LNK603-606/613-616
Auto-Restart and Open-Loop Protection
LinkSwitch-II Functional Description
In the event of a fault condition such as an output short or an
open loop condition the LinkSwitch-II enters into an appropriate
protection mode as described below.
The LinkSwitch-II combines a high voltage power MOSFET
switch with a power supply controller in one device. Similar to
the LinkSwitch-LP and TinySwitch-III it uses 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-II 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 ~450 ms (auto-restart
on-time (tAR-ON) the converter enters into auto-restart, wherein
the power MOSFET is disabled for 2 seconds (~18% auto-
restart duty cycle). 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 450 msec 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 increases, 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
Constant Voltage (CV) Operation
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-II also contains a “di/dt” correction
feature to minimize CC variation across the input line range.
As the FEEDBACK pin approaches VFBth 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
CCCV characteristic. The controller regulates the feedback pin
voltage to remain at 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.
Output Cable Compensation
6.0 V Regulator
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. Cable drop
compensation for a 24 AWG (0.3 W) cable is selected with
CBP = 1 mF and for a 26 AWG (0.49 W) cable with CPB = 10 mF.
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-II to operate continuously from the
current drawn from the DRAIN pin. A bypass capacitor value of
either 1 mF or 10 mF is sufficient for both high frequency
decoupling and energy storage.
3
www.powerint.com
Rev. F 01/10
POWERINT [ Power Integrations ]
