Multiphase N-Channel High-Side Switch Without a Second DC Supply
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Solution Overview
Problem
In GaN semiconductor processes, p-channel FETs are not available or have poor performance, necessitating the use of n-channel FETs with a second DC supply voltage, which is inefficient due to high forward voltage requirements, making charge pumps impractical.
Innovation Solution
A multiphase n-channel switch is implemented using multiple FETs in parallel, driven by AC signals of different phases to generate the required DC voltage without a second supply, ensuring one FET is always on, reducing ripple and eliminating the need for a second DC supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a second DC supply voltage is used to drive n-channel FET gates above VDD, then the FET can be turned on, but the circuit complexity increases and efficiency decreases due to high forward voltage requirements
Solution Approach 1:
The patent divides the single n-channel FET into multiple parallel FETs (typically 3-5 devices), each controlled by AC signals of different phases. This segmentation allows each FET to operate with lower voltage swings while collectively providing the necessary gate drive voltage above VDD, eliminating the need for a second DC supply.
Solution Approach 2:
The patent employs periodic AC signals with different phases (e.g., 0°, 120°, 240° for three-phase) to drive the gates of parallel FETs. The periodic switching creates a stepped voltage waveform that effectively generates the required DC voltage above VDD without needing a second supply, while maintaining continuous conduction through proper phase staggering.
2Device complexity
If a charge pump is used to generate the second supply voltage internally, then the second DC supply is eliminated, but the forward voltage requirement becomes prohibitively high for GaN diodes
Solution Approach 1:
The patent extracts the voltage multiplication function from the charge pump approach and implements it directly through the parallel FET switching action. By taking out the need for intermediate voltage conversion stages and using the FETs themselves to generate the stepped voltage waveform, the system achieves voltage multiplication without the high forward voltage losses inherent in GaN diode-based charge pumps.
Solution Approach 2:
The patent introduces AC coupling capacitors as intermediaries between the parallel FETs and the load. These capacitors smooth the stepped voltage waveform generated by the phase-shifted FET switching, providing a clean DC output without requiring high-voltage diodes or complex charge pump circuits. The capacitors mediate the transition from pulsed FET output to stable DC voltage.
3Ease of operation
If p-channel FETs are used for high-side switching, then the gate voltage can be kept between GND and VDD, but p-channel devices are not available or have poor performance in GaN processes
Solution Approach 1:
The patent inverts the conventional high-side switching approach by using n-channel FETs (which are available in GaN) instead of p-channel FETs. By arranging multiple n-channel FETs in parallel with phase-shifted AC gate drives, the system achieves the functionality of a single high-side switch while using only n-channel devices, which are the standard for GaN technology.
Solution Approach 2:
The patent changes the gate voltage parameters from the conventional DC level (between GND and VDD for p-channel) to AC signals with specific phase relationships. The AC signals have amplitudes and phase shifts designed to create the equivalent effect of a p-channel FET, with the combined output providing gate drive voltages above VDD when needed, while using only n-channel GaN FETs.
Data Source
AI summary
A power supply switch for a gallium nitride integrated circuit. The switch includes two or more parallel n-channel transistor switches (FETs). The FETs are controlled by AC gate waveforms of different phases. The use of multiple AC-controlled FETs allows effective DC operation of a bootstrap inverter circuit without requiring a second DC supply voltage.