Class D Half-Bridge Switching Circuit With Resonant Dead-Time Reduction
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Solution Overview
Problem
High switching frequencies in class D audio power amplifiers lead to increased power dissipation and distortion due to switching losses and 'dead time' effects, which negatively impact efficiency and linearity.
Innovation Solution
A circuit design that exploits the resonance of inductance and capacitance in the bridge structure, using parasitic capacitances of power transistors and conductive lines on the printed circuit board, to reduce power dissipation and improve linearity without adding external components, by controlling auxiliary switches to minimize simultaneous high voltages and currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If switching frequency is increased to improve productivity, then switching speed increases, but power dissipation and distortion increase due to switching losses and dead time effects
Solution Approach 1:
The auxiliary switches are activated before the main power switches to pre-discharge the parasitic capacitances of the power transistors. This preliminary action removes the charge that would otherwise cause high current peaks and power dissipation during the switching transition, enabling faster switching frequencies without the usual penalty of increased energy loss.
Solution Approach 2:
Auxiliary switches are introduced as intermediary elements between the control signal and the main power switches. These auxiliary switches control the discharge paths of the parasitic capacitances, mediating the switching process to avoid direct high-stress transitions through the main power devices, thereby reducing power dissipation while maintaining high switching frequency.
2Productivity
If switching frequency is increased to improve productivity, then switching speed increases, but distortion increases due to dead time effects
Solution Approach 1:
By pre-discharging the parasitic capacitances before the main switching event, the auxiliary switches eliminate the voltage plateau effect that causes distortion. This preliminary discharge ensures that when the main power switches transition, there are no residual charges to cause asymmetric voltage levels or dead time variations, thereby maintaining signal fidelity even at high switching frequencies.
Solution Approach 2:
The parasitic capacitances, which normally cause harmful effects like voltage plateaus and distortion, are converted into beneficial elements. By controlling their discharge through auxiliary switches, the patent transforms these parasitic elements into a mechanism that actively assists the switching process, enabling high-frequency operation with low distortion by precisely managing when and how these capacitances are discharged.
3Loss of energy
If auxiliary switches are added to reduce power dissipation, then efficiency improves, but device complexity increases
Solution Approach 1:
The auxiliary switches utilize the inherent parasitic capacitances of the power transistors themselves as the capacitive elements to be discharged. This self-service approach eliminates the need for separate external capacitors or complex RC networks, reducing component count and circuit complexity while still achieving the goal of reduced power dissipation through controlled discharge of these self-provided capacitive elements.
4Loss of energy
If external inductive/capacitive components are added to achieve resonance, then power dissipation reduces, but device complexity and component count increase
Solution Approach 1:
The patent exploits the parasitic capacitances that already exist in the power transistor structures and the PCB trace inductances as the resonant elements. By using these self-provided L and C elements, the circuit achieves resonant operation and reduced power dissipation without requiring any external inductors or capacitors, thereby avoiding the complexity and space requirements of additional discrete components.
Solution Approach 2:
The patent changes the operational parameters of the existing circuit elements by controlling the timing and sequence of auxiliary switch activation. This parameter control enables the parasitic capacitances and trace inductances to function as a resonant circuit, achieving the desired reduction in power dissipation through resonant operation without adding external resonant 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
The solution effectively reduces power dissipation and distortion, achieving improved efficiency and linearity, with reduced component count and no external inductive/capacitive components, while maintaining performance comparable to lower switching frequencies.
Implementation Method 1
One or more embodiments may exploit the resonance of an inductance and associated capacitances made possible by switching auxiliary switches
Data Source
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AI summary
A switching circuit for use e.g. in class D switching audio amplifiers comprises a switching circuit stage comprising first (HB1) and second (HB2) half bridges with output nodes (Vout1, Vout2) configured for supplying an electrical load (L) via respective filter networks (Lo1, C1; LO2, C2). Control circuitry is provided to control alternate switching sequences of the transistors in the half bridges wherein a first pair of transistors comprising the high-side transistor (H2 resp. H1) in one (HB2 resp. HB1) of the half bridges and the low-side transistor (L1 resp. L2) in the other (HB1, resp. HB2) of the half bridges is switched to a non-conductive state, and a second pair of transistors comprising the high-side transistor (H1 resp. H2) in the other (HB1 resp. HB2) of the half bridges and the low-side transistor (L2 resp. L1) in the one (HB2 resp. HB1) of the half bridges is switched to a conductive state. A current flow line is provided between the output nodes (Vout1, Vout2) of the half bridges (HB1, HB2) comprising an inductance (Laux) between two switches (S1, S22). First and second capacitances (Caux1, Caux2) are coupled with the output nodes (Vout1, Vout2) of the half bridges (HB1, HB2). The control circuitry switches the first (S1) and second (S2) switches to the conductive state at intervals in the alternate switching sequences of the transistors in the half bridges between switching the first pair of transistors to a non-conductive state and switching the second pair of transistors to a conductive state.