Active Clamp Flyback Converter Control for Resonance Current Limiting
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Solution Overview
Problem
Conventional active clamp flyback converters face issues with high frequency oscillations due to steep current changes caused by the recovery current of switches and parasitic inductance, difficulty in synchronous rectification, and increased transformer core loss due to negative excitation current flow.
Innovation Solution
The active clamp flyback converter includes a controller that outputs a second ON signal to control the clamp switch during a half cycle or more of the resonance period, limiting the resonance current with the excitation current of the primary winding, and sets the ON timing before the excitation current reaches zero to prevent negative excitation current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If the clamp switch is turned off during the resonance current period (complementary control or conventional non-complementary control), then the OFF period of the main switch is compensated, but the secondary side current turns off abruptly causing high frequency oscillation
Solution Approach 1:
The clamp switch is turned on before the resonance current becomes zero (at the zero-crossing point of the excitation current), allowing the resonance current to continue flowing smoothly through the clamp switch and capacitor, preventing abrupt current termination and the associated high frequency oscillations
2Power
If the clamp switch is turned on when the excitation current reaches zero (conventional non-complementary control), then the excitation current can flow in negative direction for zero volt switching, but the transformer core loss increases due to increased magnetic flux change
Solution Approach 1:
The clamp switch is activated in advance at the zero-crossing point of the excitation current, allowing the resonance current to naturally decay to zero before the excitation current becomes negative, thus achieving zero volt switching without subjecting the transformer core to reverse flux excitation and minimizing core losses
3Device complexity
If the clamp switch ON period is shorter than half the resonance period, then the control is simpler, but the resonance current is not properly limited causing steep current changes
Solution Approach 1:
The control circuit monitors the excitation current and automatically determines the optimal turn-on timing of the clamp switch at its zero-crossing point, providing feedback-based control that naturally limits the resonance current without requiring complex timing circuits or additional components
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 reduces switching loss, minimizes core loss, and enables efficient energy regeneration, facilitating easy synchronous rectification with a single current waveform on the secondary side.
Implementation Method 1
a resonance operation with a resonance period (Tcalk) in a resonance circuit including the clamp capacitor Cac and a leakage inductance Llk
Implementation Method 2
the resonance current flowing in a resonance circuit including the clamp capacitor Cac and a leakage inductance Llk generated when the clam switch is turned on is limited by an excitation current of an excitation inductance Lm of the primary winding Np
Data Source
AI summary
An active clamp flyback converter includes a main switch, a primary winding that is electrically connected in series with the main switch, a clamp switch that is electrically connected to a connection point between the main switch and the primary winding, a clamp capacitor that is connected in series with the clamp switch, and a controller that outputs a first ON signal to control the main switch and a second ON signal to control the clamp switch during a period when the main switch is off. The controller outputs the second ON signal during a half cycle or more of a resonance period, in which the resonance current flowing in a resonance circuit comprising the clamp capacitor and a leakage inductance generated when the clam switch is turned on is limited by an excitation current of an excitation inductance of the primary winding.