Transistor Drive Circuit Clamping for Gate Voltage Undershoot
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Solution Overview
Problem
Existing drive circuits using a normally-off-type transistor connected in series with a normally-on-type transistor suffer from undershooting issues in the drive voltage of the higher-side transistor when the lower-side transistor is in the OFF state, which cannot be sufficiently mitigated by conventional Zener diodes due to their limited reaction time.
Innovation Solution
A drive circuit with a clamping transistor and a capacitor is introduced between the normally-on-type and normally-off-type transistors, along with a control circuit to manage the gate voltages, ensuring rapid response to prevent undershooting by maintaining stable voltage levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Zener diode is used to curb undershooting in drive voltage, then the drive voltage stability is improved, but the reaction time is insufficient and undershooting cannot be sufficiently curbed
Solution Approach 1:
A capacitor is introduced as an intermediary energy storage element between the Zener diode and ground. The capacitor rapidly charges during voltage spikes and discharges during voltage drops, providing immediate response to undershooting events. This intermediary component bridges the gap between the Zener diode's voltage clamping function and the need for faster response time.
Solution Approach 2:
The capacitor is pre-charged to the Zener voltage during normal operation, so that when undershooting occurs, the capacitor can immediately discharge to counteract the voltage drop without waiting for the Zener diode to react. This preliminary energy storage enables the circuit to respond proactively rather than reactively to voltage fluctuations.
2Reliability
If a clamping transistor and capacitor are added to prevent undershooting, then the response speed and voltage stability are improved, but the device complexity increases
Solution Approach 1:
The clamping transistor serves multiple functions: it acts as a switch to connect or disconnect the capacitor from the circuit, provides additional voltage clamping capability, and can be controlled by the same control signal that drives the main transistor. This multi-functionality reduces the need for separate dedicated components for each function.
Solution Approach 2:
The capacitor is merged with the existing Zener diode protection circuit rather than being a completely separate protection mechanism. The clamping transistor's control terminal is merged with the gate control circuitry, allowing unified control of the main transistor and the protection capacitor. This integration reduces overall circuit complexity compared to having entirely separate protection circuits.
Data Source
AI summary
A drive circuit of an embodiment is a drive circuit that normally-off drives a transistor circuit constituted of a normally-on-type first transistor and a normally-off-type second transistor which are connected in series. The drive circuit of the embodiment has a clamping transistor that is disposed between a wiring, which is connected between the first transistor and the second transistor, and a drive terminal of the first transistor, and a capacitor that is disposed between the clamping transistor and the wiring and is connected to the clamping transistor in series.