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MC33368DG bảng dữ liệu(PDF) 10 Page - ON Semiconductor |
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10 / 16 page MC33368 http://onsemi.com 10 Table 1. Design Equations Calculation Formula Notes Converter Output Power PO + VO IO Calculate the maximum required output power. Peak Indicator Current IL(pk) + 22 PO h Vac(LL) Calculated at the minimum required ac line voltage for output regulation. Let the efficiency η = 0.92 for low line operation. Inductance LP + t VO 2 –Vac(LL) h Vac(LL)2 2 VO PO Let the switching cycle t = 40 ms for universal input (85 to 265 Vac) operation and 20 ms for fixed input (92 to 138 Vac, or 184 to 276 Vac) operation. Switch On−Time t(on) + 2PO LP h Vac2 In theory, the on−time t(on) is constant. In practice, t(on) tends to increase at the ac line zero crossings due to the charge on capacitor C5. Let Vac = Vac(LL) for initial t(on) and t(off) calculations. Switch Off−Time t(off) + t(on) VO 2 Vac Sin q –1 The off−time t(off) is greatest at the peak of the ac line voltage and approaches zero at the ac line zero crossings. Theta (q) represents the angle of the ac line voltage. Minimum Switch Off−Time t(off) min + LP IL(pk) VO The off−time is at a minimum at ac line crossings. This equation is used to calculate t(off) as Theta approaches zero. Delay Time td + –R10 C7 ln VCC –2 VCC The delay time is used to override the minimum off−time at the ac line zero crossings by programming the Frequency Clamp with C7 and R10. Switching Frequency f + 1 t(on) ) t(off) The minimum switching frequency occurs at the peak of the ac line voltage. As the ac line voltage traverses from peak to zero, t(off) approaches zero producing an increase in switching frequency. Peak Switch Current R7 + VCS IL(pk) Set the current sense threshold VCS to 1.0 V for universal input (85 to 265 Vac) operation and to 0.5 V for fixed input (92 to 138 Vac, or 184 to 276 Vac) operation. Note that VCS must be less than 1.4 V. Multiplier Input Voltage VM + Vac 2 R5 R3 ) 1 Set the multiplier input voltage VM to 3.0 V at high line. Empirically adjust VM for the lowest distortion over the ac line voltage range while guaranteeing startup at minimum line. Converter Output Voltage VO + Vref R2 R1 ) 1 –IIB R1 The IIB R1 error term can be minimized with a divider current in excess of 100 mA. Converter Output Peak−to−Peak Ripple Voltage DVO(pp) + IL(pk) 1 2 p fac C3 2 ) ESR2 The calculated peak−to−peak ripple must be less than 16% of the average dc output voltage to prevent false tripping of the Overvoltage Comparator. Refer to the Overvoltage Comparator Text. ESR is the equivalent series resistance of C3. Error Amplifier Bandwidth BW + gm 2 p C1 The bandwidth is typically set to 20 Hz. When operating at high ac line, the value of C1 may need to be increased. NOTE: The following converter characteristics must be chosen: VO = Desired output voltage. Vac(LL) = AC RMS minimum required operating line voltage for output regulation. IO = Desired output current. DVO = Converter output peak−to−peak ripple voltage. Vac = AC RMS operating line voltage. |
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