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FARM2T13 Datasheet(PDF) 8 Page - Vicor Corporation |
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FARM2T13 Datasheet(HTML) 8 Page - Vicor Corporation |
8 / 11 page FARM™ Rev 6.7 vicorpower.com Page 8 of 11 06/2015 800 927.9474 FARMxxxx The power fail warning time ( Δt) is defined as the interval between B OK and converter shut down (EN) as illustrated in Fig. 13. The Bus-OK and Enable thresholds are 205 V and 190 V, respectively. A simplified relationship between power fail warning time, operating power, and bus capacitance is obtained by inserting these constants: C = 2P Δt / (205 2 – 1902) C = 2P Δt / (5,925) It should be noted that the series combination (C1, C2, see Fig. 9) requires each capacitor to be twice the calculated value, but the required voltage rating of each capacitor is reduced to 200 V. Allowable ripple voltage on the bus (or ripple current in the capacitors) may define the capacitance requirement. Consideration should be given to converter ripple rejection and resulting output ripple voltage. For example, a converter whose output is 15 V and nominal input is 300 V will provide approximately 56 dB ripple rejection, i.e., 10 V p-p of input ripple will produce 15 mV p-p of output ripple (see Fig. 17). Equation 3 is again used to determine the required capacitance. In this case, V1 and V2 are the instantaneous values of bus voltage at the peaks and valleys (see Fig. 13) of the ripple, respectively. The capacitors must hold-up the bus voltage for the time interval ( Δt) between peaks of the rectified line as given by: Δt = (π – θ) / 2πf (4) Where: f = line frequency θ = rectifier conduction angle The approximate conduction angle is given by: θ = Cos -1 (V 2/V1) (5) Another consideration in hold-up capacitor selection is their ripple current rating. The capacitors’ rating must be higher than the maximum operating ripple current. The approximate operating ripple current (rms) is given by: I rms = 2P/Vac (6) Where: P = total output power Vac = operating line voltage Calculated values of bus capacitance for various hold-up time, ride-through time, and ripple voltage requirements are given as a function of operating power level in Figures 14, 15, and 16, respectively. APPLICATION NOTE (CONT.) Example In this example, the output required from the DC-DC converter at the point of load is 12 Vdc at 320 W. Therefore, the output power from the FARM would be 375 W (assuming a converter efficiency of 85%). The desired hold-up time is 9 ms over an input range of 90 to 264 Vac. Determining Required Capacitance for Power Fail Warning. Figure 14 is used to determine capacitance for a given power fail warning time and power level, and shows that the total bus capacitance must be at least 820 μF. Since two capacitors are used in series, each capacitor must be at least 1,640 μF. Note that warning time is not dependent on line voltage. A hold-up capacitor calculator is available on the Vicor website, at: www.vicorpower.com/powerbench/product-calculators. Determining Ride-through Time. Figure 15 illustrates ride- through time as a function of line voltage and output power, and shows that at a nominal line of 90 Vac, ride-through would be 68 ms. Ride-through time is a function of line voltage. Determining Ripple Voltage on the Hold-up Capacitors. Figure 16 is used to determine ripple voltage as a function of operating power and bus capacitance, and shows that the ripple voltage across the hold-up capacitors will be 12 V p-p. Determining the Ripple on the Output of the DC-DC Converter. Figure 17 is used to determine the approximate ripple rejection of the DC-DC converter and indicates a ripple rejection of approximately 60 dB for a 12 V output. Since the ripple on the bus voltage is 12 Vac and the ripple rejection of the converter is 60 dB, the output ripple of the converter due to ripple on its input (primarily 120 Hz) will be 12 mV p-p. For more information about designing an autoranging AC input power supply using the FARM and Vicor DC-DC converter modules, contact Vicor Applications Engineering at the nearest Vicor Technical Support Center, or send email to apps@vicorpower.com. • • • |
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