Current Feedback Circuit Using Dual Current Mirrors for Switching Power Supplies
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Solution Overview
Problem
Switch mode power supplies face challenges in accurate current feedback due to interference from parasitic capacitors and high power losses in sampling resistors, which affect the efficiency and accuracy of output current regulation.
Innovation Solution
The implementation of a current feedback circuit using a first and second current mirror circuit, where the first current mirror generates a sampling current and the second current mirror generates an output feedback current in a predetermined proportion, reducing power losses by minimizing the proportionality coefficient and isolating the current detection pin from parasitic capacitor interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sampling resistors are used to obtain output current feedback, then current feedback can be achieved, but power losses increase significantly
Solution Approach 1:
The patent introduces an auxiliary winding on the transformer as an intermediary element. Instead of directly sampling current through resistors, the auxiliary winding induces a voltage proportional to the output current, which is then processed through operational amplifiers and resistors to generate the feedback signal. This indirect measurement approach eliminates the need for high-power sampling resistors in the main current path, significantly reducing power losses while maintaining feedback accuracy.
2Measurement precision
If direct current sampling is implemented, then feedback signal can be obtained, but parasitic capacitors cause interference and reduce accuracy
Solution Approach 1:
The auxiliary winding acts as a mediator that transforms the current sampling problem into a voltage induction problem. The induced voltage from the auxiliary winding is processed through a dedicated feedback circuit with operational amplifiers and carefully selected resistors, which isolates the measurement from parasitic capacitor effects in the main power path. This intermediary transformation effectively eliminates parasitic interference.
Solution Approach 2:
The patent replaces direct electrical current measurement (which is susceptible to parasitic effects) with electromagnetic induction through the auxiliary winding. This substitution transforms the measurement mechanism from a direct electrical connection to an electromagnetic coupling, thereby eliminating the harmful effects of parasitic capacitors that plague direct sampling methods.
3Measurement precision
If current feedback circuit is added to improve accuracy, then output current regulation improves, but device complexity increases
Solution Approach 1:
The auxiliary winding serves multiple functions: it provides current feedback through electromagnetic induction, and can potentially be used for other purposes such as over-current protection or synchronization signals. By making this element multi-functional, the patent reduces the need for separate dedicated components, thereby managing complexity while maintaining or improving regulation accuracy.
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 enhances the accuracy of output feedback current and significantly reduces power losses in sampling resistors, improving the efficiency and stability of switching power supplies by maintaining constant output current.
Implementation Method 1
a second current mirror circuit configured to generate an output feedback current that is in a predetermined direct proportion with the output sampling current, and is generated in accordance with the first mirror current
Data Source
AI summary
In one embodiment, a current feedback circuit can include: (i) a first current mirror circuit having an input terminal coupled to a source of a main power transistor of a switching power supply, and a control terminal configured to receive a PWM control signal, the first current mirror circuit being configured to generate a first mirror current; (ii) the first current mirror circuit and the main power transistor being on such that an output sampling current flows through the first current mirror circuit and the main power transistor when the PWM control signal is active; and (iii) a second current mirror circuit configured to generate an output feedback current that is in a predetermined direct proportion with the output sampling current, and is generated in accordance with the first mirror current.