TK75003 Ver la hoja de datos (PDF) - Toko America Inc
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TK75003 Datasheet PDF : 14 Pages
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TK75003
DESIGN CONSIDERATIONS (CONT.)
T1 is the time required for the leakage inductance of the
flyback transformer to completely discharge its stored
energy into the voltage clamp. T1 can be calculated as:
T1 = (IPK x LLEAK ) / VOVERSHOOT
(7)
where IPK is the peak current in the MOSFET at turn-off and
LLEAK is the inductance of the flyback transformer measured
at winding n1.
T2 is the conduction time of the output diode D2 and T is the
switching period.
From Equation 4 the resistance RAUX or the voltage VAUX
can be calculated.
Example: calculate the value of RAUX with the following
typical values:
VOUT = 12 V
LLEAK = 2 µH
IAUX = 18 mA
n1 = 31
VD2 = VD3 = 1 V
VOVERSHOOT = 20 V
T2 = 2 µs
n2 = 6
IPK = 1 A
VAUX = 13.5 V
T = 5 µs
n3 = 7
Equations 5, 6 and 7 yield V1 = 19.7 V, V2 = 15.2 V, and
T1 = 100 ns. Substituting those values into Equation 4 and
solving for RAUX yields:
VIN
RST
VAUX
D3
n3
D2
+
n2
CAUX
VCC
CT
CT
DRV
OC
FB
GND
RS
R1
TL431
+
VOUT
-
STABILIZING
RAMP
SWITCH
CURRENT
SIGNAL
0.98 V
FEEDBACK
VOLTAGE
0
(a)
(b)
FIGURE 1: TK75003 IN A FLYBACK POWER SUPPLY
(a) SCHEMATIC (b) VOLTAGE AT FEEDBACK PIN
IAUX
VAUX
RAUX
VCC
CT
DRV
FB
GND
CAUX
D3
n3
V1
+
V
Vn3
_
T1 T2
T
RAUX = 20.6 Ω
Rounding the result to the nearest 5% standard value
gives RAUX = 20 Ω.
FIGURE 2: SUBCIRCUIT FOR CALCULATING THE
VALUE OF RAUX
January 1999 TOKO, Inc.
Page 9
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