Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 16

Application Note
Feedback Resistors
R sets the minimum HT voltage at start-up that will allow the
FB1
controller to continue to run. 73% of root(2) × VAC
and will generally be satisfactory for most designs but an alternative
value may be entered in [B160] for special circumstances. It also
sets the brown-out level which is 43% of this value.
R , in conjunction with R sets the output voltage and is calculated
FB2 FB1
in [E162]. Two resistors in series can be used to make up R
high value and one low value. The low value may be trimmed in
Feedback Resistors
V_UV+
RFB1
RFB2
Figure 22. Feedback Resistor Values.
NO LOAD POWER ESTIMATOR
EFF_NOLOAD
VAC_INPUT
PB_NOLOAD
P_PRELOAD
P_STARTUPRES
PSW
P_NOLOAD_TOTAL
Figure 23. No-Load Power Estimation.
16
Rev. B 10/17
value to obtain an accurate output voltage. In fact the spreadsheet
will tend to result in a slightly high output voltage as the high current
is the default value of output diode volt drop is used, but the chip measures the
MIN
output voltage when the secondary discharge current is close to zero.
Step 11 – No-Load Power Estimator
Here, all the no-load losses are calculated and summed to give a
no-load power estimate. Aiming for a 27 mW target should result in a
, one
FB1 30 mW design with adequate margin.
92.9 V
7500 Ohm
2370 Ohm
0.60 %/100
230 Volts
4.1 mW
4.4 mW
7.2 mW
6.6 mW
28 mW
DC voltage at which power supply will start up
Initial estimate for top feedback resistor (std value, use 1% tolerance)
Initial estimate for bottom feedback resistor (std value, use 1% tolerance)
Assumed efficiency at no load (0.6 if no better data available)
AC input voltage for no load power estimation
Bias winding power consumption estimate at no load
Preload resistor power consumption at no load
Energy dissipated by the startup resistor
Power losses of the switch and clamp
Estimated no load power consumption. Affected by Vcs_min
AN-69
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