文档介绍：SWITCHING POWER SUPPLY DESIGN:
CONTINUOUS MODE
FLYBACK CONVERTER
Written by Michele hi
@
Application Engineer
National Semiconductor
Typical Flyback power supply:
D4
N=18T
R5 C10
10 220p
16V
R1 D2 N=18T N=6T
D5
D3
Q1
D1
20 - 55V 10V
R3
10K
C1 C2 R6 13 9,10,11
200K C9 C8
1 1 C3 Vin SW
100V 100V 68u 68u
2 4V 4V
FB
3 LM5000-3
SHDN
R4
Byp FS P GND
12 15 14 1 4,5,6,
7,8,16
R2
R7 1K
10K
C4 C6
100p
C11 C5 C7
1000p
LM5000 FLYBACK CONVERTER
Notes:
Write down the power supply requirements on : Xxx :=
Get the results on: Rsultsxx :=
This Mathcad file helps the calculation of the ponents of a typical continuous
mode switching power supply.
Input voltage:
­ 6
- Minimum input voltage: Vimin := 22×volt msec := 10 ×sec
- Maximum input voltage: Vimax := 55×volt
- Nominal input voltage: Vinom := 36×volt
Output:
- Nominal output voltage, maximum output ripple, minimum output current, maximum output
current
Vo1 := ×volt Vrp1 := 100×mV Io1min := ×amp Io1max := 2.×amp
Vo2 := 0×volt Vrp2 := 120×mV Io2min := ×amp Io2max := ×amp
Vdfw := ×volt ( diode's forward drop voltage)
Po := Vo1 + Vd ×Io1 + Vo2 + Vd ×Io2 Po = watt
min ( fw) min ( fw) min min
Pomax := ( Vo1 + Vdfw) ×Io1max + ( Vo2 + Vdfw) ×Io2max Pomax = watt
1
- Switching Frequency: fsw := 300×kHz T := T = msec
fsw
- Transformer's Efficiency: h := (Guessed value)
- Maximum drop voltage across the switching mosfet during the on time:
- On resistance of the Mosfet: Rdson := ×ohm
Pomax
Vdson := ×Rdson Vdson = volt
h × Vimin
1) Define Primary/secondary turns ratio: Nps1
Primary/secondary turns ratio can be selected promise between maximum voltage
across the switching mosfet and desired max.-min. duty cycle.
- Nominal desired on Duty Cycle: Dnom :=
æ Vinom ­ Vdson ö Dnom
N