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FARM1E12 Datasheet(PDF) 8 Page - Vicor Corporation |
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FARM1E12 Datasheet(HTML) 8 Page - Vicor Corporation |
8 / 11 page FARM™ Rev 6.9 vicorpower.com Page 8 of 11 06/2017 800 927.9474 FARMxxxx Application Note (Cont.) The power fail warning time (∆t) 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 205V and 190V, respectively. A simplified relationship between power fail warning time, operating power, and bus capacitance is obtained by inserting these constants: It should be noted that the series combination (C1, C2, See Figure 9) requires each capacitor to be twice the calculated value, but the required voltage rating of each capacitor is reduced to 200V. 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 15V and nominal input is 300V will provide approximately 56dB ripple rejection, i.e., 10Vp-p of input ripple will produce 15mVp-p of output ripple (See Figure 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 Figure 13) of the ripple, respectively. The capacitors must hold-up the bus voltage for the time interval (∆t) between peaks of the rectified line as given by: The approximate conduction angle is given by: 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: 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. Example In this example, the output required from the DC-DC converter at the point of load is 12VDC at 320W. Therefore, the output power from the FARM would be 375W (assuming a converter efficiency of 85%). The desired hold-up time is 9ms over an input range of 90 to 264VAC. 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 820µF. Since two capacitors are used in series, each capacitor must be at least 1,640µF. Note that warning time is not dependent on line voltage. A hold-up capacitor calculator is available on the Vicor website, at: vicorpower.com/hubcalc. 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 90VAC, ride-through would be 68ms. 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 12Vp-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 60dB for a 12V output. Since the ripple on the bus voltage is 12VAC and the ripple rejection of the converter is 60dB, the output ripple of the converter due to ripple on its input (primarily 120Hz) will be 12mVp-p. For more information about designing an autoranging AC input power supply using the FARM and Vicor DC-DC convert- er modules, contact Vicor Applications Engineering at the nearest Vicor Technical Support Center, or send email to: apps@vicorpower.com. Storage Vicor products, when not installed in customer units, should be stored in ESD safe packaging in accordance with ANSI/ ESD S20.20, “Protection of Electrical and Electronic Parts, Assemblies and Equipment” and should be maintained in a temperature controlled factory/ warehouse environment not exposed to outside elements controlled between the temperature ranges of 15°C and 38°C. Humidity shall not be condensing, no minimum humidity when stored in an ESD compliant package. C = 2Pt / (2052 – 1902) C = 2Pt / (5,925) (4) t = (π – θ) / 2πf Where: f = line frequency θ = rectifier conduction angle (5) θ = Cos-1V 2 /V1 (6) I RMS = 2P/VAC Where: P = operating power level V AC = operating line voltage |
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