Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 9

AN-69
Step 4 – Select Reflected Voltage and Secondary Turns
These are the main optimization inputs that effect efficiency and the
minimum DC bulk capacitor voltage the converter will operate at,
under full load. The spreadsheet will produce a reasonably optimized
design, but further improvement may be possible. To optimize for
efficiency enter an alternative value for the reflected output voltage,
VOR in [B49], the default is 100 V. Whilst trying different values,
observe the changes in KCRMV [E65] and aim to get between 0.95
and 0.98, though 0.945 to 1.05 is acceptable. At the same time
ensure VCRMV-VMIN [E56-E57] is less than 15 V. The higher VCRMV
the better for efficiency at the cost of some twice line frequency
ripple in the output at ICC and VO (maximum power point). Fixing
MAIN OPTIMIZATION INPUTS
Turns and ratio
VOR
NS
NP
Figure 8. VOR and Secondary Turns Selection - Main Optimization Values.
DC INPUT VOLTAGE PARAMETERS
VCRMV
VMIN
VMAX
VBROWN
Figure 9. DC Input Parameter Entry and VCRMV Optimization Watch Value.
PRIMARY WAVEFORM PARAMETERS
F_RES
KCRMV
IRMS
IP
IOCP
IAVG
IP_CRMV
F_CRMV
FVMIN
VCS_VMIN
RCS
Figure 10. Figure 10: Primary Waveform Parameters and KCRMV Optimization Watch Value.
SECONDARY WAVEFORM PARAMETERS
ISP
ISRMS
IRIPPLE
Figure 11. Secondary Waveform Design Parameters.
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the number of secondary turns in [B50] while sweeping the VOR
value [B49] offers further optimization possibilities.
The default primary inductance tolerance is 10%, an alternative value
can be entered in [B88]. Tighter tolerance allows better average
efficiency across production.
F_RES is the idle ring frequency of the transformer whilst in the
application, so includes the effects of BJT collector capacitance and
and snubber capacitance. Use the default 400 kHz if no other figure
is available. Once the application has been tested, the true figure can
be entered and further optimization performed as required. Usually a
lower F_RES in the application will have an adverse effect, but it would
have to be significantly lower to have a measurable effect, say -25%.
100.00 Volts Reflected Output Voltage. Use Goal Seek to get VCRMV to desired value.
Number of Secondary Turns. Adjusting up or down along with VOR may improve
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efficiency.
105 Primary Winding Number of Turns
94 Volts Vbulk at CRMV, at max LP tolerance. Higher value typically more efficient.
Bulk cap "trough" voltage at VACMIN. Leave blank to calculate from capacitance
82.6 Volts
and load.
Maximum DC Input Voltage
375 Volts
51 Volts Bulk voltage it loses regulation
400 kHz Anticipated resonant frequency on the primary side (180<Ftrf<1200)
0.95 Ratio of primary switch off time to secondary conduction plus first valley time
0.25 Amps Primary RMS Current (calculated at load=IO, VMIN)
0.60 Amps Peak Primary Current (calculated at load=IO, VMIN)
Pulse by pulse current limit. Appears during large load transients and brownout
0.79 Amps
operation.
0.16 Amps Average Primary Current (calculated at load=IO, VMIN)
0.56 Amps Ipeak when Vin=Vcrm
65000 Hz Fsw when Vin=Vcrm
62410 Hz Fsw at VMIN. If < 65kHz, is in frequency reduction mode
0.273 Volts Vcs_pk at VMIN and load = IO
0.453 Ohm Calculated RCS value. Changes with Icc and VOR
10.53 Amps Peak Secondary Current @ VMIN
3.67 Amps Secondary RMS Current @ VMIN
3.08 Amps Output Capacitor RMS Ripple Current @ VMIN
Application Note
Rev. B 10/17
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