IGBT Over-Voltage Protection Circuit with Forced Turn-On
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Solution Overview
Problem
Existing over-voltage protection circuits for pulse width modulation (PWM) controlled drive transistors, such as IGBTs, are inadequate in preventing damage from over-voltage conditions, particularly in resonant tank circuits used in applications like induction heating/cooking systems.
Innovation Solution
An over-voltage protection circuit that includes a sense circuit to detect voltage across the drive transistor, a comparator to generate a signal indicative of an over-voltage condition, and drive circuitry to force the IGBT to turn on irrespective of the PWM signal, with features like soft-start, pulse width reduction, and shutdown mechanisms to manage excessive over-voltage conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a voltage-limiting element is connected between the DC supply node and the drive transistor to protect against over-voltage, then the drive transistor is protected from over-voltage damage, but the voltage spike may be insufficient to quickly turn on the drive transistor, causing the protection to be too slow to prevent damage
Solution Approach 1:
The patent introduces a capacitor connected between the PWM input terminal and ground as an intermediary element. This capacitor responds rapidly to voltage spikes by discharging through the drive transistor, creating a forced turn-on effect that operates faster than the voltage-limiting element alone. The capacitor acts as a mediator that bridges the gap between the slow voltage-limiting protection and the need for fast transistor response.
Solution Approach 2:
The capacitor is pre-charged during normal operation and stands ready to immediately discharge when a voltage spike occurs. This preliminary charging action ensures that when an over-voltage event happens, the capacitor can instantly provide the current needed to force the transistor on, without waiting for the voltage-limiting element to react.
2Reliability
If the drive transistor is forced to turn on immediately upon detecting an over-voltage condition, then the transistor is protected from damage, but the load circuit experiences disruption and potential operational issues
Solution Approach 1:
The patent implements a dynamic response system where the capacitor's discharge duration and the forced turn-on period are time-limited. After the over-voltage event is cleared, the system automatically resets and returns to normal PWM-controlled operation. This dynamic behavior allows the system to adapt between protection mode and normal operation mode, minimizing disruption to the load circuit.
Solution Approach 2:
The protection mechanism operates in periodic cycles: during normal operation the capacitor charges, during over-voltage events it discharges to force transistor on, and after the event it resets. This periodic charging-discharging-reset cycle provides continuous protection while allowing normal operation to resume automatically once the threat is gone.
3Speed
If a capacitor is connected between the PWM input terminal and ground to force the drive transistor on during voltage spikes, then the transistor turns on quickly for protection, but the capacitor may cause false triggering or interfere with normal PWM signal operation
Solution Approach 1:
The patent carefully selects the capacitor's electrical parameters (capacitance value, voltage rating) to create a distinct separation between normal PWM operating conditions and over-voltage spike conditions. The capacitor is designed to remain charged during normal operation and only discharge when the voltage exceeds the normal operating range, providing a clear parameter-based distinction that prevents false triggering.
Solution Approach 2:
The capacitor serves as a pre-positioned energy reservoir that cushions the system against voltage spikes. By having the capacitor already charged and ready before a spike occurs, the system can absorb the shock of the voltage spike without false triggering, as the capacitor's discharge is a controlled response to genuine over-voltage conditions rather than normal signal variations.
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
Effectively suppresses over-voltage conditions by forcing the IGBT to turn on and reducing the collector-emitter voltage, incrementally adjusting PWM signal duty cycles, and disabling the PWM signal when necessary to prevent IGBT damage and ensure system safety.
Implementation Method 1
a sense circuit configured to sense a voltage across the first and second conduction terminals
Implementation Method 2
a comparator circuit configured to compare the sensed voltage to a received voltage threshold and generate a signal indicative of an over-voltage condition
Data Source
Figure 1~2
Figure 3A~3B
Figure 4~5
AI summary
A drive transistor 14 is connected to a resonant load 16 in a low-side drive configuration. The voltage across the conduction terminals of the drive transistor is sensed and compared to an over-voltage threshold. An over-voltage signalOVC is asserted in response to the comparison. The drive transistor is controlled by a PWM control signal in normal mode. In response to the assertion of the over-voltage signal, the drive transistor is forced to turn on (irrespective of the PWM control signal) to relieve the over-voltage condition. Operation of the circuit may be disabled or forced into soft start mode in response to the assertion of the over-voltage signal. Additionally, the pulse width of the PWM control signal may be reduced in response to the assertion of the over-voltage signal.