Power Transistor Gate Pre-bias Circuit for Reverse Recovery Loss Reduction
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Solution Overview
Problem
Switching losses in transistors, particularly MOS transistors, increase with higher switching frequencies and cycle rates due to body diode activation during dead time in switched power converters.
Innovation Solution
A control circuit and method that predict when the drain potential falls below the source potential by more than the body diode's threshold voltage, applying a pre-bias potential to the gate to establish a conducting channel that bypasses the body diode, reducing losses by ensuring the channel conducts instead of the diode during these times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If switching frequency of the transistor is increased, then productivity of the power converter is improved, but switching losses increase due to body diode activation during dead time
Solution Approach 1:
The control circuit applies a pre-bias potential to the gate terminal before the body diode becomes forward-biased during dead time. This preliminary action ensures the conducting channel is already established, preventing body diode activation and the associated reverse recovery losses that would otherwise occur at higher switching frequencies
Solution Approach 2:
The control circuit detects when the drain potential is approaching the source potential and applies a pre-bias potential to counteract the impending body diode forward bias condition. This preliminary anti-action prevents the harmful reverse recovery effect before it can occur, enabling higher switching frequencies without proportional increases in switching losses
2Reliability
If dead time is added between transistor switching, then reliability is improved by avoiding shoot through current, but switching losses increase due to body diode conduction
Solution Approach 1:
During the dead time interval, the control circuit applies a pre-bias potential to the gate terminal in advance, establishing a conducting channel before the body diode can become forward-biased. This maintains reliability by keeping both transistors off during dead time while simultaneously reducing losses by preparing the channel to take over current immediately when needed
Solution Approach 2:
The pre-bias potential acts as an intermediary state that bridges the gap during dead time. It maintains the transistor in a controlled state that prevents body diode conduction while being ready to quickly establish full conduction when the transistor needs to switch on, thus mediating between the conflicting requirements of reliability and loss reduction
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 approach significantly reduces switching losses by minimizing body diode conduction and reverse recovery losses, enhancing power efficiency in transistors, especially in high-frequency and high-cycle-rate applications.
Implementation Method 1
applying a pre-bias potential to the gate of the power transistor in dependence of (notably at) the predicted time instant, such that a conducting channel between the drain and the source is provided
Data Source
AI summary
The present document describes a control circuit and a method for controlling a power transistor, wherein the power transistor has a drain, a gate and a source. The power transistor has a body diode. The control circuit is configured to predict a time instant at which a drain potential at the drain falls below a source potential at the source of the power transistor by more than a diode threshold voltage of the body diode. Furthermore, the control circuit is configured to apply a pre-bias potential and/or provide a pre-bias current to the gate of the power transistor in dependence the predicted time instant, such that a conducting channel between the drain and the source is provided, which at least partially takes over current which would otherwise flow through the body diode.