DC-DC Current Sensing With On-Resistance Compensation
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Solution Overview
Problem
Current sensing circuits in power converters face challenges in providing accurate current measurements independent of operational conditions such as temperature and supply voltage, affecting the reliability of power switch control.
Innovation Solution
A current sensing circuit is designed with a sensing replica of the pass device and a sensing resistor, where the sensing replica has an on-resistance larger than the pass device by a scaling factor N, and a compensation circuit transforms the voltage drop across the sensing resistor into an output current, making the measurement independent of the pass device's on-resistance, thereby compensating for temperature and supply voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sensing resistor is used to measure current through the pass device, then current measurement is enabled, but the measurement accuracy deteriorates due to dependency on pass device on-resistance variations with temperature and supply voltage
Solution Approach 1:
A sensing replica transistor is created that copies the electrical characteristics of the pass device. The sensing replica has the same gate-source voltage and current characteristics as the pass device, allowing it to replicate the on-resistance behavior. This copying approach enables indirect measurement that compensates for operational condition variations.
Solution Approach 2:
The sensing replica acts as an intermediary between the pass device and the measurement circuit. Instead of directly measuring current through the pass device (which suffers from on-resistance variations), the measurement is performed on the sensing replica which has been designed to have stable, predictable characteristics. The sensing replica mediates the measurement process to eliminate dependency on pass device operational conditions.
2Ease of operation
If the sensing replica has larger on-resistance than the pass device by scaling factor N, then the voltage drop at the sensing resistor provides current indication, but the voltage drop becomes dependent on the pass device on-resistance
Solution Approach 1:
The sensing replica transistor copies the electrical characteristics of the pass device including its on-resistance behavior. By designing the sensing replica with the same gate control and channel characteristics, it replicates the on-resistance variations that would occur in the pass device under different operational conditions.
Solution Approach 2:
The dimensions of the sensing replica transistor are scaled by factor N relative to the pass device, which changes its on-resistance parameter. This scaling allows the sensing replica to have a larger, more measurable voltage drop while maintaining the same relative on-resistance characteristics as the pass device, enabling accurate current indication.
3Reliability
If compensation circuit is added to remove dependency on pass device on-resistance, then measurement independence is achieved, but device complexity increases
Solution Approach 1:
Instead of adding complex compensation circuits to correct for on-resistance variations, the patent uses a sensing replica that inherently copies the pass device characteristics. This copying approach provides natural compensation without requiring additional active compensation elements, reducing overall circuit complexity while achieving measurement independence.
Solution Approach 2:
The sensing replica transistor serves dual purposes: it provides the measurement function and simultaneously compensates for operational condition variations through its inherent characteristic copying. The replica essentially self-compensates for temperature and voltage effects by replicating the same physical phenomena that affect the pass device, eliminating the need for external compensation mechanisms.
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
The solution provides reliable and independent current indications, reducing the impact of operational conditions on the accuracy of load current measurements, enhancing the stability and efficiency of power switch control.
Implementation Method 1
a voltage drop at the sensing resistor provides an indication of the current through the pass device
Implementation Method 2
The sensing replica may exhibit an on-resistance which is larger than the on-resistance of the pass device by a pre-determined scaling factor N
Implementation Method 3
a transformation circuit configured to transform the voltage drop at the sensing resistor into an output current, such that the output current is indicative of the voltage drop at the sensing resistor divided by a compensation resistance
Data Source
Figure 1
Figure 2a
Figure 2b~3
AI summary
The present document relates to a current sensing circuit. In particular, the present document relates to a current sensing circuit which provides reliable indications of the current through a transistor. A current sensing circuit (220) configured to provide an indication of a load current through a pass device (21) is described. The current sensing circuit (220) comprises a sensing replica (22) of the pass device (21) and a sensing resistor (23) arranged in series with the sensing replica (22). The sensing resistor (23) is arranged such that a voltage drop at the sensing resistor (23) provides an indication of the load current through the pass device (21). The voltage drop at the sensing resistor (23) is dependent on an on-resistance of the pass device (21). Furthermore, the current sensing circuit (220) comprises a transformation circuit (25, 26, 227, 228, 29) configured to transform the voltage drop at the sensing resistor (23) into an output current, such that the output current is indicative of the voltage drop at the sensing resistor (23) divided by a compensation resistance of a compensation circuit (227). The compensation resistance exhibits a dependency on the on-resistance of the pass device (21) which corresponds to the dependency on the on-resistance of the pass device (21) of the voltage drop at the sensing resistor (23).