Flyback Converter ZVS via Low-Side Switch Timing
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Solution Overview
Problem
Active clamp flyback (ACFB) converters operating in transition mode face challenges with zero voltage switching (ZVS) due to distorted waveforms in the secondary winding, which increase conduction loss and complexity, particularly because they require a high-side primary switch and corresponding driver, adding cost and complexity, and often use expensive and less reliable wide bandgap FETs.
Innovation Solution
The implementation of flyback converters without a high-side primary switch, using a controller to redirect the drive signal to a secondary switch via an isolated driver, reducing current flow in the primary winding during demagnetization and eliminating the need for a high-side driver, allowing for ZVS on low-side primary switches using silicon FETs, which reduces parasitic capacitance and conduction loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If active clamp flyback converters operate in transition mode with high-side primary switch, then zero voltage switching can be achieved, but device complexity and cost increase
Solution Approach 1:
The patent removes the high-side primary switch from the converter circuit, extracting the problematic component that caused complexity and reliability issues. The solution achieves ZVS by operating the low-side switch at the resonant valley point without requiring the high-side switch, thereby eliminating the associated driver circuitry and complexity while maintaining the zero voltage switching benefit
Solution Approach 2:
Instead of using the conventional approach of controlling both high-side and low-side switches to achieve ZVS, the patent inverts the strategy by controlling only the low-side switch timing. The low-side switch is turned on at the resonant valley point when the drain-source voltage is naturally zero, achieving ZVS through timing control rather than through the complex high-side switch mechanism
2Speed
If wide bandgap FETs are used for high-side switch, then switching performance improves, but cost and reliability worsen
Solution Approach 1:
The patent extracts and removes the high-side switch entirely from the circuit, eliminating the need to use expensive wide bandgap FETs. By achieving ZVS through low-side switch timing control at the resonant valley point, the solution removes the component that necessitated the use of unreliable wide bandgap devices
Solution Approach 2:
The patent replaces the expensive wide bandgap FET requirement with standard silicon FETs that are cheaper and more reliable. The solution uses conventional, proven technology components rather than expensive cutting-edge devices, achieving the same performance benefit through a different approach
3Loss of energy
If secondary winding waveform distortion occurs, then conduction loss increases, but simplifying the circuit to reduce loss is challenging
Solution Approach 1:
The patent uses feedback control by monitoring the drain-source voltage of the low-side switch and detecting the resonant valley point. The controller adjusts the switching timing based on this feedback to ensure the switch turns on at the optimal moment when voltage is zero, minimizing conduction loss while maintaining simple circuit topology
Solution Approach 2:
The patent prepares for minimal conduction loss by turning on the low-side switch in advance at the resonant valley point before the main switching event. This preliminary timing action ensures that the switch is already in the on state when the current begins to flow, avoiding the high-voltage-high-current overlap that causes conduction loss
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 simplifies the control of synchronous rectification, reduces conduction loss, and maintains a predictable waveform in the secondary winding, enabling efficient ZVS with silicon FETs while minimizing cost and complexity, improving the overall efficiency and reliability of the converter.
Implementation Method 1
The transformer includes a primary winding and a secondary winding across which voltage ratios are scaled. The transformer also provides galvanic isolation between the input and corresponding outputs.
Implementation Method 2
The flyback converter controls transistors and/or switches to charge and/or discharge inductors and/or capacitors to maintain a desired output voltage.
Data Source
AI summary
Methods, apparatus, systems, and articles of manufacture for zero voltage switching of flyback converters are disclosed. An example apparatus includes a first driver to operate a first switch to direct a first current to flow to a first winding of a transformer, and a second driver to operate a second switch to direct a second current to flow to a second winding of the transformer and operate the second switch to cause the second current to discharge a voltage of the first switch.