SMPS Switching Controller Frequency Modulation EMI Reduction
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Solution Overview
Problem
Conventional quasi-resonant switching mode power supplies (SMPS) experience increased electromagnetic interference (EMI) as the input AC voltage rises, due to reduced DC-link voltage ripple, which limits the modulation range of the switching frequency and increases switching losses.
Innovation Solution
The SMPS employs a switching controller that adjusts the switching frequency of the switching transistor by setting threshold periods and varying delay times, based on feedback voltage and current signals, to maintain a consistent frequency modulation range regardless of input AC voltage levels, thereby reducing EMI and maintaining switch efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the input AC voltage increases in a conventional quasi-resonant SMPS, then the DC-link voltage ripple decreases, but the modulation range of the switching frequency decreases and EMI increases
Solution Approach 1:
The patent applies dynamics by making the switching frequency adjustable and variable rather than fixed. The controller dynamically modifies the switching frequency based on operating conditions, specifically using variable delay times (first delay time and second delay time) to modulate the switching frequency of the switching transistor. This dynamic frequency adjustment maintains effective EMI reduction across varying input voltages while preserving switching efficiency.
Solution Approach 2:
The patent changes the parameter of switching frequency through variable delay times. By adjusting the delay time between switching events, the switching frequency is modulated within a specific range. This parameter change approach allows the system to maintain consistent EMI reduction performance regardless of input AC voltage variations, resolving the contradiction between switching losses and EMI.
2Object-affected harmful factors
If the switching frequency is modulated to reduce EMI, then switching losses are minimized, but the modulation range is limited by DC-link voltage ripple
Solution Approach 1:
The system uses dynamic frequency modulation where the switching frequency is continuously adjusted based on operating conditions. The controller employs variable delay times that can be adapted to different input voltage levels, maintaining an effective modulation range across varying conditions. This dynamic approach ensures the modulation range is not limited by DC-link voltage ripple variations.
Solution Approach 2:
The patent implements periodic switching action with variable period intervals. By controlling the switching transistor to operate at variable frequencies within a specific range, the system achieves effective EMI reduction. The periodic switching with adjustable timing allows the modulation range to remain effective regardless of input voltage changes.
3Device complexity
If a fixed switching frequency is used, then the control is simple, but EMI increases and switching losses are not minimized
Solution Approach 1:
The patent employs periodic switching action with variable frequency control. The switching transistor is turned on and off at variable frequencies determined by delay time control, creating a modulated switching pattern. This periodic variable-frequency approach achieves EMI reduction and switching loss minimization while maintaining relatively simple control implementation through delay time adjustment.
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 effectively minimizes EMI and maintains switch efficiency by ensuring a consistent frequency modulation range across varying input AC voltages, reducing switching losses and adhering to EMI regulations.
Implementation Method 1
A SMPS can rectify an input AC voltage to an input DC voltage
Implementation Method 2
A switching transistor is coupled to a primary coil of a transformer for generating power which is transferred to a second side of the transformer
Data Source
AI summary
In one embodiment, an SMPS includes a rectifier for generating an input DC voltage from an input AC voltage. A switching transistor is coupled to a primary coil of a transformer for generating power which is transferred to a second side of the transformer according to an operation of the switching transistor. A switching controller receives a feedback voltage corresponding to an output voltage, a sensing signal corresponding to a current flowing through the switching transistor, and a first signal corresponding to a voltage difference between a first electrode and a second electrode of the switching transistor. The switching controller controls an on/off operation of the switching transistor. The switching controller sets a threshold period whenever the first signal has a value greater than a reference value, thereby setting a plurality of threshold periods during operation of the switching mode power supply. For each threshold period, the switching controller turns on the switching transistor at a point after a variable delay time from a previous point at which the switching transistor was turned on.