LED String Segment Switching for AC Grid Compliance
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Solution Overview
Problem
Existing LED lighting systems face inefficiencies due to flicker and high crest factor when directly connected to the power grid, leading to excessive harmonic distortion and reduced power factor, which compromises electric efficiency and increases thermal oscillations.
Innovation Solution
The system employs a design that uses LEDs as current regulators, achieving a quasi-constant current supply with a crest factor of 1.14, minimizing flicker and harmonic distortion by shaping the sink current into a trapezoidal form compliant with EN61000-3-2 standards, using modular components like LEDs arranged in an N x M topology and modulation techniques such as amplitude, PWM, and random modulation to optimize efficiency and compliance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If LEDs are directly connected to the power grid, then the system structure is simplified and components are reduced, but the current becomes heavily distorted with THD exceeding 78% and power factor less than 0.5
Solution Approach 1:
The LED string is divided into multiple segments that can be independently controlled. Each segment is switched on or off based on the instantaneous voltage level, allowing the total harmonic distortion to be reduced while maintaining direct AC connection simplicity.
Solution Approach 2:
The LED segments are switched periodically according to the AC voltage waveform. By controlling which segments are active at different voltage levels, the system achieves sinusoidal current approximation with THD below 15% while maintaining direct AC connection.
2Reliability
If the number of LEDs is dimensioned to resist maximum grid voltage, then the system can handle peak voltages, but the conducting time is only half the total time causing significant flicker
Solution Approach 1:
The LED string is segmented into multiple groups with different voltage thresholds. This allows progressive activation of segments as voltage increases, ensuring continuous lighting throughout the entire AC cycle and eliminating flicker while maintaining voltage resistance.
Solution Approach 2:
The system changes the operating parameters of different LED segments based on the instantaneous voltage level. By adjusting which segments are active according to voltage magnitude, the system maintains constant current supply throughout the full AC cycle, eliminating the 50% duty cycle flicker.
3Object-generated harmful factors
If known functional solutions simulate sinus shape of sink current, then harmonic limits are met, but the crest factor exceeds 1.41 causing flicker and thermal oscillations
Solution Approach 1:
The system uses periodic switching of LED segments synchronized with the AC voltage waveform. This natural synchronization achieves sinusoidal current approximation with crest factor of 1.41 or lower, eliminating the excessive thermal oscillations caused by higher crest factors in other solutions.
Solution Approach 2:
The LED segments themselves serve as the switching elements, with each segment's forward voltage characteristic naturally determining its activation point. This self-service approach achieves sinusoidal current control without external PWM controllers, maintaining crest factor at acceptable levels.
4Object-generated harmful factors
If discrete amplitude modulation is used to approximate step sine function, then EN61000-3-2 compliance may be achieved, but significant flicker occurs with light off for +/- 1.3ms around grid voltage zero crossing
Solution Approach 1:
Multiple LED segments with progressively increasing voltage thresholds ensure that as one segment turns off, another turns on. This overlapping activation maintains continuous light output throughout the entire AC cycle, eliminating the 2.6ms total blackout period of discrete AM solutions.
Solution Approach 2:
The system continuously adjusts which segments are active based on instantaneous voltage, maintaining constant current through the LED string throughout the full AC cycle. This parameter-based control eliminates the discrete on/off transitions that cause flicker in other solutions.
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 results in an electric efficiency of up to 99%, minimal flicker, and compliance with EN61000-3-2 harmonic pollution standards, eliminating the need for bulky components like electrolytic capacitors and transformers, allowing for ultra-thin and efficient lighting solutions.
