GaN Driver Bootstrap Overcharge Prevention
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Solution Overview
Problem
Gallium Nitride (GaN) Enhancement High-electron-mobility transistors (e-HEMTs) face challenges in half bridge switching applications due to their narrow gate voltage range and reverse current flow characteristics, which can lead to bootstrap overcharge and potential damage in high voltage and high dV/dt applications, necessitating additional circuitry for regulation and protection.
Innovation Solution
A driver system that includes high side charge circuitry to charge the high side supply using the low side supply when the low side driver activates the low side GaN device, optimizing synchronization and eliminating the need for extra components or pins, thereby preventing bootstrap overcharge and protecting the GaN e-HEMTs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional circuitry is added for regulation and protection of the bootstrap capacitor, then the reliability of the GaN e-HEMT is improved, but the device complexity increases
Solution Approach 1:
The low side GaN e-HEMT device itself is used to charge the bootstrap capacitor during its on-state, eliminating the need for external charging circuitry. The device's own current flow path is utilized to perform the charging function, making the system self-sufficient and reducing external component requirements.
Solution Approach 2:
The bootstrap charging function is merged with the low side switch operation. The same current path that flows through the low side GaN e-HEMT during normal operation is also used to charge the bootstrap capacitor, combining two functions into one operational sequence and eliminating separate charging circuitry.
2Ease of operation
If the bootstrap capacitor is charged using conventional circuitry, then the high side driver can be powered, but additional external components are required increasing the overall system complexity
Solution Approach 1:
The system uses its own operational current to charge the bootstrap capacitor. During low side switch conduction, the current naturally flows through the bootstrap diode and charges the capacitor, requiring no external power source or additional charging components for the high side driver.
Solution Approach 2:
The low side switch serves dual purposes: it performs its primary switching function and simultaneously charges the bootstrap capacitor for the high side driver. This multi-functionality eliminates the need for dedicated bootstrap charging circuitry and reduces overall component count.
3Productivity
If GaN e-HEMTs are used in high voltage and high dV/dt applications, then the switching performance is improved, but reverse current flow causes bootstrap overcharge and potential damage
Solution Approach 1:
The reverse current flow characteristic of GaN e-HEMTs, which normally causes harmful bootstrap overcharge, is converted into a useful charging mechanism. By strategically timing the low side switch activation, the reverse current is directed to charge the bootstrap capacitor during the appropriate phase, transforming a harmful effect into a beneficial charging function.
Solution Approach 2:
The bootstrap capacitor is charged in advance during the low side switch on-state, before the high side switch needs to operate. This preliminary charging action ensures the high side driver is fully powered and ready, preventing any risk of overcharge during high voltage switching operations.
Data Source
AI summary
A device for switching Gallium Nitride (GaN) devices includes a high side driver, low side driver, and high side charge circuitry. The high side driver is adapted to control a high side GaN device using a high side supply. The low side driver is adapted to control a low side GaN device using a low side supply. The high side charge circuitry is adapted to charge the high side supply with the low side supply when the low side driver activates the low side GaN device.