Switched-Capacitor Gate Driving for MOSFET False Turn-On Suppression
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Solution Overview
Problem
Existing gate driving circuits for transistors, particularly in bridge-leg configurations, suffer from spurious voltages that can lead to shoot-through currents and device failure, especially in high-speed devices like SiC MOSFETs, due to unavoidable delays and non-zero impedance in the gate path, which conventional methods fail to fully address.
Innovation Solution
A gate driving circuit incorporating a switched capacitor circuit to generate a negative pulse voltage that counteracts spurious voltages, using a combination of transistors and capacitors to momentarily provide a negative voltage source, improving switching performance by ensuring the gate-source voltage remains below the threshold voltage during off-states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a steady negative voltage source is used to bias the gate-source voltage during off state, then the MOSFET can be protected from false turn-on due to positive spurious voltage, but the circuit requires an additional voltage supply and the steady negative voltage reduces the MOSFET lifetime and increases body diode forward voltage
Solution Approach 1:
The patent applies periodic action by using a switched capacitor circuit that generates negative voltage pulses only during specific switching transitions (turn-off and turn-on events) rather than maintaining a continuous negative bias. The capacitor charges during dead time and discharges to generate negative pulses when needed, then returns to zero voltage, eliminating the need for a separate negative voltage supply while providing protection during critical moments
Solution Approach 2:
The patent implements self-service by using the existing dead time in the gate driver circuit to charge the capacitor, and then using that stored energy to generate the protective negative voltage pulses. The circuit leverages its own operational characteristics (dead time and switching events) to generate the protective function without requiring external negative voltage supplies or additional control circuits
2Object-affected harmful factors
If passive impedance reduction methods are used to reduce spurious voltage magnitude, then the gate path impedance is reduced, but the spurious voltage cannot be eliminated due to non-zero impedance of the low impedance path and internal gate resistance
Solution Approach 1:
The patent applies preliminary anti-action by generating a negative voltage pulse before the positive spurious voltage can cause false turn-on. The switched capacitor circuit is timed to discharge and create a negative voltage spike at the gate exactly when the MOSFET is turning off, counteracting the positive spurious voltage before it can exceed the threshold voltage and cause unwanted conduction
Solution Approach 2:
The patent introduces a switched capacitor circuit as an intermediary element between the gate driver and the MOSFET gate. This intermediary circuit actively generates negative voltage pulses to counterbalance the spurious positive voltages, providing a mediating protective function that neither passive impedance reduction alone nor direct gate control can achieve
3Speed
If wide-bandgap devices like SiC MOSFETs are used to achieve high switching speed, then the switching performance is improved, but the spurious voltage issue becomes worse due to their high switching speed
Solution Approach 1:
The patent applies preliminary anti-action by anticipating and counteracting the spurious voltage effects that are exacerbated by high switching speeds. The switched capacitor circuit is designed to generate negative voltage pulses synchronized with the high-speed switching events of SiC MOSFETs, providing protective action precisely when the rapid voltage changes generate the largest spurious voltages
Solution Approach 2:
The patent implements dynamics by using a switched capacitor circuit that dynamically responds to the high-speed switching events. The circuit actively switches the capacitor charge and discharge states in synchronization with the MOSFET switching, creating time-varying negative voltage pulses that adapt to the dynamic spurious voltage conditions generated by high-speed operation
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
The proposed circuit effectively counters both positive and negative spurious voltages, enhancing switching speed and reducing power loss, with performance comparable to conventional drivers using steady negative voltages while being easily implementable with standard gate driver ICs.
Implementation Method 1
a switched capacitor circuit connected to the first gate driver and adapted to generate temporarily a negative voltage to counteract a spurious voltage occurred at the first gate driver
Data Source
AI summary
An apparatus for driving a MOSFET, which includes a first gate driver adapted to generate a driving signal for a MOSFET, a switched capacitor circuit connected to the first gate driver and adapted to generate temporarily a negative voltage to counteract a spurious voltage occurred at the first gate driver; and a second gate driver comprising adapted to drive the switched capacitor circuit. The first gate driver and the second gate driver are powered by a same voltage source and controlled by a same control logic. The apparatus is adapted to momentarily generate a negative voltage that could counteract the spurious voltage generated by switching devices, and improve the switching performance.