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CS5212EDR14G 데이터시트(PDF) 8 Page - ON Semiconductor |
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CS5212EDR14G 데이터시트(HTML) 8 Page - ON Semiconductor |
8 / 13 page CS5212 http://onsemi.com 8 APPLICATIONS INFORMATION APPLICATIONS AND COMPONENT SELECTION Inductor Component Selection The output inductor may be the most critical component in the converter because it will directly effect the choice of other components and dictate both the steady−state and transient performance of the converter. When selecting an inductor the designer must consider factors such as DC current, peak current, output voltage ripple, core material, magnetic saturation, temperature, physical size, and cost (usually the primary concern). In general, the output inductance value should be as low and physically small as possible to provide the best transient response and minimum cost. If a large inductance value is used, the converter will not respond quickly to rapid changes in the load current. On the other hand, too low an inductance value will result in very large ripple currents in the power components (MOSFETs, capacitors, etc) resulting in increased dissipation and lower converter efficiency. Also, increased ripple currents will force the designer to use higher rated MOSFETs, oversize the thermal solution, and use more, higher rated input and output capacitors − the converter cost will be adversely effected. One method of calculating an output inductor value is to size the inductor to produce a specified maximum ripple current in the inductor. Lower ripple currents will result in less core and MOSFET losses and higher converter efficiency. The following equation may be used to calculate the minimum inductor value to produce a given maximum ripple current (α ⋅ IO,MAX). The inductor value calculated by this equation is a minimum because values less than this will produce more ripple current than desired. Conversely, higher inductor values will result in less than the maximum ripple current. LoMIN + (Vin * Vout) @ Vout (a @ IO,MAX @ Vin @ fSW) α is the ripple current as a percentage of the maximum output current (α = 0.15 for ±15%, α = 0.25 for ±25%, etc) and fsw is the switching frequency. If the minimum inductor value is used, the inductor current will swing ± α/2% about Iout. Therefore, the inductor must be designed or selected such that it will not saturate with a peak current of (1 + α/2) ⋅ IO,MAX. Power dissipation in the inductor can now be calculated from the RMS current level. The RMS of the AC component of the inductor is given by the following relationship: IAC + IPP 12 where IPP = α ⋅ IO,MAX. The total IRMS of the current will be calculated from: IRMS + IOUT2 ) IAC2 The power dissipation for the inductor can be determined from: P + IRMS2 RL Input Capacitor Selection and Considerations The input capacitor is used to reduce the current surges caused by conduction of current of the top pass transistor charging the PWM inductor. The input current is pulsing at the switching frequency going from 0 to peak current in the inductor. The duty factor will be a function of the ratio of the input to output voltage and of the efficiency. DF + VO VI 1 Eff The RMS value of the ripple into the input capacitors can now be calculated: IIN(RMS) + IOUT DF * DF2 The input RMS is maximum at 50% DF, so selection of the possible duty factor closest to 50% will give the worst case dissipation in the capacitors. The power dissipation of the input capacitors can be calculated by multiplying the square of the RMS current by the ESR of the capacitor. Output Capacitor The output capacitor filters output inductor ripple current and provides low impedance for load current changes. The effect of the capacitance for handling the power supply induced ripple will be discussed here. Effects of load transient behavior can be considered separately. The principle consideration for the output capacitor is the ripple current induced by the switches through the inductor. This ripple current was calculated as IAC in the above discussion of the inductor. This ripple component will induce heating in the capacitor by a factor of the RMS current squared multiplied by the ESR of the output capacitor section. It will also create output ripple voltage. The ripple voltage will be a vector summation of the ripple current times the ESR of the capacitor, plus the ripple current integrating in the capacitor, and the rate of change in current times the total series inductance of the capacitor and connections. The inductor ripple current acting against the ESR of the output capacitor is the major contributor to the output ripple voltage. This fact can be used as a criterion to select the output capacitor. VPP + IPP CESR The power dissipation in the output capacitor can be calculated from: P + IAC2 CESR where: IAC = AC RMS of the inductor CESR = Effective series resistance of the output capacitor network. MOSFET & Heatsink Selection Power dissipation, package size, and thermal solution drive MOSFET selection. To adequately size the heat sink, |
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