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MC34262 数据表(PDF) 10 Page - ON Semiconductor |
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MC34262 数据表(HTML) 10 Page - ON Semiconductor |
10 / 19 page MC34262, MC33262 http://onsemi.com 10 Table 1. Design Equations Notes Calculation Formula Calculate the maximum required output power. Required Converter Output Power PO = VO IO Calculated at the minimum required ac line voltage for output regulation. Let the efficiency h = 0.92 for low line operation. Peak Inductor Current IL(pk) = 22 PO hVac(LL) 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. Inductance LP = t 2 2 VO PO VO − Vac(LL) h Vac(LL)2 In theory the on−time ton is constant. In practice ton tends to increase at the ac line zero crossings due to the charge on capacitor C5. Let Vac = Vac(LL) for initial ton and toff calculations. Switch On−Time h Vac2 ton = 2 PO LP The off−time toff 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. Switch Off−Time VO − 1 toff = 2 Vac ⎪Sin q⎜ ton The minimum switching frequency occurs at the peak of the ac line voltage. As the ac line voltage traverses from peak to zero, toff approaches zero producing an increase in switching frequency. Switching Frequency f = ton + toff 1 Set the current sense threshold VCS to 1.0 V for universal input (85 Vac to 265 Vac) operation and to 0.5 V for fixed input (92 Vac to 138 Vac, or 184 Vac to 276 Vac) operation. Note that VCS must be <1.4 V. Peak Switch Current R7 = IL(pk) VCS 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. Multiplier Input Voltage + 1 Vac VM = R5 2 R3 The IIB R1 error term can be minimized with a divider current in excess of 50 mA. Converter Output Voltage − IIB R2 VO = Vref R2 + 1 R1 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. Converter Output Peak to Peak Ripple Voltage 2 2 pfac C3 + ESR2 DVO(pp) = IO 1 The bandwidth is typically set to 20 Hz. When operating at high ac line, the value of C1 may need to be increased. (See Figure 25) Error Amplifier Bandwidth BW = gm 2 p C1 The following converter characteristics must be chosen: VO − Desired output voltage IO − Desired output current DVO − Converter output peak−to−peak ripple voltage Vac − AC RMS line voltage Vac (LL) − AC RMS low line voltage |
类似零件编号 - MC34262_05 |
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类似说明 - MC34262_05 |
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