Rdson Current Sensing With Temperature Compensation for DC/DC Converters
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Solution Overview
Problem
Existing DC/DC converters face challenges in accurately monitoring output current, particularly in multiphase converters, due to inefficiencies in current sensing methods that introduce power loss, require multiple pins, and are difficult to scale, while variations in temperature and supply voltage lead to inaccurate current sensing.
Innovation Solution
A current sensing system utilizing a switch, current paths, current mirror, buffers, and variable resistors, with temperature compensation and auto-zeroing mechanisms to adjust resistor ratios, compensating for variations in drain-source on-resistance (Rdson) to enhance accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If traditional current sensing methods are used in DC/DC converters, then current monitoring is achieved, but power loss increases and scalability is reduced
Solution Approach 1:
The patent uses a current mirror circuit to create a copy of the sense current rather than directly measuring it through power-consuming resistors. The current mirror replicates the current flowing through the low-side transistor, allowing accurate sensing without introducing additional power loss through sense resistors in the main current path.
Solution Approach 2:
The patent replaces traditional resistor-based current sensing with a transistor-based current mirror system. This substitution eliminates the need for physical sense resistors that dissipate power, using instead a field-effect transistor circuit that senses current through voltage measurements at the transistor terminals.
2Measurement precision
If multiple pins are used for current sensing, then measurement capability is improved, but device complexity increases
Solution Approach 1:
The patent makes the existing drain and source terminals of the low-side power transistor serve dual functions: they continue to carry the main power current while simultaneously serving as sensing terminals for current measurement. This eliminates the need for separate sense pins, as the same terminals perform both power transmission and measurement functions.
Solution Approach 2:
The patent merges the power current path and the sensing function into a single integrated system. The current mirror circuit combines the power transistor with sensing capability, allowing the same physical terminals to handle both high-power current flow and low-level measurement signals without requiring separate dedicated sense pins.
3Temperature
If temperature variations occur, then operating range is extended, but current sensing accuracy deteriorates
Solution Approach 1:
The patent incorporates a feedback mechanism where the differential amplifier continuously monitors the voltage difference between the drain and source terminals and adjusts its output accordingly. This feedback loop compensates for temperature-induced variations in transistor characteristics, maintaining accurate current sensing across different operating temperatures by dynamically adjusting the sensing signal.
Solution Approach 2:
The patent exploits the temperature-dependent characteristics of the transistor by using the differential amplifier to detect and compensate for parameter changes. The system allows the transistor operating parameters to change with temperature while using the differential measurement to track and account for these changes, converting a potential source of error into a measurable signal that can be compensated.
4Adaptability or versatility
If supply voltage varies, then adaptability is improved, but current sensing accuracy deteriorates
Solution Approach 1:
The patent uses the differential amplifier to maintain an equipotential reference by comparing the drain and source terminal voltages differentially. This differential measurement approach rejects common-mode voltage variations, allowing the system to maintain accurate current sensing even when the supply voltage changes, as both terminals experience similar voltage shifts that cancel out in the differential measurement.
Data Source
AI summary
An apparatus includes a first variable resistor having a first resistor control terminal and a second variable resistor having a second resistor control terminal. The apparatus also includes a temperature sensing circuit having a temperature sensing output, the temperature sensing circuit configured to provide a device temperature indication at the temperature sensing output. Additionally, the apparatus includes a controller having a controller input, a controller output, the controller input coupled to the temperature sensing output, the controller output coupled to the first resistor control terminal and to the second resistor control terminal, the controller configured to produce a control signal at the controller output responsive to the device temperature indication.