Class-D Power Amplifier Bootstrap Circuit Without Diode Voltage Drop
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Solution Overview
Problem
The bootstrap configuration in class-D power amplifiers results in a lower bootstrap voltage due to inevitable voltage drops across diodes, leading to reduced gate-source voltage for highside power MOSFETs, increasing Ron resistance and requiring suboptimal recharging of the bootstrap capacitor.
Innovation Solution
A switch circuit replaces the diode, allowing the bootstrap capacitor to be recharged to a voltage close to the regulated source, with a latch circuit maintaining stable logical levels and a low ON resistance, avoiding voltage drops and enabling simple control using the same gate signal for the lowside power MOSFET.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bootstrap diode is used to charge the bootstrap capacitor, then the capacitor can be recharged, but a voltage drop occurs reducing the bootstrap voltage below the regulated source voltage
Solution Approach 1:
The patent replaces the diode-based charging mechanism with a transistor-based switch circuit. The transistor Mboot acts as a controllable switch that replaces the diode Dboot, allowing the bootstrap capacitor to be charged to the full regulated source voltage Vreg without the inherent voltage drop of a diode. The transistor is controlled by the gate signal Vgatelow to enable charging when the lowside power MOSFET is ON and to prevent discharge when it is OFF.
2Loss of energy
If the bootstrap voltage is lower than the regulated source voltage, then the bootstrap capacitor can be charged, but the gate-source voltage for the highside power MOSFET is reduced increasing Ron
Solution Approach 1:
The transistor switch circuit replaces the diode to eliminate the voltage drop, allowing the bootstrap voltage Vboot to match the regulated source voltage Vreg. This ensures the highside power MOSFET receives full gate-source voltage for optimal operation with minimal Ron resistance.
3Device complexity
If a diode is used for bootstrap charging, then the circuit is simple, but the voltage drop across the diode increases at lower temperatures
Solution Approach 1:
The patent replaces the temperature-sensitive diode with a transistor-based switch circuit. The transistor Mboot, controlled by the gate signal Vgatelow, provides a low-resistance charging path that is not subject to the same temperature-dependent voltage drop characteristics as diodes, maintaining efficient charging across varying temperatures.
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 solution maintains a bootstrap voltage close to the regulated source, reducing Ron resistance and optimizing the operation of the class-D power stage by eliminating the large voltage drop associated with diodes, thereby enhancing the amplifier's performance.
Implementation Method 1
a switch circuit coupled between the first voltage terminal and the second voltage terminal, said switch circuit being controlled by the second gate signal
Implementation Method 2
The latch circuit is advantageous because it maintains its logical levels i.e. its output voltages, stable as far as the control signals are stable
Data Source
AI summary
The invention refers to a power amplifier comprising a first transistor (MH) having a first main channel coupled between a positive power supply terminal (Vdd) and an output terminal (Vout), said first transistor having a control terminal driven by a first gate signal (Vgatehigh) provided by a high driver circuit, which is biased from a first voltage terminal (Vboot). The power amplifier further comprises a second transistor (ML) having a second main channel coupled between the output terminal and a negative power supply terminal (Vss), said second transistor having a second control terminal driven by a second gate signal (Vgatelow) provided by a low driver circuit, which is biased from a second voltage terminal (Vreg), and a switch circuit (10) coupled between the first voltage terminal (Vboot) and the second voltage terminal (Vreg), said switch circuit being controlled by the second gate Signal (Vgatelow).