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TK75003D Datasheet(PDF) 8 Page - TOKO, Inc

Part # TK75003D
Description  PWM CONTROLLER
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Manufacturer  TOKO [TOKO, Inc]
Direct Link  http://www.toko.com
Logo TOKO - TOKO, Inc

TK75003D Datasheet(HTML) 8 Page - TOKO, Inc

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Page 8
January 1999 TOKO, Inc.
TK75003
DESIGN CONSIDERATIONS
SELECTING A START-UP RESISTOR
Figure 1 shows the typical application of the TK75003 in an
off-line flyback power supply (input full-wave bridge and
capacitor not shown). The IC starts when the voltage
across the capacitor CAUX reaches the UVLO on Voltage
VIN(ON) of the IC. The starting resistor RST can be designed
as follows:
RST(MAX) = (VIN(MIN) - VCC(ON,MAX) - 2 V) / ICC(START,MAX)
(1)
At 85 Vrms line voltage, and taking into account the
specified maximum values of the UVLO on voltage and the
start-up supply current ICC(START), the maximum allowed
value of the starting resistor is:
RST(MAX) = (85
2 - 16 - 2 ) / 1.0 mA = 102.2 k
(2)
A practical choice for the starting resistor is RST = 100 kΩ.
The worst-case dissipation of the resistor appears at high
line and at the minimum VCC voltage. At 265 Vrms line
voltage and 9 V VCC
, the dissipation is 2.2 W, so a 3 W
resistor should be used. Note that 1.0 mA reflects the worst
case ICC(START) at the edge of UVLO release.
SELECTING THE TRANSFORMER TURNS RATIO
During steady-state operations, the auxiliary supply voltage
is generated by the auxiliary winding n3 and the rectifier
diode D3. In the flyback power supply, neglecting the effect
of the leakage inductance of the transformer, the number
of turns of the auxiliary winding can be calculated from the
following equation:
n3 = n2 [(VAUX + VD3) / (VOUT + VD2)]
(3)
where VD2 and VD3 are the forward voltage drops of the
output rectifier diode and the auxiliary rectifier diode. The
voltage VAUX should be selected such that it stays between
the specified worst-case upper and lower limits of the IC,
considering the component tolerances, ripple, and other
second-order effects. The upper limit for VAUX is the
minimum voltage of the built-in clamp (16 V). The lower
limit for VAUX is the maximum UVLO off voltage (12.0 V). It
is prudent to choose the mean value of those two voltages
(i.e., 14.0 V), as VAUX.
COMPENSATING FOR LEAKAGE INDUCTANCE
The leakage inductance of the flyback transformer causes
a voltage overshoot at turn-off of the MOSFET. The
magnitude and duration of the overshoot depends on the
leakage inductance, the peak current at turn-offs, and the
voltage-clamping circuit employed to limit the overshoot.
The overshoot tends to increase the auxiliary voltage. The
simplest solution to reduce that increase is to add a
resistor RAUX in series with the rectifier diode D3. The
optimal value of the resistor can be calculated from the
subcircuit shown in Figure 2.
The average current flowing in RAUX is equal to the current
IAUX drawn by the IC. The following equation can be written
from the equality:
IAUX = (1 / RAUX) x ([(V1 - VD3 - VAUX) x (T1 / T)] + [(V2 - VD3 - VAUX) x (T2 / T)])
(4)
The voltage V
1 can be calculated as follows:
V1 = (VOUT + VD2) x (n1 / n2) + [VOVERSHOOT x ( n3 / n1)]
(5)
where V
OVERSHOOT is the additional voltage appearing
across the MOSFET due to the leakage inductance.
The voltage V
2 can be calculated as follows:
V2 = (VOUT + VD2) x ( n3 / n2)
(6)


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