LNK623-626
Output Regulation
When designing a board for the LinkSwitch-CV based power
supply, it is important to follow the following guidelines:
The LNK626 regulates the output using ON/OFF control,
enabling or disabling switching cycles based on the sampled
voltage on the FEEDBACK pin. The output voltage is sensed
using a primary referenced winding on transformer T± eliminating
the need for an optocoupler and a secondary sense circuit. The
resistor divider formed by R3 and R6 feeds the winding voltage
into U±. Standard ±5 resistor values were used to center the
nominal output voltages. Resistor R1 and C1 reduce pulse
grouping by creating an offset voltage that is proportional to the
number of consecutive enabled switching cycles.
Single Point Grounding
Use a single point (Kelvin) connection at the negative terminal of
the input filter capacitor for the LinkSwitch-CV SOURCE pin and
bias winding return. This improves surge capabilities by
returning surge currents from the bias winding directly to the
input filter capacitor.
Bypass Capacitor
The BYPASS pin capacitor should be located as close as
possible to the SOURCE and BYPASS pins.
Key Application Considerations
Feedback Resistors
Output Power Table
Place the feedback resistors directly at the FEEDBACK pin of
the LinkSwitch-CV device. This minimizes noise coupling.
The data sheet maximum output power table (Table ±)
represents the maximum practical continuous output power
level that can be obtained in a Flyback converter under the
following assumed conditions:
Thermal Considerations
The copper area connected to the source pins provide the
LinkSwitch-CV heat sink. A rule of thumb estimate is that the
LinkSwitch-CV will dissipate ±05 of the output power. Provide
enough copper area to keep the source pin temperature below
±±0° C to provide margin for part to part RDS(ON) variation.
±. The minimum DC input voltage is ±00 V or higher at 90 VAC
input. The value of the input capacitance should be large
enough to meet these criteria for AC input designs.
2. Secondary output of 1 V with a Schottky rectifier diode.
3. Assumed efficiency of 805.
4. Continuous conduction mode operation (KP = 0.4).
1. Reflected Output Voltage (VOR) of ±±0 V.
6. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper to keep the SOURCE pin tempera-
ture at or below ±±0 °C for P package and ±00 °C for D
packaged devices.
Secondary Loop Area
To minimize leakage inductance and EMI, the area of the loop
connecting the secondary winding, the output diode and the
output filter capacitor should be minimized. In addition,
sufficient copper area should be provided at the anode and
cathode terminal of the diode for heatsinking. A larger area is
preferred at the quiet cathode terminal. A large anode area can
increase high frequency radiated EMI.
7. Ambient temperature of 10 °C for open frame designs and
an internal enclosure temperature of 60 °C for adapter
designs.
Electrostatic Discharge Spark Gap
In chargers and adapters ESD discharges may be applied to
the output of the supply. In these applications the addition of a
spark gap is recommended. A trace is placed along the
isolation barrier to form one electrode of a spark gap. The other
electrode, on the secondary side, is formed by the output return
node. The arrangement directs ESD energy from the secondary
to the primary side AC input. A ±0 mil gap is placed near the
AC input. The gap decouples any noise picked up on the spark
gap trace to the AC input. The trace from the AC input to the
spark gap electrode should be spaced away from other traces
to prevent unwanted arcing occurring and possible circuit
damage.
Note: Higher output power are achievable if the efficiency is
higher than 805, typically for high output voltage designs.
Bypass Pin Capacitor
A ± μF Bypass pin capacitor (C4) is recommended. The
capacitor voltage rating should be equal to or greater than
6.8 V. The capacitor’s dielectric material is not important. The
capacitor must be physically located close to the
LinkSwitch-CV BYPASS pin.
Circuit board layout
LinkSwitch-CV is a highly integrated power supply solution that
integrates on a single die, both the controller and the high
voltage MOSFET. The presence of high switching currents and
voltages together with analog signals makes it especially
important to follow good PCB design practice to ensure stable
and trouble free operation of the power supply.
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Rev. E 09/09
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