Valley-Mode Switching Schemes for Power Converters
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Solution Overview
Problem
Conventional valley-mode switching schemes for switch-mode power converters are inadequate due to fluctuations in resonant characteristics caused by input line voltage or output current changes, leading to increased electromagnetic interference (EMI), incompatibility with fixed switching frequency control schemes, and unsuitability for slow power switches like bipolar junction transistors (BJTs).
Innovation Solution
An improved valley-mode switching scheme that determines a desired switch turn-on time based on pulse width modulation or pulse frequency modulation control schemes and predicts local minimum voltage points, allowing the power switch to turn on at a subsequent valley, independent of resonant characteristics, enabling low-voltage operation and compatibility with conventional feedback control techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional valley-mode switching schemes turn on the switch at the first valley in a switching cycle, then switching losses are reduced, but switching frequency becomes variable and EMI increases
Solution Approach 1:
The controller predicts the timing of future valleys based on current resonant characteristics and operating conditions, then generates the turn-on signal at the appropriate predicted time rather than reacting to the actual valley occurrence. This preliminary action allows the switching frequency to be predetermined while still achieving valley-mode switching, thus reducing EMI while maintaining low switching losses
2Loss of energy
If conventional VMS schemes turn on the switch at the first valley, then switching losses are reduced, but compatibility with fixed switching frequency control schemes is lost
Solution Approach 1:
The controller performs preliminary calculation of the desired turn-on time based on the fixed switching frequency and control scheme requirements, then predicts which valley will occur at or near that time, and finally generates the turn-on signal at that predetermined time. This separates the frequency determination (fixed) from the valley timing (predicted), enabling compatibility with fixed frequency control schemes while maintaining VMS benefits
Solution Approach 2:
The controller continuously monitors operating conditions such as input voltage and output current that affect resonant characteristics, uses this feedback to update valley timing predictions, and adjusts the turn-on signal timing accordingly. This feedback mechanism maintains accuracy of valley-mode switching while preserving fixed switching frequency operation
3Loss of energy
If conventional VMS schemes use high switching frequencies (e.g., 130 kHz) to achieve valley-mode switching, then switching losses are reduced, but suitability for slow power switches like BJTs is lost
Solution Approach 1:
The controller predicts valley timing in advance based on resonant characteristics and generates the turn-on signal at the optimal time, allowing slow switches like BJTs sufficient time to respond. This eliminates the need for high switching frequencies while still achieving valley-mode switching, making the system compatible with slow power switches while maintaining low switching losses
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 improved scheme reduces switching losses and EMI, supports the use of BJTs, and maintains control scheme flexibility, allowing for low-voltage operation without restricting control schemes, thus enhancing efficiency and reducing component count and costs.
Implementation Method 1
The resonant characteristics of a switch-mode power converter fluctuate in time based on operating conditions. For example, fluctuations in an input line voltage or in an output current drawn from the switch-mode power converter by an external load can affect the resonant behavior of the switch-mode power converter and therefore affect timing for valleys.
Data Source
AI summary
An improved valley-mode switching (VMS) scheme and circuitry for implementing the improved VMS switching scheme in a switch-mode power converter are disclosed. For a given switching cycle, a desired switch turn-on time is determined based on a pulse width modulation, pulse frequency modulation, or other suitable power converter control scheme. Also, one or more times corresponding to local minimums (valleys) are predicted for the voltage across a power switch of the switching power converter. The power switch is turned on at a valley immediately subsequent or otherwise subsequent to the desired switch time determined according to the power converter control scheme. Thus, the improved VMS scheme enables low-voltage switch operation to reduce switching loss and EMI noise without restricting the control scheme of the power converter.