Bidirectional Current Sensing in Bridge Circuits
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Solution Overview
Problem
Conventional current sensing methods for high power bridge circuits require intrusive external elements and cannot accurately measure bidirectional currents, leading to power dissipation and limitations in driver IC functionality.
Innovation Solution
The low side switch in the half/full bridge circuit is used as the current sensing element, with the voltage at the common node shifted to enable sensing of both positive and negative currents, allowing for non-intrusive, fully integrated current measurement within the driver IC.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external sensing elements (resistors) are inserted into the power circuit for current sensing, then current measurement capability is achieved, but power dissipation increases and device complexity increases
Solution Approach 1:
The low-side MOSFET serves dual functions: switching and current sensing. The sensing capability is inherent to the MOSFET's own characteristics (RDSON), eliminating the need for separate sensing elements. This self-service approach reduces power dissipation and simplifies the overall circuit design.
Solution Approach 2:
The low-side MOSFET is designed to perform multiple functions simultaneously: it acts as both the switching element and the current sensing element. By utilizing the MOSFET's inherent RDSON characteristic for sensing, the circuit achieves multi-functionality without requiring additional components.
2Measurement precision
If external sensing elements are inserted into the power circuit, then current sensing is enabled, but device complexity and pin requirements increase
Solution Approach 1:
The current sensing function is merged with the existing low-side MOSFET switching function. The sensing circuitry is integrated within the driver IC, combining multiple functions into a single device and reducing the number of external components and pins required.
Solution Approach 2:
The MOSFET's inherent RDSON characteristic provides the sensing mechanism, eliminating the need for external sensing elements. This reduces device complexity and integrates the sensing capability directly into the power circuit without additional pins or components.
3Measurement precision
If conventional sensing circuitry is used in the driver IC, then current measurement is possible, but bidirectional current measurement capability is lost
Solution Approach 1:
The sensing circuit is designed to dynamically adapt to bidirectional current flow. By using the MOSFET's RDSON characteristic and implementing appropriate voltage shifting and rectification circuitry, the system can accurately measure currents in both positive and negative directions, enhancing versatility.
Solution Approach 2:
The circuit employs voltage shifting techniques to accommodate bidirectional current measurement. By dynamically adjusting the reference voltage level and using absolute value circuitry, the system can accurately sense currents regardless of direction, transforming the sensing capability to handle parameter changes in current polarity.
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 approach eliminates the need for external sensing elements, reduces power dissipation, and enables accurate bidirectional current sensing, enhancing the reliability and efficiency of high power MOSFET switched output circuits.
Implementation Method 1
Since the RDSON of the MOSFET switching element is known, and the drain voltage of the switch is the same as the VS (common node) voltage, the current in the circuit can be ascertained by sensing the VS voltage while the low side switch is on.
Data Source
AI summary
An apparatus and method for determining the output current in a bridge-connected switched transistor output circuit including high-side and low-side transistor switches, typically MOSFETS. The voltage at a common node between the high and low side switches is sensed, and offset in a first circuit by a fixed amount so that the voltage is positive for all positive or negative output currents of interest. The output current is actually determined in a second circuit which receives the offset voltage signal only predetermined times in relation to the on-time of the low side switch. The first circuit includes a current reference source/level shifter and a current mirror circuit formed of a plurality of transistors in a particular circuit configuration. The second circuit is coupled to an output of the first circuit by a gated NMOS transistor at the desired times to provide the current measurement signal. The second circuit includes a second current reference source having substantially the same electrical characteristics as the first current reference source, and a second plurality of transistors respectively matched to the input side circuit transistors, and connected in the same circuit configuration. In the second circuit, the offset signal is compared with high and low reference signals to provide an indication if the output current exceeds an overcurrent limit, either positively or negatively. The sensing circuit is advantageously integrated with the output circuit gate driver in a single IC.