Common-Mode Current Cancellation via Variable Capacitor Network
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Solution Overview
Problem
Common-mode current interference in isolated power supply systems due to parasitic capacitance mismatch between transformer windings, which can cause signal interference and increase system size, cost, and complexity.
Innovation Solution
A compensation circuit with a variable capacitor diode network and digital-to-analog converter provides a compensation signal to the isolated ground plane to reduce or eliminate common-mode current, using drive signals and adjusting amplitude and polarity based on DC bias voltage, implemented in a push-pull DC-DC converter system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a grounded shield is placed between the primary and secondary of the transformer to minimize common-mode current, then common-mode current is reduced, but the transformer size, cost, and complexity increase
Solution Approach 1:
The invention extracts the common-mode current cancellation function from the transformer structure itself and implements it separately through a compensation circuit. Instead of adding a grounded shield within the transformer, the patent uses a discrete compensation circuit that generates an equal and opposite common-mode current to cancel the harmful current, thereby reducing common-mode current without increasing transformer complexity
Solution Approach 2:
The invention introduces a compensation circuit as an intermediary between the transformer and ground. This compensation circuit includes a capacitor and resistor network that generates a compensation current serving as a mediator to counteract the common-mode current, avoiding the need to modify the transformer structure directly
2Object-affected harmful factors
If a potentiometer and resistor-capacitor network are used to inject compensation current into the isolated ground, then common-mode current is minimized, but the parts count and cost increase
Solution Approach 1:
The invention merges the compensation current generation function with the existing driver circuitry by using the same drive signals that already exist in the push-pull converter. The compensation circuit utilizes the primary side drive signals to generate the compensation current, eliminating the need for separate potentiometers and reducing parts count
Solution Approach 2:
The invention makes the existing drive signals serve multiple functions: they both drive the primary winding of the transformer and serve as the input to the compensation circuit. This multi-functionality reduces the need for additional components, as the same signal sources are utilized for both power transfer and common-mode current cancellation
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
Effectively reduces residual current in the secondary winding of the transformer, minimizing signal interference and system complexity while maintaining isolation, with no added DC current and high operating bandwidth.
Implementation Method 1
The compensation circuit may include a variable capacitor diode network that receives the drive signals, and may further include a biasing element that provides a direct-current (DC) bias voltage to the variable capacitor diode network
Implementation Method 2
The transformer may provide isolation between the non-isolated subsystem and the isolated subsystem by blocking DC signals
Implementation Method 3
The common-mode current that flows from the non-isolated primary to the isolated secondary through the parasitic capacitance inside the transformer
Data Source
AI summary
A residual current (e.g. common-mode current) may be present in an isolated subsystem. The isolated subsystem may include the secondary winding of a transformer while a first subsystem may include the primary winding of the transformer. The first subsystem may also include a compensation circuit. A driver circuit may generate drive signals provided to the primary winding of the transformer and also to the compensation circuit. The compensation circuit may include a variable capacitor network (e.g. a variable capacitor diode network) that receives the drive signals and also receives a bias voltage, and generates a cancellation signal according to the drive signals and the bias voltage. The compensation circuit may provide the cancellation signal to the ground plane of the isolated subsystem through a capacitor that couples the variable capacitor diode network to the ground plane, in order to reduce or cancel the residual current present in the isolation subsystem.