Sensor-less Buck Regulator Using Replica Transistor Current Sensing
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Solution Overview
Problem
Conventional direct and indirect sensing methods in synchronous buck converters suffer from power dissipation, size, complexity, and cost issues, as well as inaccurate regulation due to variations in drain-to-source on resistance and temperature effects.
Innovation Solution
A synchronous buck converter with a replica transistor whose drain is coupled to the low-side switch transistor, using a current sensing amplifier to drive a scaled current into the replica transistor, allowing for 'sensor-less' load current sensing and an average current mode control loop that samples the scaled current at the mid-point of the on-time to drive an error amplifier and pulse-width modulator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sense resistor is used in series with the low-side switch to measure output current, then direct sensing of load current is achieved, but power dissipation increases, size increases, complexity increases, and cost increases
Solution Approach 1:
The patent creates a replica transistor that copies the electrical characteristics of the low-side switch transistor. The replica transistor's drain-to-source voltage is made substantially equal to that of the low-side switch, allowing indirect sensing of load current without requiring a physical sense resistor in series with the switch. This copying approach eliminates the need for additional sensing components while maintaining measurement capability.
Solution Approach 2:
The patent introduces a current sensing amplifier as an intermediary device that measures the drain-to-source voltage of the low-side switch transistor and converts it into a usable signal for control. This intermediary approach allows the system to sense current through voltage measurement rather than direct current measurement, avoiding the need for power-dissipating sense resistors.
2Device complexity
If indirect sensing is used by measuring drain-to-source voltage of low-side switch and multiplying by presumed on resistance, then sensing complexity is reduced, but regulation accuracy deteriorates due to device-to-device variation and temperature effects
Solution Approach 1:
The patent implements a feedback mechanism where the current sensing amplifier continuously monitors the drain-to-source voltage of the low-side switch and adjusts the control signal accordingly. The error amplifier compares the sensed voltage with a reference and provides feedback to the pulse-width modulator, ensuring accurate regulation despite variations in on-resistance due to temperature or device-to-device differences.
Solution Approach 2:
The replica transistor is designed to closely copy the electrical behavior of the actual low-side switch, including temperature-dependent characteristics. By measuring the voltage across the replica transistor rather than relying on presumed resistance values, the system achieves accurate sensing that automatically compensates for temperature effects and manufacturing variations.
3Loss of energy
If a replica transistor with higher on resistance is used for sensing, then sensing power dissipation is reduced, but voltage drop across sensing element increases
Solution Approach 1:
The current sensing amplifier is configured to measure only the necessary portion of the voltage drop across the replica transistor's drain-to-source junction. By using high-impedance measurement inputs, the amplifier draws minimal current, effectively measuring the voltage without adding significant power dissipation. The system accepts some voltage drop but minimizes the additional power loss through efficient measurement techniques.
Data Source
AI summary
A synchronous buck converter is provided in which a replica transistor has its drain coupled to a drain of the low-side switch transistor. A current sensing amplifier drives a scaled current into the replica transistor such that the drain-to-source voltage of the replica transistor substantially equals the drain-to-source voltage of the low-side switch. The replica transistor is much smaller than the low switch transistor such that the replica transistor's on resistance is higher by a scale factor Kf as compared to the on resistance for the low-side switch transistor.