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TK75003D Datasheet(PDF) 8 Page - TOKO, Inc |
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TK75003D Datasheet(HTML) 8 Page - TOKO, Inc |
8 / 14 page 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 Ω (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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