TK75003 Ver la hoja de datos (PDF) - Toko America Inc

Número de pieza

componentes Descripción

Fabricante

TK75003

PWM CONTROLLER

Toko America 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:

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

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 kΩ

(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,

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 V1 can be calculated as follows:

V1 = (VOUT + VD2) x (n1 / n2) + [VOVERSHOOT x ( n3 / n1)]

(5)

where VOVERSHOOT is the additional voltage appearing

across the MOSFET due to the leakage inductance.

The voltage V2 can be calculated as follows:

V2 = (VOUT + VD2) x ( n3 / n2)

(6)

Page 8

January 1999 TOKO, Inc.

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