Phase Cut Dimming with Anti-Series MOSFETs and Unipolar Zero Detection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing phase cut dimmers struggle to efficiently control and protect low power loads, such as small LED lamps, due to uncontrollable leakage currents and high power consumption, which limits their ability to dim lights to low levels and requires complex and costly protection circuits.
Innovation Solution
A phase cut dimming apparatus using a pair of MOSFETs with intrinsic body diodes, a unipolar zero detector, and a low power detection circuit for over-current and over-temperature protection, along with a low pass filter and a low power DC power supply, to minimize power consumption and enable effective dimming of low power loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional filters and control circuits are used in phase cut dimmers, then EMI filtering and dimming control are achieved, but uncontrollable leakage current flows through the load even when the AC switch is completely off
Solution Approach 1:
The patent extracts the harmful leakage current path by introducing a synchronous rectification circuit that actively manages the current flow through the filter. The control circuit detects the AC waveform and generates complementary gate signals to ensure the MOSFETs conduct during both half-cycles, converting the uncontrollable leakage current into a controllable rectified current that can be directed through the load or bypassed as needed.
Solution Approach 2:
The patent implements feedback control by using the AC switch node voltage as a reference to generate synchronous rectification signals. The control circuit continuously monitors the AC waveform and adjusts the MOSFET gate signals accordingly, creating a closed-loop system that adapts to the instantaneous voltage state and eliminates leakage current by ensuring current flows only when intentionally commanded.
2Illumination intensity
If the AC switch is completely turned off to achieve maximum dimming, then dimming to low levels is enabled, but uncontrollable currents Ifil and Iaux remain flowing through the load
Solution Approach 1:
The patent ensures continuous useful action by making the load current continuous through synchronous rectification. Even when the AC switch is off for dimming, the MOSFETs in the synchronous rectification circuit conduct during both positive and negative half-cycles of the AC waveform, maintaining a continuous current path through the load that follows the rectified voltage waveform rather than interrupting completely.
Solution Approach 2:
The patent replaces the mechanical/physical limitation of the AC switch (which cannot conduct in reverse direction) with electronic control of MOSFETs that can be actively switched to conduct in both directions. This substitution of mechanical switching with electronically controlled semiconductor switching enables continuous current flow through synchronous rectification, eliminating the dead zones where leakage current would otherwise flow uncontrollably.
3Ease of operation
If thyristors are used as AC switches, then leading edge dimming is achieved, but the switch cannot be turned off at any phase angle
Solution Approach 1:
The patent changes the fundamental parameter of switch controllability by replacing the latching特性 of thyristors with the controllable on/off特性 of MOSFETs. The MOSFETs can be turned on and off at any phase angle by gate signaling, enabling both leading edge and trailing edge dimming modes. The synchronous rectification circuit further enhances this by coordinating MOSFET switching with the AC waveform phase, achieving precise phase-angle control that thyristors cannot provide.
4Reliability
If complex protection circuits are added for over-current and over-temperature protection, then device protection is improved, but power consumption and cost increase
Solution Approach 1:
The patent achieves multi-functionality by using the existing synchronous rectification MOSFETs and control circuitry to perform both dimming control and protection functions. The same gate drive circuit that controls the MOSFETs for synchronous rectification also monitors their current and temperature status, and can shut them off for protection without requiring separate dedicated protection circuits. This universal use of components eliminates additional power consumption and cost.
Solution Approach 2:
The patent implements self-service protection where the control circuit automatically monitors the health status of the MOSFETs and takes protective action without external intervention. The protection circuitry uses the existing voltage and current sensing nodes to detect over-current and over-temperature conditions, and the control algorithm automatically shuts down the MOSFETs when thresholds are exceeded, making the system self-protecting without adding complex external protection devices that would consume additional power.
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 solution allows for efficient dimming of low power loads with reduced power consumption and effective protection, enabling dimming to low levels while maintaining low power usage and simplicity, thus addressing the limitations of existing dimmers.
Implementation Method 1
The filter FIL is to block the transmission of high frequency EMI (Electromagnetic Interference) between the AC power supply/load and the dimmer
Implementation Method 2
an AC switch formed of a pair of MOSFETs or IGBTs can be easily turned on and off by gate signaling
Implementation Method 3
each MOSFET with intrinsic body diode connected anti-parallel to drain and source electrodes respectively
Implementation Method 4
a unipolar zero detector to detect a unipolar zero level transition of a rectified AC voltage across the AC switch
Implementation Method 5
detecting an over-current condition of the AC switch by monitoring a negative drain voltage of the MOSFETs with respect to the common source terminal; detecting an over-temperature condition of the AC switch by monitoring a positive drain voltage
Implementation Method 6
In particular for phase cut power control, such as dimmers, AC switches are commonly implemented by thyristors or anti-series connected pair of MOSFETs or IGBTs
Data Source
AI summary
A phase cut dimmer, coupled between an AC power source and a load, to control the amount of power delivered from the power source to the load, comprising: a dimmer switch to turn on and off the current from the power source to the load; the dimmer switch being an AC switch formed of a first MOSFET and a second MOSFET connected in anti-series; the two source electrodes of the MOSFETs being connected together to form a common source terminal and the two gate electrodes of the MOSFETs being connected together to form a switch control terminal; a line filter bypassing very low current at the line frequency; a rectifier to convert the AC voltage from the power source to pulsating DC voltage which is coupled to the dimmer switch; a very low power unipolar zero detector extracting a zero marker signal from the pulsating DC voltage; the unipolar zero detector comprising a voltage-controlled bleeder impedance, a comparator, a pulse generator, wherein the voltage-controlled bleeder impedance is coupled to the pulsating DC voltage whereby the bleeder impedance is controlled proportional to the instantaneous value of the voltage; the pulsating DC voltage is compared to a reference close to zero level by the comparator, triggering the pulse generator to generate the zero marker signal when the voltage is lower than the reference; a very low power adjustable timer to control the dimmer switch in synchronization with the zero marker signal.