POWER DEVICES
Figure 3: Converter efficiency at VIN (no air flow)
= 24V with convection cooling The controller U1 equally balances the inductor currents among all
phases (extender and primary). This is done by measuring the output currents of each channel via the corresponding current sense ISPx, ISNx pins, and the ISP and ISN terminals connected to SENSE+ and SENSE– of U2. The INTVCC
and bootstrap voltage (BOOST) signals are
also factored into the control equation. The schematics in Figure 1 and Figure 2 illustrate a minimum
configuration for a boost converter featuring up to five phases. The LT8551 can be used to scale up nearly any single-phase boost controller up to 18 unique phases, with a corresponding multiplication of available output power. In >5-phase configurations, one LT8551 is the master, with additional LT8551 controllers acting as slaves. The CLK1 signal of the master synchronises primary and slave controllers, whereas the CLK2 signal defines the phase angles for the consequent phases, with up to 18 unique angles. The 18-phase limitation does not necessarily limit the number of channels - if channels are allowed to share the same phase angle, then the number of power phases is essentially unlimited. The power train configuration shown in Figure 1 and Figure 2
comprises N-channel power MOSFETs Q1 through Q20, inductors L1 through L5, and input and output filters. The efficiency of the converter is shown in Figure 3, with a maximum output current of 30A, an output voltage VOUT
= 48V, and an input voltage VIN current should be reduced to lower than VIN = 24V. The load to limit the input current
and thermal stress. The load current derating curve is presented in Figure 4. The LT8551 includes internal inductor current balancing
Figure 5: An LT8551-based demonstration circuit DC2896A-B
circuits that provide excellent current sharing between the phases, from ±6% to ±10% at maximum. To reduce thermal stress on both controllers, especially at higher
voltages, use the auxiliary power source (AUX). One solution can be found in the LT8551’s schematics. A photo of the DC2896A-B evaluation circuit, with designated
primary and expanded phases, is shown in Figure 5. The thermal image of the expanded phases is shown in Figure 6.
Figure 6: An LT8551-based demonstration circuit thermal image with convection cooling (no air flow). VIN
= 24V, VOUT = 48V at 25A The LT8551 phase expander gives power supply designers a flexible
tool to build high power, high efficiency boost converters by expanding switching phases until the desired limit of power is reached. High frequency (up to 1MHz) helps minimise power components’ size, and integrated gate drivers, along with accurate inductor current monitoring and balancing, help prevent saturation and evenly spread heat across a board’s surface.
Analog Devices Incorporated
www.analog.com Figure 4: Converter input voltage vs. load current derating curve NOVEMBER 2021 | ELECTRONICS TODAY 17
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