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ISL6219A Datasheet(PDF) 12 Page  Intersil Corporation 

ISL6219A Datasheet(HTML) 12 Page  Intersil Corporation 
12 / 17 page 12 General Design Guide This design guide is intended to provide a highlevel explanation of the steps necessary to create a multiphase power converter. It is assumed that the reader is familiar with many of the basic skills and techniques referenced below. In addition to this guide, Intersil provides complete reference designs that include schematics, bills of materials, and example board layouts for all common microprocessor applications. Power Stages The first step in designing a multiphase converter is to determine the number of phases. This determination depends heavily on the cost analysis which in turn depends on system constraints that differ from one design to the next. Principally, the designer will be concerned with whether components can be mounted on both sides of the circuit board; whether throughhole components are permitted on either side; and the total board space available for power supply circuitry. Generally speaking, the most economical solutions will be for each phase to handle between 15 and 20A. Allsurfacemount designs will tend toward the lower end of this current range and, if throughhole MOSFETs can be used, higher perphase currents are possible. In cases where board space is the limiting constraint, current can be pushed as high as 30A per phase, but these designs require heat sinks and forced air to cool the MOSFETs. MOSFETs The choice of MOSFETs depends on the current each MOSFET will be required to conduct; the switching frequency; the capability of the MOSFETs to dissipate heat; and the availability and nature of heat sinking and air flow. LOWER MOSFET POWER CALCULATION The calculation for heat dissipated in the lower MOSFET is simple, since virtually all of the heat loss in the lower MOSFET is due to current conducted through the channel resistance (rDS(ON)). In Equation 7, IM is the maximum continuous output current; IL,PP is the peaktopeak inductor current (see Equation 1); d is the duty cycle (VOUT/VIN); and L is the perchannel inductance. An additional term can be added to the lowerMOSFET loss equation to account for additional loss accrued during the dead time when inductor current is flowing through the lowerMOSFET body diode. This term is dependent on the diode forward voltage at IM, VD(ON); the switching frequency, fS; and the length of dead times, td1 and td2, at the beginning and the end of the lowerMOSFET conduction interval respectively. Thus the total power dissipated in each lower MOSFET is approximated by the summation of PL and PD. UPPER MOSFET POWER CALCULATION In addition to rDS(ON) losses, a large portion of the upper MOSFET losses are due to currents conducted across the input voltage (VIN) during switching. Since a substantially higher portion of the upperMOSFET losses are dependant on switching frequency, the power calculation is somewhat more complex. Upper MOSFET losses can be divided into separate components involving the upperMOSFET switching times; the lowerMOSFET bodydiode reverse recovery charge, Qrr; and the upper MOSFET rDS(ON) conduction loss. When the upper MOSFET turns off, the lower MOSFET does not conduct any portion of the inductor current until the voltage at the phase node falls below ground. Once the lower MOSFET begins conducting, the current in the upper MOSFET falls to zero as the current in the lower MOSFET ramps up to assume the full inductor current. In Equation 9, the required time for this commutation is t1and the associated power loss is PUP,1. Similarly, the upper MOSFET begins conducting as soon as it begins turning on. In Equation 10, this transition occurs over a time t2, and the approximate the power loss is PUP,2. A third component involves the lower MOSFET’s reverse recovery charge, Qrr. Since the inductor current has fully commutated to the upper MOSFET before the lower MOSFET’s body diode can recover all of Qrr, it is conducted through the upper MOSFET across VIN. The power dissipated as a result is PUP,3 and is simply Finally, the resistive part of the upper MOSFET’s is given in Equation 12 as PUP,4. In this case, of course, rDS(ON) is the on resistance of the upper MOSFET. 1 f S  2V ∆ ID 0.025  1 – t DV 1 f S  2V ∆ ID 0.025  ≤ < (EQ. 6) P LOW 1 , r DS ON () I M N  2 1d – () IL PP , 2 1d – () 12  + = (EQ. 7) P LOW 2 , V DON () fS I M N  IPP 2  + t d1 I M N  I PP 2  – t d2 + = (EQ. 8) P UP 1 , V IN IM N  IL PP , 2  + t 1 2  f S ≈ (EQ. 9) P UP 2 , V IN I M N  IL PP , 2  – t 2 2  f S ≈ (EQ. 10) P UP 3 , V IN Qrr fS = (EQ. 11) P UP 4 , r DS ON () I M N  2 d I PP 2 12  + = (EQ. 12) ISL6219A 
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