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ISL6244CRZ-T Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6244CRZ-T Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 25 page  10 FN9106.3 December 28, 2004 Interleaving The switching of each channel in a multi-phase converter is timed to be symmetrically out of phase with each of the other channels. In a 3-phase converter, each channel switches 1/3 cycle after the previous channel and 1/3 cycle before the following channel. As a result, the three-phase converter has a combined ripple frequency three times greater than the ripple frequency of any one phase. In addition, the peak-to- peak amplitude of the combined inductor currents is reduced in proportion to the number of phases (Equations 1 and 2). Increased ripple frequency and lower ripple amplitude mean that the designer can use less per-channel inductance and lower total output capacitance for any performance specification. Figure 13 illustrates the multiplicative effect on output ripple frequency. The three channel currents (IL1, IL2, and IL3), combine to form the AC ripple current and the DC load current. The ripple component has three times the ripple frequency of each individual channel current. Each PWM pulse is terminated 1/3 of a cycle, or 1.33 µs for fS = 250kHz, after the PWM pulse of the previous phase. The peak-to-peak current waveforms for each phase is about 7A, and the dc components of the inductor currents combine to feed the load. To understand the reduction of ripple current amplitude in the multi-phase circuit, examine the equation representing an individual channel’s peak-to-peak inductor current. In Equation 1, VIN and VOUT are the input and output voltages respectively, L is the single-channel inductor value, and fS is the switching frequency. The output capacitors conduct the ripple component of the inductor current. In the case of multi-phase converters, the capacitor current is the sum of the ripple currents from each of the individual channels. Compare Equation 1 to the expression for the peak-to-peak current after the summation of N symmetrically phase-shifted inductor currents in Equation 2. Peak-to-peak ripple current decreases by an amount proportional to the number of channels. Output- voltage ripple is a function of capacitance, capacitor equivalent series resistance (ESR), and inductor ripple current. Reducing the inductor ripple current allows the designer to use fewer or less costly output capacitors. Another benefit of interleaving is to reduce input ripple current. Input capacitance is determined in part by the maximum input ripple current. Multi-phase topologies can improve overall system cost and size by lowering input ripple current and allowing the designer to reduce the cost of input capacitance. The example in Figure 14 illustrates input currents from a three-phase converter combining to reduce the total input ripple current. The converter depicted in Figure 14 delivers 52A to a 1.20V load from a 19V input. The RMS input capacitor current is 6.5A. Compare this to a single-phase converter also stepping down 19V to 1.20V at 52A. The single-phase converter has 11.96A RMS input capacitor current. The single-phase converter must use an input capacitor bank with twice the RMS current capacity as the equivalent three- phase converter. Figures 28, 29 and 30 in the section entitled Input Capacitor Selection can be used to determine the input-capacitor RMS current based on load current, duty cycle, and the number of channels. They are provided as aids in determining the optimal input capacitor solution. Figure 31 shows the single phase input-capacitor RMS current for comparison. FIGURE 13. PWM AND INDUCTOR-CURRENT WAVEFORMS FOR 3-PHASE CONVERTER 1 µs/DIV PWM2, 5V/DIV PWM1, 5V/DIV IL2, 7A/DIV IL1, 7A/DIV IL1 + IL2 + IL3, 7A/DIV IL3, 7A/DIV PWM3, 5V/DIV I PP V IN V OUT – () V OUT Lf S VIN ------------------------------------------------------ = (EQ. 1) I CPP , V IN NV OUT – () V OUT Lf S VIN ------------------------------------------------------------ = (EQ. 2) FIGURE 14. CHANNEL INPUT CURRENTS AND INPUT- CAPACITOR RMS CURRENT FOR 3-PHASE CONVERTER INPUT-CAPACITOR CURRENT, 15A/DIV 1 µs/DIV CHANNEL 1 INPUT CURRENT 15A/DIV CHANNEL 2 INPUT CURRENT 15A/DIV CHANNEL 3 INPUT CURRENT 15A/DIV ISL6244 |
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