LNK603-606/613-616
Output Filter
Capacitors
Output
Diode Snubber
Input Stage
Primary Clamp
R1
C1
R8
C6
T1
C2
R3
R4
D5
C3
D7
S
S
S
S
R1
U1
LinkSwitch-II
R5
L2
Feedback
Resistors
R2
FB
BP
D
D1
R6
C7
D2
D3
C4
C5
D4
Bypass
Capacitor
C8
D3
R9
Bypass Supply
Components
RF1
Preload
Resistor
Spark
Gap
AC
Input
DC
Output
PI-5110-050508
Figure 5.
PCB Layout Example Showing 5.1 W Design Using P Package.
Secondary Loop Area
Addition of a Bias Circuit for Higher Light Load Efficiency
and Lower No-load Input Power Consumption.
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.
The addition of a bias circuit can decrease the no-load input
power from ~200 mW down to less than 30 mW at 230 VAC
input. Light load efficiency also increases which may avoid the
need to use a Schottky barrier vs PN junction output diode
while still meeting average efficiency requirements.
Electrostatic Discharge Spark Gap
An trace is placed along the isolation barrier to form one
electrode of a spark gap. The other electrode on the secondary
is formed by the output return node. The spark gap directs
ESD energy from the secondary back 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.
The power supply schematic shown in Figure 4 has the bias
circuit incorporated. Diode D6, C5 and R4 form the bias circuit.
As the output voltage is less than 8 V, an additional transformer
winding is needed, AC stacked on top of the feedback winding.
This provides a high enough voltage to supply the BYPASS pin
even during low switching frequency operation at no-load.
In Figure 4 the additional bias winding (from pin 2 to pin 1) is
stacked on top of the feedback winding (pin 4 to pin 2). Diode
D6 rectifies the output and C5 is the filter capacitor. A 10 uF
capacitor is recommended to hold up the bias voltage at low
switching frequencies. The capacitor type is not critical but the
Drain Clamp Optimization
LinkSwitch-II senses the feedback winding on the primary side
to regulate the output. The voltage that appears on the feed-
back winding is a reflection of the secondary winding voltage
while the internal MOSFET is off. Therefore any leakage
inductance induced ringing can affect output regulation. Optimizing
the drain clamp to minimize the high frequency ringing will give
the best regulation. Figure 6 shows the desired drain voltage
waveform compared to Figure 7 with a large undershoot due to
the leakage inductance induced ring. This will reduce the
output voltage regulation performance. To reduce this adjust
the value of the resistor in series with the clamp diode.
voltage rating should be above the maximum value of VBIAS
.
The recommended current into the BYPASS pin is equal to IC
supply current (~0.5 mA) at the minimum bias winding voltage.
The BYPASS pin current should not exceed 3 mA at the maximum
bias winding voltage. The value of R4 is calculated according to
(VBIAS – VBP)/IS2, where VBIAS (10 V typ.) is the voltage across C5, IS2
(0.5 mA typ.) is the IC supply current and VBP (6.2 V typ.) is the
6
Rev. F 01/10
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