Amperometric Transformer Current Sensing Circuit for DC Component Detection
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Solution Overview
Problem
Current sensing techniques for high-frequency discontinuous currents in Switch Mode Power Supplies face limitations, particularly in sensing the DC component due to volt-second balance issues, leading to increased core size, noise, and high-frequency distortions in magnetic core solutions.
Innovation Solution
A circuit incorporating an active switch or diode across the secondary winding of an amperometric transformer, coupled with a capacitor and sensing resistor, allows for DC component sensing by storing voltage offsets during zero current periods, reducing magnetic flux and the number of transformer turns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a damping element (RC network with diode) is inserted across the secondary winding to preserve the DC component, then the DC component can be sensed, but the magnetic flux in the core increases requiring bigger core size and more turns
Solution Approach 1:
The patent introduces a capacitor as an intermediary element connected in parallel with the sensing resistor. This capacitor blocks the DC component from being shorted by the sensing resistor while allowing AC components to pass through normally. The capacitor acts as a mediator that preserves the DC voltage across the secondary winding without requiring additional damping elements with diodes, thus avoiding the increase in magnetic flux and core size.
Solution Approach 2:
The patent changes the electrical parameters of the sensing circuit by introducing a capacitor with specific capacitance value. This parameter change modifies the impedance characteristics of the sensing branch, allowing the DC component to be preserved across the secondary winding while maintaining normal AC sensing performance. The capacitor's reactance varies with frequency, automatically providing the desired frequency-dependent behavior without increasing core size.
2Measurement precision
If a damping element with diode is used to reset the core during zero current period, then the DC component is preserved, but high-frequency noise and distortions are generated in the measured current
Solution Approach 1:
The capacitor serves as an intermediary that prevents the diode from directly influencing the current waveform. By placing the capacitor in parallel with the sensing resistor, the circuit allows the diode to conduct during reverse voltage periods without allowing its switching action to directly affect the measured current. The capacitor smooths out the high-frequency noise generated by the diode's switching, preserving the DC component while filtering out the harmful high-frequency distortions.
Solution Approach 2:
The introduction of the capacitor changes the frequency response characteristics of the sensing circuit. The capacitor's reactance is high at low frequencies (preserving DC) and decreases at higher frequencies (attenuating noise). This parameter change allows the circuit to selectively pass desired signals while blocking harmful high-frequency noise and distortions generated by the diode's switching action.
3Power
If the volt second balance condition is maintained in the amperometric transformer, then the transformer operates efficiently, but the average voltage on the sensing resistor becomes zero making DC component sensing impossible
Solution Approach 1:
The patent segments the sensing function into two separate paths: one for AC components through the sensing resistor, and another for DC components through the capacitor. The sensing resistor continues to provide AC current sensing while the capacitor blocks DC from being shorted, allowing the DC voltage to be measured across the secondary winding. This segmentation allows both AC and DC components to be sensed simultaneously without violating the volt-second balance condition.
Solution Approach 2:
The capacitor acts as an intermediary that decouples the DC and AC sensing paths. It allows the sensing resistor to maintain its low-impedance path for AC signals while simultaneously blocking DC signals from being shorted. This intermediary element enables the transformer to maintain volt-second balance for efficient operation while allowing DC component detection through the voltage that appears across the capacitor.
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 solution enables accurate, low-noise current measurement with reduced core size and cost, while maintaining the ability to sense the average current value without additional filtering.
Implementation Method 1
the current to be sensed may be measured by feeding it into the primary winding of an amperometric transformer, the secondary winding whereof is coupled to a current sensing resistor
Implementation Method 2
A circuit incorporating an active switch or diode across the secondary winding of an amperometric transformer, coupled with a capacitor and sensing resistor, allows for DC component sensing by storing voltage offsets during zero current periods
Data Source
Figure 1
Figure 2
AI summary
A current sensing circuit for sensing an intermittent current having a Zero Current Period includes : - an amperometric transformer (10) having a primary winding (10a) for said current (IHF) to be sensed to flow therethrough and a secondary winding (10b), - a sensing resistor (12) coupled to the secondary winding (10b) of the transformer (10), - an offset capacitor (14) coupled with sensing resistor (12) between the sensing resistor (12) and ground, and - a switch element (16) acting across the coupling of the sensing resistor (12) and the offset capacitor (14), said switch element (16) being electrically conductive during said zero current period (ZCP) or a fraction thereof.