AN-29
• The Quick Selection Curves assume that the AC Input
voltage waveform is a pure sine wave. If the input voltage
waveformisdistorted,theresultantpeakvoltageontheinput
bulk capacitor may be much lower than anticipated. This
causes the TOPSwitch-GX device to reach current limit or
duty cycle limit at less than the maximum possible load.
Other Key Considerations
We have seen how to use the information provided by the
TOPSwitch-GX Quick Selection Curves. However there are
otherkeyfactorstoconsiderwhencompletingthepowersupply
design.Thesecanproduceresultsthatdifferfromthepredictions
of the Quick Selection Curves.
Therefore, in locations where significant line distortion is
expected, the designer should provide a suitable design margin.
This can be accomplished by derating maximum output power
or increasing the input capacitance.
Factors which can lower the performance:
• Input capacitor tolerance and aging should be taken into
account. LowercapacitancedecreasestheDCinputvoltage,
increasing primary RMS currents and hence giving larger
conduction losses in the device chosen.
• Some wattmeters give erroneous readings when the current
has a high crest factor. It is important to use an instrument
designedforthepurpose. TheVoltechPM100isanexample.
• In production, the primary inductance of the transformer
will also have a significant tolerance. Inductances higher
than those in Tables 1 to 4 will cause the power supply to
operate beyond recommended design guidelines (KRP too
low). Values of primary inductance significantly lower than
those in Tables 1 to 4 would lead to higher peak and RMS
drain current in the TOPSwitch-GX MOSFET. This causes
an increase in device dissipation and also causes the device
to reach current limit at less than maximum load.
• Minimum line frequency is important. A low line frequency
requireslargercarryoverperiodsfortheinputbulkcapacitor,
causing high voltage ripple across it. If the line frequency
expected to be lower than 50 Hz, the input capacitor should
be sized appropriately or the maximum output power be
derated.
TYPICAL 5 V OUTPUT POWER SUPPLY COMPONENT PARAMETERS
SINGLE VOLTAGE INPUT (230 VAC ± 15%)
PARAMETER
Units
242Y
243Y
244Y
245Y
246Y
247Y
248Y
249Y
Maximum Transformer
Primary Inductance Lp
µH
3190
1593
1062
797
531
398
319
265
Transformer Leakage
Inductance
Secondary Trace
Inductance
%/Lp
nH
1.5
20
1.5
20
1.5
20
1.5
20
1.5
19
1.5
16
1.5
13
1.5
10
Transformer Resonant
Frequency (secondary
open)
kHz
750
800
850
900
950
1000
1050
1100
Transformer Primary
AC Resistance
Transformer Secondary
AC Resistance
Output Capacitor
Equivalent Series
Resistance @100 kHz
mΩ
mΩ
mΩ
4600
12
2400
1600
1200
1000
800
1
600
0.75
2
400
0.5
1
6
9
4
6
3
5
2
4
18
3
Output Inductor DC
Resistance
Common Mode Inductor
DC Resistance (both legs)
mΩ
mΩ
6
370
2
4.5
340
2
3.5
310
2
3
280
2
2.5
250
2
2
220
2
1.5
190
2
1
160
2
Core Loss
%/PIN
Table 4. Typical Power Supply Component Parameters for a TOPSwitch-GX Flyback Power Supply with a Single Input (5 V output).
D
2/03
5
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