Universal AC Power Factor Correction Benefits from Adaptive Cell Topology

Because of the wide variation in AC line voltages around the world, universal power supplies are typically designed to accept 85 VRMS to 264 VRMS inputs. This range has imposed significant design tradeoffs on the power factor corrected AC rectification typically used in front end converters. The Vicor PFM converter, with Adaptive Cell™ topology at its heart, provides a breakthrough in AC front end power factor correction that addresses these tradeoffs to provide a consistently efficient solution. It does so by introducing innovation at three different levels – Component, Cell and Brick.

At component level, it utilises Double Clamp ZVS, while its Adaptive Cell converter architecture dynamically interconnects two such Double Clamp ZVS ‘Cells’ to meet prevailing line voltage levels. This structure is contained within the PFM VI Chip, which powers the VI BRICK AC Front End module. This also includes input rectification, EMI and transient suppression filters in a convenient PCB mount package with standardised baseplate.

Power factor correction control designs have traditionally been referenced to boost converter technology. Output voltage is regulated by varying the amplitude of the input current, which closely follows the shape of the line voltage.  One of the main concerns is the rating of power semiconductors and magnetic components for worst case scenarios which never concurrently occur.

The first aspect of Vicor’s solution, the Double Clamp, Zero Voltage Switching Converter (DC-ZVS) comprises a full bridge primary and a single ended rectifier secondary, as shown in Figure 1. Using both the inductor and the capacitor CCL as ‘double clamp’ energy storage elements allows switching at either zero-voltagei.e. ‘zero-voltage-switching’. This avoids switching losses and enables very high switching frequency. More importantly, it also allows switches with lower voltage ratings, resulting in isolated converters with power densities exceeding 700 W/in2 (43 W/cm3).

Schematic Zero Voltage Switching Converter

Figure 1: Double Clamp, Zero Voltage Switching Converter Schematic Diagram

Adaptive Cell architecture addresses the issue of wide input voltage variation. It manages two DC-ZVS converters at their highest efficiency for all the input line voltages. Figure 2 shows the two-cell arrangement. Their primary stages can be configured either in series or parallel to the input line through switches T1, T2 and T3. Power sharing is guaranteed for any condition.

Schematic Adaptive Cell

Figure 2: Adaptive Cell™ dynamic architecture schematic diagram

During the lower half of the possible input voltage range, from 85 VRMS to 145 VRMS, T1 and T3 are closed. This configures the primaries in parallel, so they divide the input current and share the same input voltage. Effective duty cycle control is applied, shaping the input current according to power factor corrected front-end requirements. When input voltage exceeds the 145VRMS threshold (with some hysteresis) and up to the higher limit of 264VRMS, T1 and T3 open, while T2 closes. Primaries are now connected in series, effectively dividing the line voltage while sharing the input current. Duty cycle control is applied to obtain power factor correction as in the previous state. The two cells configurations allow identical converters operations across each half of the input line range. Both input current and voltage range requirements are reduced by 50% compared to single cell architecture.

The Vicor PFM module’s DC-ZVS engine and Adaptive Cell architecture bring several significant benefits. Converter efficiency and thermal design are minimally affected by line voltage. Reduced rating, higher figure of merit components can be used. This increases efficiency and reduces thermal gradients, improving robustness and reliability, and maximising system availability. High-frequency, Zero Voltage Switching allows low-cost, high-density filters. The noise harmonic spectrum is also reduced, easing filter tuning.

Avanced controls within the PFM reduce susceptibility to line voltage disturbances and line harmonic propagation, improving overall power quality. The module meets SELV standards, and offers isolated output through magnetic coupling. It also meets all applicable international EMI and EMC compliance standards.

The PFM module merges state-of-the art technologies at various levels: topology, architecture, high-performance analogue controls, supervisory digital controls and low- profile, surface-mount packaging. These technologies enable high density, optimized rectification power system designs across the universal AC input line.

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One Response to:
Universal AC Power Factor Correction Benefits from Adaptive Cell Topology

  1. […] The use of an active power factor correcting circuit results in few discontinuities in the input current and consequently low distortion and harmonic content imposed on the input current drawn from the line. However Vicor has recently introduced a modular VI Brick® AC Front End, based on their new dynamic converter architecture, called Adaptive Cell. […]

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