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Digital Consideration of a Two-Stage Rail Grade DC-DC Converter

Fig. 1. Proposed two-stage dc-dc module with digital controller.
In this proposed system, the two-phase interleaved buck converter is used for voltage regulation. Under critical conduction mode, the voltage regulation must change the on-time of the main switch and the frequency simultaneously. Meanwhile, the negative coupled inductors are used to help magnetic integration. This negative coupling affects the static inductance and the resonant inductance, i.e, resonant with junction capacitance for zero voltage switching (ZVS). With the selected negative coupling coefficient (-0.7), the input voltage losing ZVS is from 68 V to 96 V. To solve this issue, the synchronous rectifier off-time is purposely delayed to inject more energy for ZVS operation. All the mentioned functions (voltage regulation and ZVS extension) are finally implemented through a 60 MHz microcontroller unit (TMS320F28027).
A 300 kW rail-grade converter is built as shown in Fig. 2. Its footprint is a standard quarter brick (2.3 inch ×1.45 inch), which is exactly the same as that of the benchmark system (SynQor's module). This standalone system, as a commercial product, includes power stages, a digital controller, an auxiliary power supply, protection circuits, and power management bus (PMBus) chips. The measured peak efficiency is 95.8 percent with a power density of 195 W/in3.

Fig. 2. Comparison between the proposed module and SynQors module.
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