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AN-6961 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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AN-6961 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 8 page AN-6961 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.2 • 4/8/09 7 Determine the Output Capacitor Co The output capacitor is determined by the requirement of sufficient hold-up time tHold: η • ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − • • = 2 min . o 2 o hold o o V V t P 2 C (11) where tHold is the output capacitor hold-up time, which is measured from the time the AC input turns off to before the output voltage falls below the minimum operating voltage of the following DC/DC stage. The output ripple voltage ΔVO is expressed as: o o o o V C P V • • = Δ ω (12) where: ω= 2 π f, f is AC line frequency, IO is the output current. Determine the Compensation Capacitor CEA As discussed previously, to achieve a low input current THD, the variation of the on-time within one input AC cycle should be very small. To achieve this, the bandwidth should be lower than 20Hz. The capacitance CEA connected between COMP and GND can be obtained as: BW 2 g C m EA • • = π (13) where BW is PFC control loop bandwidth and establishes it as 20Hz. The error amplifier is a trans-conductance amplifier that converts voltage to current with a 125µmho output conductance. Design Example This section shows a design example of a 90W (19V/4.74A) adaptor. From the specification, all critical components are treated and final measurement results are given. The basic design specification are shown as following: • AC Input Voltage Range Vrms: 90 ~ 264 VAC • Rated Output Power PO: 90 W • Minimum Switching Frequency fs,min: 35KHz • High Regulated Output Voltage: 400V (at 180 ~ 264VAC) • Low Regulated Output Voltage: 250V (at 90 ~ 132VAC) Based on the given design guideline, the critical parameters are calculated and summarized in Table 1: Table 1. Critical System Parameters Lb 530µH ton,fix(90Vrms) 13.86µs ipk 3.327A ton,fix(132Vrms) 6.44µs Nb 65T ton,fix(180Vrms) 3.46µs Naux 7T ton,fix(264Vrms) 1.61µs CO 68µF/450V ΔVO(VO = 250V) 14.043V CEA 1μF ΔVO(VO = 400V) 8.77V Reference Circuit The complete circuit diagram is shown in Figure 19 and the Bill of Materials for the PFC stage is shown in Table 2. Table 2. BOM List of PFC Stage Reference Components Reference Components F1 4A/250V C2 0.33μ/275V R1 510K C3 Open R2 510K C4 Open R3 10K C5 Open R4 1M C7 0.47μF/400V R5 18.7K C8 0.47μF/400V R6 1M C11 2.2μF/50V R7 430K C12 68μF/450V R8 1M C24 104pF R13 Open C25 Open R14 24K C27 Open R15 68K(47K) C28 Open(22p) R16 10 Ω C29 221pF R17 0 Ω Q1 2N-7002 R18 0.18 Ω/2W Q2 2SK-2482/ TO-220 R55 Open BD1 KBP205G R56 Open ZD1 ZD24V R57 Open D2 R860/TO-220 R58 Open D3 1N4148 R59 0 Ω L1 1mH MOV1 470V/7D L2 13mH TR1 055 L4 RM-10/ 530µH U1 FAN6961 L5 400μH C1 224pF |
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