Cycloconverter Fault Switching to Prevent AC FET Avalanche
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Solution Overview
Problem
Conventional cycloconverters face issues with FET avalanche due to residual energy dumping during switch operation, which can damage the FETs, and existing solutions are inadequate, especially with new generation Si FETs and GaN FETs having low or no avalanche ratings.
Innovation Solution
A power converter system comprising a pair of AC FETs and a controller that detects faults and manages switch operations to deplete residual energy to the grid without causing FET avalanche, by maintaining AC FETs in a prior switching state and allowing energy to dissipate through body diodes, thereby preventing avalanche currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional cycloconverters dump remanent energy across AC FETs during switch cessation, then energy is depleted from the resonant tank, but the FETs experience avalanche current that can damage them
Solution Approach 1:
The patent introduces a body diode as an intermediary component between the resonant tank and the AC FETs. When a fault is detected, the body diode becomes forward-biased and provides a dedicated current path for the resonant tank energy to flow through, acting as a mediator that prevents the avalanche current from directly impacting the FETs. This intermediary structure allows energy depletion while protecting the sensitive semiconductor devices.
Solution Approach 2:
The patent converts the potentially harmful body diode (which can cause shoot-through currents during normal operation) into a protective element during fault conditions. By detecting faults and intentionally allowing the body diode to conduct the resonant tank current, the previously problematic component becomes the solution that saves the FETs from avalanche damage.
2Reliability
If FETs with high avalanche current rating are used to overcome avalanche damage, then FET reliability improves, but device cost and supply chain constraints increase
Solution Approach 1:
The patent employs standard, readily available AC FETs with typical avalanche ratings rather than requiring specialized high-avalanche-rated devices. The protection mechanism allows the use of conventional, cost-effective FETs that are easily sourced from standard semiconductor suppliers, eliminating the need for expensive customized or specialty components.
3Reliability
If switches are turned off at tank current zero-cross to prevent avalanche, then FET protection improves, but ADC measurement error and driver propagation delay still cause reduced magnitude avalanche currents
Solution Approach 1:
The patent implements preliminary fault detection that triggers protection before the actual avalanche event occurs. By detecting fault conditions in advance and preemptively managing the switch states and body diode conduction, the system prevents avalanche currents from developing, eliminating the need for precise zero-cross timing that is susceptible to measurement errors and propagation delays.
4Reliability
If AC FETs are opened during fault conditions to stop current flow, then FET protection improves, but remanent energy in the resonant tank cannot be depleted
Solution Approach 1:
The patent dynamically adjusts the switching states of the AC FETs based on fault detection. Instead of a static open or closed state, the system transitions to a specific configuration where the body diode is forward-biased, creating a dynamic current path that allows energy depletion while maintaining FET protection. The switching strategy adapts to the fault condition to achieve both protection and energy dissipation.
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 reduces or eliminates FET avalanche occurrences during switch cessation, ensuring the safe depletion of residual energy to the grid without damaging the FETs, even with low avalanche-rated FETs.
Implementation Method 1
maintaining AC FETs in a prior switching state and allowing energy to dissipate through body diodes
Implementation Method 2
without causing the first pair of AC FETs or the second pair of AC FETs to avalanche
Data Source
AI summary
A power converter is provided herein and comprises a cycloconverter comprising a first pair of AC FETs and a second pair of AC FETs and a controller configured to detect when at least one of a to-be fault or an ongoing fault occurs and open or close at least one of the first pair of AC FETs or the second pair of AC FETs such that remanent energy stored in a resonant tank of the power converter is depleted to a grid without causing the first pair of AC FETs or the second pair of AC FETs to avalanche.