Implementation Method 1
LEDs experience a pronounced un-linear current to voltage characteristic and thus requiring constant current supply
Implementation Method 2
shaping the sink current into a trapezoidal form compliant with EN61000-3-2 standards, using modular components like LEDs arranged in an N x M topology
Implementation Method 3
modulation techniques such as amplitude, PWM, and random modulation to optimize efficiency and compliance
Implementation Method 4
modulation techniques such as amplitude, PWM, and random modulation to optimize efficiency and compliance
Implementation Method 5
The functional approach of LEDs supply or the so called 'Direct AC drive' use the voltage drop across LEDs during the regulating process
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
An EN61000-3-2 compliant lighting apparatus sized for minimum crest factor criteria or for maximum efficiency, that sinks from the grid rectangular current pulses AM, high frequency PWM, instrinsic PWM or random modulated, consisting of a rectifier bridge, a constant current source series-connected with a sequence of [N x M] high power [HP] and high brightness [HB] LEDs, series-connected and organized in N segments times M LEDs each, segments that can be introduced or removed from the circuit by short-circuiting them by an individual switch, except for the segment to the current source, a resistor voltage divider setting the voltage switching tresholds at which the LED segments are introduced or removed from the circuit and a voltage reference modulated or not by a signal un-correlated to the network voltage. Upon connecting the grid to the input terminals A and B of the circuit displayed in Figure 5, the grid alternate voltage is full wave rectified by the rectifier bridge and is applied to the string of N x M luminiscent diodes [LED] series-connected with the constant current source. Upon increasing the UAB voltage between 0 and up to reaching the conduction value of the segment with the highest SN parameter that is permanently present in circuit, in series with the constant current source, the current sink by the apparatus. At the hereinafter increase of voltage at AB terminals, the current through SN segment and through the constant current source increases exponentially up to th value set by the constant current source, through the switch made by transistor QN-1 by-passing segments SN-1... S1. Observing the serial presence of the constant current source, the voltage drop across segment SN is limited and consistent to the constant current set by the current source. For UAB voltages lower than the maximum voltage drop on a series of LEDs supplied at that constant current, the full input voltage drops at the terminals of segment SN; As input voltages exceed the above mentioned maximum drop voltage, the balance will drop on the constant current source. When the input voltage exceeds the threshold values set via the R1, R2+...+RN resistive divider, the switch consisting the transistor QN-1 and controlled by the corresponding comparator, gets switched off, thus introducing the segment SN-1 in the circuit, in series with SN. The sink current drops to a value set by UAB voltage, quadruplicated to MN+MN-1 number of diodes from segments SN si SN-1 present in series and their curent to voltage curve. Hereinafter, as the UAB voltage successively exceeds the threshold values set by the R1, R2, RN resistive dividers, the schematic introduces successively in the circuit, in series with the already existing segments, additional segments that undertake voltage drops from the constant current source. Upon the decrease UAB voltage at AB terminals, the process is carried out in a reverse way, namely upon each drop in UAB voltage, below the threshold levels set by the resistive dividers made of R1, R2,... RN, the schematic successively removes from the circuit and in a reverse order as above, LED segments, by-passing them with the appropriate switch. The power conversion efficiency and the crest factor are being set by simply fixing the N [the number of segments] and the switching threshold levels,. Following the above described successive switch, I have ensured a sequence of constant amplitude current pulses, approximately rectangular, starting at approximately 0.2mS and lasting up to approximately 9.8ms from the zero crossing of grid voltage is sinking through the LEDs. To get EN61000-3-2 compliance, the sink current is amplitude, HF-PWM, LF intrinsec PWM or random modulated so that the envelope of real time or average sink current follows the„ i trapezoid" trapezoidal profile, indicated in Figure 4. If setting the driver for maximum efficiency, the harmonic spectrum of the sink current does not comply with EN61000-3-2, its harmonic amplitudes between approximately 1kHz and 3kHz, being some 3% higher than the limit. For EN61000-3-2 compliance the spectrum of sink current is scattered by modulating the reference voltage by low frequency uncorrected to the grid. Spectrum scattering through such method reduces the amplitude of the harmonic components by some 8dB thus offering a safe margin to the limit. Comparing the maximum theoretical performances achievable through the various modulation methods, it is noticed that for a given structure of 10 x 10 LEDs, they are grouped. Under these circumstances, the election of the modulation method is technical and price based. In the real life the efficiency is affected by power losses in the current source, in switches and on rectifier bridge. A superficial calculation, sets these additional losses in the area of 1.5% which affects the theoretical efficiency to approximately 92%. Nevertheless the efficiency losses can be offset by simply gliding the switching thresholds to the point where the harmonics between 1kHz and 3kHz drop below the 3% limit.