AN-35
V
V
Peak Power
Curve
Peak Power
Curve
Reduced RLF or
increased leakage
inductance
Increased RLF or
reduced leakage
inductance
Reference
Reference
Reduced RLF
or increased
leakage
Higher RLF
or lower
leakage
inductance
inductance
Auto-restart
Auto-restart
I
I
PI-2958-081602
Figure 5. Effect on Output Characteristic when RLF or Leakage Inductance Changes.
VFB − VC(IDCT)
V
RFB =
IDCT
56.7 V − 5.75 V
2.3 mA
(24)
Peak Power
Curve
=
= 22 kΩ
Select the nearest standard value. Resistor RFB can then be
adjusted to center the output voltage. The example in
Figure 3 uses a 20.5 kΩ value for RFB (R1), centering the output
voltage VO near 5.5 V at nominal output current IO.
Increased RFB
Reference
Increased
R
FB to adjust
for LLEAK
Note that RFB power dissipation, a significant component of
Auto-restart
LinkSwitch standby power, should always be calculated:
I
PI-2959-071902
2
(25)
P
= 2.3 mA × R = 111 mW
(
)
RFB
FB
Figure 6. Increasing RFB to Adjust for High Leakage Increases No
load Voltage and Consumption.
For applications that do not need to comply with strict standby
power requirements, higher values of VOR can be used, also
increasing the power capability of LinkSwitch.
Clamp Resistor (RLF)
ThevalueforRLF,whicheffectivelyfilterstheleakageinductance
spikefromthereflectedvoltagewaveform,isverifiedempirically
through iteration. RLF has a direct effect on both the average
value and slope of both the CV and CC curves as shown in
Figure5andcanthereforebeusedtotunetheoutputcharacteristic
to some extent.
Clamp Diode (DCLAMP
)
DiodeDCLAMP shouldbeanultra-fastorfastrecoverydiodewith
at least 600 V breakdown voltage. Fast types typically offer a
slightcostadvantageandalsoreduceEMI,sotheyarepreferred.
Note that normal recovery diodes (1N400X or similar types),
which may allow excessive drain voltage ringing, should not be
used.
In the CV region, increasing RLF increases the average output
voltage, while reducing the slope of the CV region (the change
in output voltage with the change in output current). In the CC
region, increasing RLF makes the average output current lower,
while tending to “bend” the curve inward slightly (fold back).
B
4/03
7
ETC [ ETC ]
