Active-Clamp Forward-Flyback Converter ZVS
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Solution Overview
Problem
Conventional active-clamp forward-flyback converters face challenges in achieving zero-voltage-switching (ZVS) under heavy and light load conditions due to insufficient resonant current, leading to inefficiencies and increased power losses, particularly under heavy loads and excessive iron losses at high frequencies.
Innovation Solution
Employing the ZVS scheme of the active-clamp flyback converter and designing the flyback sub-circuit to operate in continuous conduction mode, with the diode reverse-biased to reduce unnecessary power losses, and using a flywheeling diode to maintain output inductor current slope, while the output inductor operates in discontinuous conduction mode to adjust the duty ratio and clamp capacitor voltage, thereby ensuring ZVS and reducing element power losses across a wide range of loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the conventional active-clamp forward-flyback converter is used to achieve continuous energy transmission and soft-switching, then the power density and conversion efficiency are improved, but the zero-voltage-switching (ZVS) cannot be effectively achieved under heavy load conditions due to insufficient resonant current
Solution Approach 1:
The patent divides the converter into two independent sub-circuits: a forward sub-circuit with a first secondary winding and a flyback sub-circuit with a second secondary winding. Each sub-circuit can operate independently, allowing the flyback sub-circuit to provide additional resonant current to support ZVS under heavy load conditions when the forward sub-circuit alone is insufficient.
Solution Approach 2:
The patent changes the operating parameters by introducing a flyback sub-circuit that operates in continuous conduction mode (CCM) while the forward sub-circuit operates in discontinuous conduction mode (DCM). This parameter change enables the system to maintain sufficient resonant current for ZVS across a wide load range, particularly under heavy load conditions.
2Reliability
If extra inductor or saturable reactor is added to increase resonant current and achieve ZVS, then the ZVS of main switch is improved, but iron loss increases significantly when operating at high frequency
Solution Approach 1:
The patent merges the forward converter and flyback converter into a single integrated circuit with shared primary winding and active-clamp components. The flyback sub-circuit's inductor serves dual purposes: providing resonant current for ZVS and functioning as the output inductor, thereby avoiding the need for separate extra inductors that would cause additional iron losses.
Solution Approach 2:
The output inductor in the flyback sub-circuit serves multiple functions: it provides resonant current for ZVS of the main switch, acts as the output filter inductor, and enables continuous conduction mode operation. This multi-functionality eliminates the need for dedicated resonant inductors that would increase iron losses at high frequencies.
3Power
If the converter operates with both forward and flyback sub-circuits active, then the energy processing capability is increased, but unnecessary element power losses occur under light load conditions
Solution Approach 1:
The patent implements dynamic operation where the flyback sub-circuit operates in continuous conduction mode (CCM) while the forward sub-circuit operates in discontinuous conduction mode (DCM). Under light load conditions, the duty ratio and clamp capacitor voltage automatically adjust, allowing the system to maintain high efficiency by reducing unnecessary power losses in the elements.
Solution Approach 2:
The patent changes operating parameters dynamically: under light load conditions, the duty ratio decreases and the clamp capacitor voltage reduces, which automatically reduces the power losses in diodes and other elements. This parameter adaptation allows the system to maintain high efficiency across different load conditions without requiring separate circuits for each load range.
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 enhances conversion efficiency under both heavy and light loads by prolonging the resonant current's negative-to-positive transition interval, reducing power losses, and allowing for the use of lower voltage-rated diodes, resulting in improved power density and cost-effectiveness.
Implementation Method 1
the current iS1 flowing through the main switch S1 resonates to the negative direction so as to discharge the parasitic capacitance of switches to be zero value
Implementation Method 2
a transformer Tr having a primary winding N1, a first secondary winding N2 and a second secondary winding N3
Implementation Method 3
the first diode D1, a second diode D2
Data Source
AI summary
The present invention discloses a forward-flyback converter with active-clamp circuit. The secondary side of the proposed converter is of center-tapped configuration to integrate a forward circuit and a flyback circuit. The flyback sub-circuit operating continuous conduction mode is employed to directly transfer the reset energy of the transformer to the output load. The forward sub-circuit operating discontinuous conduction mode can correspondingly adjust the duty ratio with the output load change. Under the heavy load condition, the mechanism of active-clamp flyback sub-circuit can provide sufficient resonant current to facilitate the parasitic capacitance of the switches to be discharged to zero. Under the light load condition, the time interval in which the resonant current turns from negative into positive is prolonged to ensure zero voltage switching function. Meanwhile, the flyback sub-circuit wherein the rectifier diode is reverse biased is inactive in order to further reduce the power losses.