Hybrid Bulk Capacitance Circuit for AC/DC Power Supplies
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Solution Overview
Problem
Existing AC/DC chargers and adapters for low power electronics face challenges in size reduction, power density, and reliability due to the large volume occupied by bulk capacitors, especially when operating over a wide range of AC input voltages, as they require physically large capacitors to meet energy storage needs, which are costly and inefficient.
Innovation Solution
A bulk capacitor circuit that selectively connects multiple capacitors of different voltage ratings based on the input voltage range, using a switch controlled by input voltage sensing circuitry to optimize capacitance and reduce volume, while ensuring reliable operation by maintaining ripple voltage within specified ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single bulk capacitor is used to operate over a wide AC input voltage range (85-265 VAC), then the capacitor must be physically large to meet energy storage requirements at minimum voltage, but this increases volume and cost
Solution Approach 1:
The bulk capacitance is segmented into multiple capacitors with different voltage ratings (e.g., a first capacitor rated for higher voltage and a second capacitor rated for lower voltage). These capacitors are connected in parallel only when the AC input voltage is below a threshold, allowing the system to use smaller capacitors overall while maintaining adaptability across the full voltage range.
Solution Approach 2:
The capacitor configuration is made dynamic through a switch controlled by a sensor that detects AC input voltage. When voltage drops below a threshold, the switch connects additional capacitors in parallel to increase total capacitance. This dynamic adjustment allows the system to optimize capacitance for each operating condition without requiring a single oversized capacitor.
2Reliability
If capacitors with higher voltage ratings are used to ensure reliable operation at maximum AC input voltage, then reliability is improved, but cost increases
Solution Approach 1:
Different capacitors are assigned different voltage ratings matched to their specific operating conditions. The first capacitor is rated for higher voltage to handle maximum AC input, while the second capacitor is rated for lower voltage and is only activated when needed. This local optimization allows each capacitor to be appropriately rated for its actual stress conditions, reducing overall cost while maintaining reliability.
3Loss of energy
If GaN power transistors are used instead of silicon MOSFETs, then switching losses are reduced and switching frequency can be increased, but device complexity increases
Solution Approach 1:
The patent transitions from silicon MOSFETs to GaN HEMTs, changing the material parameter to achieve lower on-resistance, higher breakdown voltage, and elimination of reverse-recovery characteristics. This parameter change enables operation at higher switching frequencies (500-600 kHz) with reduced switching losses, despite the increased device complexity.
Data Source
AI summary
A bulk capacitor circuit for an AC input AC/DC Switching Mode Power Supply, such as an AC/DC adapter/charger without active power factor correction, is provided, comprising a plurality of bulk capacitors having different voltage ratings, and driver and control circuitry comprising AC input voltage sensing and comparator circuitry, which enables selective connection of one or more of the plurality of bulk capacitors, responsive to a sensed AC input voltage range. A startup circuit provides power to the driver circuit initially, so that the AC input voltage can be determined before power-up and enabling of the DC/DC converter. This solution provides for a reduction in capacitor volume, with associated improvement in the power density of an isolated AC/DC power supply, while the startup circuit ensures that an appropriate bulk capacitance is connected at startup for low line AC input, to maintain the ripple voltage in an appropriate range for reliable operation.