advertisement High Efficiency 2-Phase Boost Converter Minimizes Input and Output Current Ripple – Design Note 371 Goran Perica Introduction Many automotive and industrial applications require higher voltages than is available on the input power supply rail. A simple DC/DC boost converter sufﬁces when the power levels are in the 10W to 50W range, but if higher power levels are required, the limitations of a straightforward boost converter become quickly apparent. Boost converters convert a low input voltage to a higher output voltage by processing the input current with a boost inductor, power switch, output diode and output capacitor. As the output power level increases, the currents in these components increase as well. INPUT 10V TO 23V 22μF w2 75k 13.3k 1 VIN LTC1871-7 7 ITH GATE 10 4 SENSE FREQ 8 5 MODE/SYNC INTVCC 6 3 VFB GND 4.7nF 12k 2 80.6k 12.4k 9 RUN 4.7μF Switching currents also increase proportional to the output-to-input voltage conversion ratio, so if the input voltage is low, the switching currents can overwhelm a simple boost converter and generate unacceptable EMI. For example, consider Figure 1, a 12V input to 24V, 10A output switching converter operating at 300kHz. The currents processed by the converter in Figure 1 are shown in the ﬁrst row of Table 1. The relatively high current levels in the switcher are reﬂected in high input and output ripple currents, which results in increased EMI. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. L1 4.7μH MBRB2545 HAT2165 w2 22μF w6 0.002Ω OUTPUT 24V 10A + 220μF VOUT 100mV/DIV 1μs/DIV DN371 F01 229k L1: COOPER HC3-4R7 ALL CERAMIC CAPACITORS ARE X7R, TDK DN371 F01b Figure 1b. Single-Phase Boost Converter Output Voltage Ripple Figure 1a. Single-Phase Boost Converter: Can be Used to Convert 12V Input to 24V, 10A Output Table 1. Dual-Phase Boost Converter Has Lower Input and Output Ripple Currents and Voltages Than Single-Phase Boost Converter INPUT RMS CURRENT INPUT RIPPLE CURRENT MOSFET RMS DRAIN CURRENT OUTPUT DIODE RMS CURRENT OUTPUT CAPACITOR RMS CURRENT OUTPUT CAPACITOR FREQUENCY OUTPUT VOLTAGE RIPPLE SINGLEPHASE BOOST CONVERTER 21.1A 4.2AP-P 15.4A 14.4A 10.5A 300kHz 212mV DUAL-PHASE BOOST CONVERTER 20.7A 0.17AP-P 2 × 7.4A 2 × 7.2A 1.9A 600kHz 65mV 09/05/371_conv The circuit shown in Figure 2 performs the same DC/DC conversion, but with greatly reduced input and output ripple, signiﬁcantly reducing EMI, and at a higher effective switching frequency, which allows the use of two 22μF output capacitors versus six 22μF output capacitors required in Figure 1. a 240W boost supply application, the power dissipation of 12.9W is relatively easy to manage in a well laid out, large multilayer PCB with some forced airﬂow. Conclusion The simple LT3782 dual-phase switching boost converter improves on single-phase alternatives by allowing high power output with lower ripple currents, reduced heat dissipation and a more compact design. The trick is the 2-phase boost topology, which interleaves two 180° out-of-phase output channels to mutually cancel out input and output ripple current—the results are shown in the second row of Table 1. Each phase operates at 50% duty cycle and the rectiﬁed output currents from each phase ﬂow directly to the load—namely the low inductor ripple current—so only a small amount of output current (shown in Table 1) is handled by the output capacitors. 96 EFECIENCY (%) 95 The centerpiece of the design in Figure 2 is the LT®3782 2-phase current mode PWM controller. Current mode operation ensures balanced current sharing between the two power converters resulting in even power dissipation between the power stages. L1 93 92 91 90 0 2 4 6 8 OUTPUT CURRENT (A) 10 12 DN371 F03 Figure 3. 12V Input to 24V Output Dual-Phase Boost Converter Efﬁciency The efﬁciency of the dual-phase converter, shown in Figure 3, is high enough that it can be built entirely with surface mount components—no need for heat sinks. In INPUT 10V TO 23V 94 UPS840 825k BGATE1 SEN1P RUN RSET DELAY 62k DCL 59k 4.7nF 4.7nF 15k LT3782 GBIAS SLOPE GBIAS1 SS GBIAS2 GND SEN2N VEE1 SEN2P VEE2 BGATE2 VC 100pF 10nF SEN1N VCC HAT2165 10Ω 0.004Ω 2.2μF 220μF 22μF + 274k 0.004Ω 22μF w2 OUTPUT 24V 10A VOUT 100mV/DIV 1μs/DIV 10Ω 10nF Figure 2b. Dual-Phase Boost Converter Output Voltage Ripple HAT2165 220k FB L2 DN371 F02b UPS840 24.9k DN371 F02 L1, L2: PULSE PB2020-153 ALL CERAMIC CAPACITORS ARE X7R TDK Figure 2a. Dual-Phase Boost Converter Reduces EMI and Ripple Currents with a Minimum Input and Output Filtering Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn371f_conv LT/TP 0905 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005

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