Flyback Converter Fixed-Frequency Pre-Magnetization
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Solution Overview
Problem
Flyback converters operating in discontinuous conduction mode with fixed frequency face challenges due to parasitic oscillations and high switching losses, especially when using synchronous rectifiers, as they require maintaining quasi-resonant operation with a constant switching frequency, which is not feasible with traditional designs.
Innovation Solution
Implementing a pre-magnetizing period and adjusted delay times to control the switching frequency, allowing the flyback converter to operate at a fixed frequency while minimizing switching losses by buffering energy and reducing the voltage amplitude during parasitic oscillations, thus enabling fixed-frequency resonant operation with reduced switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the flyback converter is operated in discontinuous conduction mode with fixed frequency, then the switching frequency is constant and easy to control, but parasitic oscillations occur and switching losses increase
Solution Approach 1:
The patent applies preliminary action by introducing a pre-magnetizing period before the main switching operation. During this pre-phase, the transformer is magnetized in advance with a controlled current, preparing the magnetic field in a way that reduces subsequent switching losses. This preliminary magnetization ensures that when the main switch operates at fixed frequency, the transformer core is already in an optimal state, reducing parasitic oscillations and energy losses during the main switching cycles.
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the duty cycle and current limits during the pre-magnetizing period. The control circuit modifies operating parameters such as the pre-magnetization current magnitude and duration, and the main switching duty cycle, to optimize the balance between fixed frequency operation and reduced switching losses. These parameter adjustments allow the system to maintain constant switching frequency while adapting to different load conditions to minimize energy loss.
2Loss of energy
If the flyback converter is operated in quasi-resonant mode, then switching losses are reduced, but the switching frequency varies
Solution Approach 1:
The patent uses preliminary action by performing a controlled pre-magnetization phase before the main resonant switching. This pre-phase sets up the transformer magnetic field in advance, creating initial conditions that enable the subsequent main switch to operate at a predetermined fixed frequency while still benefiting from reduced switching losses. The preliminary current ramp-up prepares the core flux in a way that facilitates low-loss switching at constant frequency.
Solution Approach 2:
The patent applies dynamics by implementing a two-stage operation: a dynamic pre-magnetizing phase followed by a fixed-frequency main switching phase. During the pre-phase, the duty cycle and current are dynamically adjusted to build the magnetic field, then the system transitions to a stable fixed-frequency operation. This dynamic transition allows the system to combine the benefits of resonant operation with the stability of fixed frequency control.
3Loss of energy
If a synchronous rectifier is used in the flyback converter, then rectification efficiency is improved, but early switch-on problems occur during fixed frequency operation
Solution Approach 1:
The patent applies preliminary action by introducing a deliberate delay period between the end of the pre-magnetizing current and the activation of the synchronous rectifier. This delay ensures that the transformer is fully magnetized and the voltage polarity is stable before the rectifier switch turns on. By timing the rectifier switch-on after this preliminary magnetization phase, the system avoids early switch-on problems while maintaining the efficiency benefits of synchronous rectification.
Solution Approach 2:
The patent employs feedback by using the pre-magnetizing current signal and voltage detection to control the timing of the synchronous rectifier switch. The control circuit monitors the transformer voltage and current conditions, and based on this feedback, it precisely times the rectifier switch activation. This feedback mechanism ensures the rectifier switches at the correct moment, preventing early switch-on while optimizing rectification 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 allows for stable, low-loss operation by ensuring the flyback converter switches on at a lower voltage level, reducing energy storage requirements and maintaining quasi-resonant operation advantages with a fixed switching frequency, thereby improving efficiency and reducing component stress.
Implementation Method 1
the transformer is magnetized when the electronic switch is closed and demagnetized when the electronic switch is opened. Magnetizing the transformer includes storing energy in the transformer, and demagnetizing the transformer includes transferring the stored energy to the secondary winding
Implementation Method 2
During this delay time, parasitic oscillations of a voltage across the electronic switch may occur
Data Source
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Figure 3
Figure 4~5
AI summary
A switching converter is described herein. In accordance with one example, the switching converter includes a transformer comprising a primary winding, a secondary winding, and at least an auxiliary winding. The switching converter further includes a first electronic switch coupled to the primary winding and configured to switch a primary current passing through the primary winding on and off in accordance with a first drive signal. A synchronous rectifier circuit is coupled between the secondary winding and an output node of the switching converter. Further, the switching converter includes a pre-magnetization circuit which comprises a capacitive circuit and a second electronic switch that is connected between the auxiliary winding and the capacitive circuit. The capacitive circuit is configured to apply a first voltage to the auxiliary winding during a pre-magnetization period when the second electronic switch is closed and to be (re-) charged with a second voltage when the second electronic switch is open; the second voltage is higher than the first voltage.