limit
The current limit circuit senses the current in the power MOSFET.
When this current exceeds the internal threshold (ILIMIT), the power
MOSFET is turned off for the remainder of that cycle. The leading
edge blanking circuit inhibits the current limit comparator for a short
time (tLEB) after the power MOSFET is turned on. This leading edge
blanking time has been set so that current spikes caused by
capacitance and rectifier reverse recovery time will not cause
premature termination of the MOSFET conduction. The LinkSwitch-3
also contains a “di/dt” correction feature to minimize CC variation across
the input line range.
Constant Voltage (CV) Operation
As the FEEDBACK pin approaches 2 V from the constant current
regulation mode, the power supply transitions into CV operation.
The switching frequency at this point is at its maximum value,
corresponding to the peak power point of the CV/CC characteristic.
The controller regulates the FEEDBACK pin voltage to remain at
FEEDBACK pin threshold (VFBTH) using an ON/OFF state-machine.
The FEEDBACK pin voltage is sampled 2.5 ms after the turn-off of the
high-voltage switch.
At light loads the current limit is also reduced to decrease the
transformer flux density and the FEEDBACK pin sampling is done
earlier.
6 V Regulator
The 6 V regulator charges the bypass capacitor connected to the
BYPASS pin to 6 V by drawing a current from the voltage on the
DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal
supply voltage node. When the MOSFET is on, the device runs off of
the energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the LinkSwitch-3 to
operate continuously from the current drawn from the DRAIN pin
however for the best no-load input power, the BYPASS pin should be
supplied current of IS1 from the bias winding at no-load conditions.
A bypass capacitor value of 1 mF is sufficient for both high frequency
decoupling and energy storage.
Output Cable Compensation
This compensation provides a constant output voltage at the end of
the cable over the entire load range in CV mode. As the converter
load increases from no-load to the peak power point (transition point
between CV and CC) the voltage drop introduced across the output
cable is compensated by increasing the FEEDBACK pin reference
voltage. The controller determines the output load and therefore the
correct degree of compensation based on the output of the state
machine. The amount of cable drop compensation is determined by
the third digit in the device part number.
3
Rev. C 03/16
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