LLC Resonant Converter Capacitive Mode Protection Circuit

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Solution Overview

Problem

LLC resonant converters face damage due to inability to achieve Zero Voltage Switching (ZVS) when operating in capacitive mode, leading to potential damage of high-side and low-side switches.

Innovation Solution

A control method that senses the current through the resonant inductor, determines the operational mode, and implements a capacitive protection mechanism by turning the high-side and low-side switches off for a specified number of cycles, then turns them on once the current sense signal reaches a zero-crossing threshold, ensuring safe operation and preventing hard-switching.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the LLC resonant converter operates in capacitive mode, then the converter can handle certain load conditions, but the switches cannot achieve Zero Voltage Switching (ZVS) and may be damaged

Engineering Contradiction:

Improveoperational mode rangeVSAvoidswitch safety

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The control circuit detects the capacitive mode condition in advance by monitoring the current through the resonant inductor. When capacitive mode is detected, the control circuit proactively turns off both switches for N switching cycles before any potential damage can occur, preventing the harmful effect rather than reacting to it after it happens.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent converts the harmful capacitive mode operation into a beneficial protective mechanism. By detecting capacitive mode conditions and responding with a controlled shutdown sequence, the system uses the detection of the problematic state as the trigger for protection, turning what would be a damaging condition into an opportunity for preventive action.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Reliability

If the high-side and low-side switches are turned off for N switching cycles to protect against capacitive mode, then switch safety is improved, but converter productivity decreases

Engineering Contradiction:

Improveswitch protectionVSAvoidconverter output

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

Instead of permanently shutting down the converter when capacitive mode is detected, the control circuit applies a partial action by turning off the switches for only N switching cycles. This limited-duration shutdown is sufficient to allow the resonant current to return to safe levels, after which normal operation can resume. The action is excessive enough to provide protection but limited enough to minimize productivity loss.

Inventive Principle:

Principle #16Partial or excessive action

Solution Approach 2:

The protective measure is applied periodically for a specific duration (N switching cycles) rather than continuously. The converter operates normally during inductive mode and only undergoes the brief shutdown sequence when capacitive mode is detected, creating a periodic pattern of normal operation interrupted by short protective pauses when needed.

Inventive Principle:

Principle #19Periodic action

3Productivity

If the converter operates without capacitive mode protection, then productivity is maintained, but the switches are at risk of damage from hard-switching

Engineering Contradiction:

Improvecontinuous operationVSAvoidswitch stress

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The control circuit continuously monitors the current through the resonant inductor and uses this feedback to determine the operational mode. When the feedback indicates capacitive mode (current direction and magnitude conditions are met), the control circuit automatically activates the protective shutdown sequence, creating a closed-loop feedback system that adapts the converter's operation to prevent damage.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The control circuit acts as an intermediary between the resonant converter's operational state and the switches. It monitors the resonant current and intervenes by controlling the switch states, preventing direct harmful interaction between the capacitive mode conditions and the switches. The control circuit mediates by introducing a protective buffer period where both switches are off.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS9912244B2Control method and circuit for resonant converters with capacitive protection

Publication Date: 2018.03.06 CHENGDU MONOLITHIC POWER SYST

AI summary

Control method and circuit for resonant converters with capacitive protection. When a resonant converter enters into a capacitive mode, a high-side switch and a low-side switch of the resonant converter are turned off. After the high-side switch and a low-side switch are turned off for N switching cycles, the high-side switch is turned on once a current sense signal flowing through a resonant inductor of the resonant converter is increased to a zero-crossing threshold during an ascent stage of the current sense signal, and the low-side switch is turned on once the current sense signal is decreased to the zero-crossing threshold during a descent stage of the current sense signal.