GaN Bootstrap Supply for Half-Bridge Zero Reverse Recovery
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Solution Overview
Problem
Conventional half-bridge topologies using GaN FETs face inefficiencies due to reverse recovery charge losses in high voltage Schottky or PN junction diodes, which become significant at higher operating frequencies, affecting the bootstrap circuit's performance.
Innovation Solution
The implementation of a depletion mode GaN FET as a bootstrap device synchronized with the low side transistor, integrated within the half-bridge device pair, addresses the inefficiencies by minimizing voltage across the bootstrap capacitor and reducing the size of the bootstrap device, while ensuring robust operation under various conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional Schottky or PN junction diodes are used in the bootstrap circuit, then the circuit can be manufactured with existing processes, but reverse recovery charge losses increase significantly at higher operating frequencies
Solution Approach 1:
The patent changes the fundamental parameter of the bootstrap device from a diode (Schottky or PN junction) to a depletion mode GaN FET. This parameter change eliminates the reverse recovery charge issue entirely, as depletion mode GaN FETs do not exhibit reverse recovery effects like conventional diodes. The solution maintains manufacturability by using GaN FETs that can be integrated with existing half-bridge device pairs.
2Loss of energy
If high voltage Schottky diodes are used to minimize forward voltage drop, then conduction losses are reduced, but reverse recovery charge is still generated affecting high frequency performance
Solution Approach 1:
The patent changes the device type from Schottky diode to depletion mode GaN FET. This parameter change simultaneously achieves low conduction losses (comparable to or better than Schottky diodes) while eliminating reverse recovery charge, thereby enabling higher operating frequencies without the trade-off present in conventional Schottky diode solutions.
3Ease of manufacture
If PN junction diodes are used in the bootstrap circuit, then manufacturing is simplified, but reverse recovery charge causes significant losses at high switching frequencies
Solution Approach 1:
The patent transitions from PN junction diodes to depletion mode GaN FETs. This parameter change maintains manufacturing feasibility through integration with half-bridge device pairs while completely eliminating reverse recovery charge losses, solving the contradiction between manufacturing simplicity and high-frequency efficiency.
4Reliability
If conventional bootstrap methods are used, then the high side transistor can be powered, but efficiency decreases at higher operating frequencies due to reverse recovery charges
Solution Approach 1:
The patent changes the bootstrap device parameter from conventional diodes to depletion mode GaN FETs. This maintains the essential function of powering the high side transistor while eliminating reverse recovery charge losses, thereby improving efficiency at higher operating frequencies without compromising reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces losses and enhances efficiency by synchronizing the bootstrap device with the low side transistor, preventing over-voltage conditions and minimizing reverse conduction voltage drops, thus improving the overall performance of the half-bridge topology at higher frequencies.
Implementation Method 1
a depletion mode transistor (e.g., an n-channel depletion mode field effect transistor (FET)) as the bootstrap device
Implementation Method 2
The gate of the bootstrap device is coupled to an output of the low side gate driver such that the bootstrap device is synchronized to a switching of the low side transistor
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
ABSTRACT An electrical circuit arranged in a half bridge topology. The electrical circuit includes a high side transistor; a low side transistor; a gate driver and level shifter electrically coupled to a gate of the high side transistor; a gate driver electrically coupled to a gate of the low side transistor; a capacitor electrically coupled in parallel with the gate driver and level shifter; a voltage source electrically coupled to an input of the gate driver and level shifter and an input of the gate driver; and, a bootstrap transistor electrically coupled between the voltage source and the capacitor. A GaN field-effect transistor is synchronously switched with a low side device of the half bridge circuit.