Small Current Sensing in Power Transistor Systems
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Solution Overview
Problem
Conventional current sensing methods in power transistor systems face challenges in accurately measuring small currents due to batch variations and non-linearities in resistances, which restrict the measuring range and require precise, high dynamic range evaluation circuits, limiting the detection of small currents.
Innovation Solution
A device and method for current sensing in power transistor systems that utilize two series circuits with different resistances, where one resistance is connected when the transistor is conducting, allowing for precise current measurement by switching off the power transistor when the current is below a threshold, and using a second transistor to sense low currents without additional measuring resistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current-sensing circuit is switched in parallel with the load circuit using a current-sensing transistor, then the load current can be ascertained through voltage drop sensing, but batch variations and non-linearities of the resistances directly affect the current dividing ratio and voltage accuracy
Solution Approach 1:
The patent changes the operating state of the power transistor from conducting to switched-off state for measurement. By switching off the power transistor, the control unit enables the second transistor to conduct, allowing current measurement through the second resistance. This parameter change (on/off state) eliminates the influence of batch variations and non-linearities present in the conducting state, thereby resolving the contradiction between measurement precision and reliability.
2Measurement precision
If conventional analog-to-digital converters with precise measuring resistors are used, then accurate current measurement is achieved, but the evaluation circuit restricts the measuring range to small currents due to offset voltages
Solution Approach 1:
The patent employs a dynamic measurement approach where the system switches between two different measurement configurations based on current magnitude. For small currents below a threshold, the power transistor is switched off and the second transistor is activated to provide appropriate measurement sensitivity. For larger currents, the normal operating configuration is used. This dynamic switching resolves the contradiction by adapting the measurement range to match the actual current level, overcoming the offset voltage limitation of conventional ADCs.
Solution Approach 2:
The patent segments the current measurement range into two distinct zones: small currents (below threshold) and larger currents (above threshold). Each zone has its own dedicated measurement path - the second transistor and second resistance for small currents, and the normal load circuit for larger currents. This segmentation allows each measurement path to be optimized for its specific range, resolving the contradiction between precision and measuring range adaptability.
3Power
If the power transistor remains continuously conducting for normal operation, then power delivery is maintained, but small currents cannot be measured with high precision
Solution Approach 1:
The patent implements periodic switching of the power transistor between conducting and switched-off states. During normal operation, the power transistor conducts to deliver power. When small current measurement is required, the control unit periodically switches off the power transistor and activates the second transistor for measurement. This periodic action between power delivery mode and measurement mode resolves the contradiction, allowing both functions to be performed at appropriate times.
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
Enables precise detection of small currents with minimized losses and system compactness, as the second transistor takes over current sensing when the power transistor is switched off, allowing for accurate measurement of currents across different magnitudes without additional components.
Implementation Method 1
Since the forward voltage in the Ohmic range supplies a voltage that is proportional to the current, it is possible to ascertain the current with the aid of the forward resistance
Data Source
AI summary
A device for current sensing of a power transistor system having a power transistor, a first series circuit which includes a first transistor and a first resistance, the first resistance disposed in a load circuit of the first transistor, a second series circuit which has a second transistor and a second resistance disposed in a load circuit of the second transistor, the first series circuit, the second series circuit and the power transistor situated in parallel with one another, the first resistance connected to the first transistor in an electrically conductive manner when the first transistor is switched on, and the second resistance connected to the second transistor in an electrically conductive manner when the second transistor is switched on, and a gate terminal of the first transistor is connected in an electrically conductive manner to a gate terminal of the power transistor when the power transistor is switched on.