Active Bootstrap Gate Driver for Faster High-Side GaN Switching
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Solution Overview
Problem
Conventional bootstrapping gate drivers for high side FETs face limitations due to diode voltage drops and slow charging of bootstrap capacitors, which restrict the gate-to-source voltage of high side FETs, leading to slower turn-on times and increased power consumption.
Innovation Solution
The implementation of an active bootstrap charging circuit using an active charging FET and a diode-connected FET, along with a second bootstrap capacitor, allows the bootstrap capacitor to be charged up to the supply voltage without diode voltage drops, enabling a larger gate-to-source voltage and faster pull-up times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a diode-connected FET is used to charge the bootstrap capacitor, then the circuit structure is simple, but the voltage drop across the diode limits the charging voltage and slows down the charging speed
Solution Approach 1:
The patent changes the charging mechanism from passive diode-based charging to active FET-based charging. The active charging FET dynamically controls the charging current, enabling faster charging speeds while the diode-connected FET maintains voltage level stability. This parameter change transforms the charging process from voltage-limited to current-controlled.
Solution Approach 2:
The patent introduces a second bootstrap capacitor as an intermediary energy storage element. This capacitor is charged to a higher voltage level and then used to charge the first bootstrap capacitor, effectively bypassing the voltage drop limitation of the diode-connected FET and enabling faster charging of the primary capacitor.
2Device complexity
If a diode-connected FET is used to charge the bootstrap capacitor, then the circuit structure is simple, but the gate-to-source voltage of the high side FET is limited
Solution Approach 1:
The patent fundamentally changes the voltage generation mechanism by using active FETs instead of passive diodes. The active charging FET and diode-connected FET work together to generate a voltage that exceeds the supply voltage Vdd, thereby increasing the gate-to-source voltage of the high side FET and improving its switching performance.
Solution Approach 2:
The second bootstrap capacitor serves as an intermediary that stores energy at a higher voltage level. This intermediary capacitor enables the first bootstrap capacitor to be charged to a voltage higher than Vdd, which in turn provides a higher gate-to-source voltage for the high side FET without requiring a more complex circuit topology.
3Use of energy by stationary object
If a larger resistance is used for the pull-up resistor, then the static current is reduced and power consumption decreases, but the turn-on time of the high side FET increases
Solution Approach 1:
The patent employs periodic action by using the second bootstrap capacitor to periodically supply charging current to the first bootstrap capacitor during the high side FET turn-on phase. This periodic energy transfer enables fast turn-on without requiring a low resistance pull-up resistor, thus maintaining low static current while achieving fast switching.
Solution Approach 2:
The second bootstrap capacitor is pre-charged to a higher voltage level before the high side FET needs to turn on. This preliminary energy storage allows the first bootstrap capacitor to be rapidly charged when needed, enabling fast turn-on time without compromising static current reduction through the use of a larger pull-up resistor.
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 provides a more robust and efficient bootstrapping gate driver with faster turn-on and turn-off times, improved power handling, and reduced power consumption, making it more resilient to voltage and temperature variations.
Implementation Method 1
Capacitor 350 is coupled between node 360 and the output. The gate terminal of FET 385 is coupled to node 375. As the voltage on the output increases, energy stored in capacitor 350 discharges through node 370, pull up resistor 345, and node 375, which increases the voltage on nodes 370 and 375. As the voltage on the output approaches the supply voltage Vdd, the voltage on node 375 increases to approximately the voltage on the output plus the voltage across capacitor 350, above the supply voltage Vdd.
Implementation Method 2
The drain terminal of active charging FET 335 is coupled to the gate terminal of diode-connected FET 325. The source terminal of diode-connected FET 325 is coupled to node 375. For a logic high on the input, the voltage at node 360 and the voltage at the output are at ground. The voltage at node 375 is approximately equal to the supply voltage Vdd plus the voltage stored on capacitor 315. Hence, active charging FET 335 turns on and charges capacitor 350 to approximately Vdd.
Data Source
AI summary
A circuit to enhance the driving capability of conventional inverting bootstrapping GaN drivers. When the inverting driver input is logic high and the driver output is off, the voltage stored on the first bootstrap capacitor for turning on the high side (pull-up) FET of the inverting driver is charged to the full supply voltage using an active charging FET, instead of using a diode or diode-connected FET in a conventional bootstrapping driver. The gate voltage of the active charging FET is bootstrapped to a voltage higher than supply voltage by a second bootstrap capacitor that connects to the inverting driver input, which is at a logic high. The second bootstrap capacitor is charged by an additional diode or diode-connected FET connected to the supply voltage when the inverting driver input is a logic low.