Soft Switching via Current Injection in Forward Converters
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Solution Overview
Problem
Existing single-ended forward power converters fail to achieve true soft switching across the secondary side, especially in continuous mode, due to limitations in magnetizing current generation and increased power dissipation, which affects efficiency and creates ringing and spikes during switching.
Innovation Solution
A current injection method is employed using a current source circuitry to inject a pulse of current into the auxiliary winding of the transformer, ensuring that the secondary switching elements turn off at zero current and primary switchers turn on at zero voltage, eliminating ringing and spikes by utilizing parasitic-capacitance-based magnetizing current and external current injection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional soft switching methodologies are used in single-ended forward power converters operating in continuous mode, then primary switching elements can turn on at zero voltage, but secondary switching elements cannot turn off at zero current, resulting in power dissipation and ringing
Solution Approach 1:
The patent applies preliminary action by injecting current into the auxiliary winding before the secondary switching elements need to turn off. This pre-injected current ensures that the secondary current reaches zero exactly when the secondary switches are turned off, enabling true soft switching without requiring complex real-time control mechanisms.
Solution Approach 2:
The patent uses an auxiliary winding as an intermediary element to achieve soft switching. By injecting current into this auxiliary winding, the patent indirectly controls the secondary side current behavior, allowing secondary switches to turn off at zero current without directly controlling the secondary switches themselves.
2Reliability
If magnetizing current generation is limited in conventional converters, then device simplicity is maintained, but true soft switching cannot be achieved across both primary and secondary sides
Solution Approach 1:
The auxiliary winding serves multiple functions: it acts as a magnetizing inductance path, provides a current injection mechanism for soft switching, and enables both primary and secondary soft switching. This multi-functionality allows the patent to achieve reliable soft switching without adding separate dedicated circuits for each function.
3Loss of energy
If higher leakage inductance transformers are used to achieve soft switching, then switching conditions improve, but power conversion efficiency decreases
Solution Approach 1:
The patent changes the approach from modifying transformer parameters (leakage inductance) to modifying operating conditions through controlled current injection. By injecting current into the auxiliary winding at specific timing, the patent achieves soft switching without requiring high leakage inductance, thereby maintaining high power conversion efficiency.
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 enables true soft switching in single-ended forward converters operating in continuous mode, enhancing efficiency by reducing power dissipation, eliminating the need for snubbers, and allowing lower leakage inductance transformers, thereby increasing overall power conversion efficiency.
Implementation Method 1
A current injection method is employed using a current source circuitry to inject a pulse of current into the auxiliary winding of the transformer
Implementation Method 2
the first magnetizing current is generated from energy stored in parasitic capacitances of the first and second switching elements
Data Source
AI summary
A method for operation of a single-ended forward converter to achieve “true soft switching” The method includes injecting, with a current source and in transformer winding, a narrow pulse of current via an injection winding of the transformer to add such pulse to the magnetizing current to exceed the current level of current passing through freewheeling rectifier to reduce that current to zero time when the freewheeling rectifier is turns off at zero current conditions. Further, the sum of the magnetizing current and the injected current provide the current required by the output inductor during the transition time. The amplitude of injected current is defined such that the sum is greater than the minimum current through the output inductor (at least by an amount that reflects into the primary winding). The amount of current reflected in the primary is chosen to be sufficiently large to discharge parasitic capacitances reflected across the primary main switch to zero during the transition time.