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MC34262 Folha de dados(PDF) 9 Page - ON Semiconductor |
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MC34262 Folha de dados(HTML) 9 Page - ON Semiconductor |
9 / 16 page MC34262 MC33262 9 MOTOROLA ANALOG IC DEVICE DATA APPLICATIONS INFORMATION The application circuits shown in Figures 19, 20 and 21 reveal that few external components are required for a complete power factor preconverter. Each circuit is a peak detecting current–mode boost converter that operates in critical conduction mode with a fixed on–time and variable off–time. A major benefit of critical conduction operation is that the current loop is inherently stable, thus eliminating the need for ramp compensation. The application in Figure 19 operates over an input voltage range of 90 Vac to 138 Vac and provides an output power of 80 W (230 V at 350 mA) with an associated power factor of approximately 0.998 at nominal line. Figures 20 and 21 are universal input preconverter examples that operate over a continuous input voltage range of 90 Vac to 268 Vac. Figure 20 provides an output power of 175 W (400 V at 440 mA) while Figure 21 provides 450 W (400 V at 1.125 A). Both circuits have an observed worst–case power factor of approximately 0.989. The input current and voltage waveforms of Figure 20 are shown in Figure 22 with operation at 115 Vac and 230 Vac. The data for each of the applications was generated with the test set–up shown in Figure 24. 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 η = 0.92 for low line operation. Peak Inductor Current IL(pk) = 22 PO ηVac(LL) Let the switching cycle t = 40 µs for universal input (85 to 265 Vac) operation and 20 µs for fixed input (92 to 138 Vac, or 184 to 276 Vac) operation. Inductance LP = t 2 2 VO PO VO – Vac(LL) η 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 η 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 ( θ) represents the angle of the ac line voltage. Switch Off–Time VO – 1 toff = 2 Vac Sin θ 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 µA. 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 πfac C3 + ESR2 ∆VO(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 π C1 The following converter characteristics must be chosen: VO IO Vac Vac(LL) — Desired output voltage — AC RMS line voltage — Desired output current — Converter output peak–to–peak ripple voltage — AC RMS low line voltage ∆VO |
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