RDSon Current Sensing in Power Conversion Devices
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Solution Overview
Problem
Current RDSon current sensing techniques in power conversion devices face challenges due to variations in MOSFET drain-to-source on-state resistance (RDSon) across different wafers and temperature fluctuations, leading to inaccurate motor torque and speed control, especially in cost-sensitive applications like electric bikes and power tools.
Innovation Solution
A power conversion device with a single current sensor for DC-link current sensing, which compensates for RDSon variations by sampling currents during specific subperiods of the switching period, allowing for accurate RDSon monitoring and diagnostic monitoring of transistor junction temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If RDSon current sensing is used to eliminate costly shunt resistors, then system cost is reduced, but measurement precision deteriorates due to RDSon variations with temperature and across wafers
Solution Approach 1:
The patent implements feedback by using the single current sensor to continuously monitor the current through the active MOSFET and comparing it with the expected current based on PWM duty cycle. The controller adjusts the PWM signal in real-time to compensate for RDSon variations, ensuring accurate current control despite temperature and manufacturing variations.
Solution Approach 2:
The patent changes the operating parameters by sampling current during specific subperiods when only one MOSFET is active in each leg, and when the current through the single current sensor equals the phase current. This allows the system to adapt to RDSon variations by measuring at optimal moments rather than continuously, maintaining precision without requiring expensive high-precision components.
2Measurement precision
If three-shunt current sensing is used for high power applications, then measurement precision is improved, but device complexity and PCB size increase significantly
Solution Approach 1:
The patent merges the functions of multiple current sensors into a single current sensor. By strategically placing one current sensor in the DC link and using synchronous sampling during specific subperiods, the system combines the current sensing function that would otherwise require three separate shunt resistors and amplifiers into a single, simpler sensing circuit.
Solution Approach 2:
The single current sensor serves multiple functions: it senses the DC link current continuously, senses the phase current during specific subperiods when MOSFETs are active, and provides data for both control and diagnostic purposes. This multi-functionality eliminates the need for dedicated shunt resistors in each phase leg.
3Device complexity
If single shunt resistor current sensing is used, then device complexity is reduced, but measurement precision deteriorates for motor control with large current variations within one PWM cycle
Solution Approach 1:
The patent employs periodic action by sampling the current at specific intervals during the PWM cycle - specifically during subperiods when only one MOSFET per leg is active. This periodic sampling approach allows the system to capture accurate current information at critical moments without requiring the amplifier to handle the full bandwidth of continuous high-frequency PWM signals.
Data Source
AI summary
A power conversion device includes: a plurality of legs, each leg including a high-side switch connected between a voltage supply node and a phase node and a low-side switch connected between the phase node and a reference node; a phase current sensor for each leg and configured to sense current flowing through the high-side switch or the low-side switch of the corresponding leg; a single current sensor connected between the reference node and the low-side switches, or between the voltage supply node and the high-side switches; and a controller. During a subperiod of a switching period, the controller is configured to sample the current sensed by at least one of the phase current sensors and a current sensed by the single current sensor such that the current in one or more of the legs is sampled during the same subperiod as the current flowing through the single current sensor.