POWER DEVICES
Phase expander helps build high power, high efficiency boost converters
Victor Khasiev, Senior applications engineer at ADI explores multiplying the power of a boost converter with a phase expander The LT8551 is an excellent choice for automotive and industrial
T
here’s a general understanding in the engineering community that multiphase functionality required when boost converters
must supply high output voltages, work from low input voltages, provide high step-up ratios, or support high load currents. Multiphase boost designs have a number of advantages over single-phase designs, including: increased efficiency, improved transient response, and reduced input and output capacitance values (due to reduced inductor ripple current and ripple current in the input and output capacitors) resulting in reduced thermal stress on the components of the entire boost converter power train. The easy part of designing a multiphase boost converter is
connecting the input supply and output rails to reduce the size and cost of the input/output filters. The difficult part is connecting the outputs of the error amplifiers and feedback pins of the phase controllers to ensure balanced current sharing and correct phase synchronisation. Both signals are extremely sensitive to noise and, even with a very careful layout, can be affected by the sharp current and voltage changes typical in boost converter applications. Some boost controllers come with multiphase functionality to solve this problem, but many do not. For controllers that do not have multiphase circuitry included, the
LT8551 multiphase boost converter phase expander from ADI solves the problem by operating alongside, and detecting the states of the switching components of the primary controller. The LT8551 duplicates its functionality, measures the primary controller inductor current, and adjusts the inductor current in each added phase.
applications due to its high input/output voltage (up to 80V) and ability to produce very high power boost converters, including those with bidirectional current flow.
Functionality of the converter Figure 1 and Figure 2 show a complete LT8551-based phase expander solution. To clarify functionality, the phase expander U1 is divided into three subcircuits: U1.1, U1.2, and U1.3. The interface, U1.1, communicates with the primary controller U2 and any external signals. The power stages U1.2 and U1.3 implement actual power conversion and control the MOSFET switches. All three sections of U1 shown in Figure 1 and Figure 2 are integrated in the LT8551 controller. The primary controller, U2, senses the output voltage through its FB
pin. It also completes the functionality of the peak current-mode control with its ITH pin as the output of the error amplifier. All high impedance circuitry (off the FB pin) and noise sensitive components (off the ITH pin) are adjacent to U2 and do not connect to external components. This approach enables compact and noise-shielded layouts. Instead of using the typical feedback and error amplifier signals to
expand phases, the LT8551 implements an elaborate (but more robust) switch state detection scheme. The subcircuit U1.1 relies on the robust signals of the gate drive voltages, BG and TG, and switching node signal, SW, of the primary controller to manage the four phases comprising the power trains driven by U1.2 and U1.3.
Figure 1: The interface of the LT8551 phase expander U1.1 to the primary controller U2. The four additional (expanded) power phases of the solution are shown in Figure 2
16 NOVEMBER 2021 | ELECTRONICS TODAY
Figure 2: An electrical schematic of the LT8551 power section, U1.2 and U1.3. The LT8551’s interface to the primary boost controller is shown in Figure 1. The VIN
= 6V to 46V, while the VOUT = 48V at 30A
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