Inductor Current Sensing in Buck Converters Using Switch Node Filtering
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Solution Overview
Problem
Current DC/DC converters face issues with switching noise and low signal-to-noise ratio in current sensing, leading to jitter and imprecise switching, especially at high duty cycles and low output voltages, due to the use of sense resistors and inductor DCR for current detection.
Innovation Solution
A current sensing technique that filters out switching noise by summing signals from power and synchronous rectifier switches to generate a signal proportional to the inductor current, using a first RC circuit to attenuate AC components and a series RC circuit to derive a noiseless AC component from the switch voltage, improving the signal-to-noise ratio and reducing jitter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a low value sense resistor is used to reduce power dissipation, then power loss is reduced, but the signal to noise ratio deteriorates causing imprecise switching
Solution Approach 1:
The patent segments the current sensing function into two separate paths: one for detecting the DC component of inductor current and another for detecting the AC component. This segmentation allows each path to be optimized independently, enabling the use of very low value sense resistors without sacrificing measurement precision, as the AC component can be derived from switch node voltage rather than sense resistor voltage.
Solution Approach 2:
The patent introduces an intermediary approach by using the switch node voltage (which has high signal amplitude) as a mediator to derive the AC component of inductor current. This intermediary signal serves as a reference to reconstruct the AC current waveform without relying on the small voltage drop across the sense resistor, thereby improving signal to noise ratio while maintaining low power dissipation.
2Reliability
If switching noise is present in the current sense circuit, then false triggering of the PWM comparator occurs, but filtering the noise may affect the accuracy of the sensed signal
Solution Approach 1:
The patent segments the current sensing function into two separate paths: one for detecting the DC component of inductor current and another for detecting the AC component. This segmentation allows each path to be optimized independently, enabling the use of very low value sense resistors without sacrificing measurement precision, as the AC component can be derived from switch node voltage rather than sense resistor voltage.
Solution Approach 2:
The patent introduces an intermediary approach by using the switch node voltage (which has high signal amplitude) as a mediator to derive the AC component of inductor current. This intermediary signal serves as a reference to reconstruct the AC current waveform without relying on the small voltage drop across the sense resistor, thereby improving signal to noise ratio while maintaining low power dissipation.
3Device complexity
If the inductor current is sensed using DCR, then no additional sense resistor is needed, but the signal to noise ratio remains low and switching noise causes jitter
Solution Approach 1:
The patent segments the current sensing function into two separate paths: one for detecting the DC component of inductor current and another for detecting the AC component. This segmentation allows each path to be optimized independently, enabling the use of very low value sense resistors without sacrificing measurement precision, as the AC component can be derived from switch node voltage rather than sense resistor voltage.
Solution Approach 2:
The patent introduces an intermediary approach by using the switch node voltage (which has high signal amplitude) as a mediator to derive the AC component of inductor current. This intermediary signal serves as a reference to reconstruct the AC current waveform without relying on the small voltage drop across the sense resistor, thereby improving signal to noise ratio while maintaining low power dissipation.
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 proposed solution significantly reduces switching noise and improves the signal-to-noise ratio, resulting in more precise duty cycle control and reduced power loss, enhancing the efficiency of the current mode regulator.
Implementation Method 1
The value K*iL includes the DC and AC components of the current through the inductor L. The AC component of K*iL contains switching noises and is filtered (attenuated) by a first RC circuit.
Implementation Method 2
This Vsw signal is applied to a series RC circuit to filter out the DC component and create a ramping AC signal that corresponds to the AC component of the inductor current, but without any switching noise.
Data Source
AI summary
In a current mode switching power supply, current through the inductor needs to be sensed to control the peak current. The inductor current includes a DC component and an AC component containing switching noise. To reduce the switching noise, the actual inductor current is sensed to generate a signal, and a first AC component is attenuated by a first RC circuit while not attenuating a first DC component. A second AC component is derived by applying the rectangular wave switch voltage, which is at the duty cycle of the regulator, to a second RC filter, which blocks a second DC component. The second AC component is much larger than the first AC component and does not contain switching noise. The large second AC component, the smaller “noisy” first AC component, and the first DC component are applied to the first RC circuit to create a low-noise inductor current signal.