LED Driver Start-Up Circuit for Fast Charging at Low Voltage
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Solution Overview
Problem
Switch mode power supplies (SMPS) used in LED luminaires experience unacceptably long start-up times, especially when dimmed, due to reduced input voltage, which is exacerbated by traditional trickle charge methods that increase power losses and are dependent on resistor sizing.
Innovation Solution
A start-up constant current source is introduced to rapidly charge the Vcc capacitor, engaging only during start-up and shutting off once the SMPS can produce its own regulated power, independent of input voltage, thereby minimizing power losses and reducing start-up time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a lower resistance value is used in the trickle charge path, then the start-up time is reduced, but the power losses during steady state operation increase
Solution Approach 1:
The patent applies dynamics by making the start-up path resistance variable rather than fixed. A transistor (Q1) is used as a variable resistor that provides low resistance during start-up to quickly charge the Vcc capacitor, then transitions to high resistance after start-up to eliminate steady-state power losses. This dynamic switching resolves the contradiction between fast start-up and low power consumption.
Solution Approach 2:
The patent uses preliminary action by providing a dedicated start-up path that pre-charges the Vcc capacitor before the main power conversion begins. The capacitor C1 stores energy during the start-up phase, allowing the controller to begin switching operations without waiting for steady-state power conversion to charge the Vcc capacitor. This preliminary charging action reduces start-up time while the subsequent disconnection prevents ongoing power losses.
2Illumination intensity
If the input voltage is reduced by a dimmer, then the light output is reduced, but the start-up time increases unacceptably
Solution Approach 1:
The patent uses dynamics to maintain constant start-up performance across varying input voltages. The transistor Q1 is biased to provide a relatively constant current to the Vcc capacitor regardless of input voltage fluctuations caused by dimmer settings. This ensures that even at minimum dimmer settings with reduced input voltage, the Vcc capacitor charges quickly and start-up time remains acceptable.
Solution Approach 2:
The patent introduces an intermediary capacitor C1 in the start-up path that decouples the Vcc charging process from the input voltage variations. This intermediary energy storage element buffers against dimmer-induced voltage reductions, allowing the Vcc capacitor to charge reliably even when the dimmer reduces input voltage to minimum levels, thus maintaining acceptable start-up times across all dimmer settings.
3Loss of energy
If a higher resistance value is used in the trickle charge path, then the power losses are reduced, but the start-up time increases
Solution Approach 1:
The patent resolves this contradiction through dynamic resistance control. During start-up, the transistor Q1 operates in its linear region providing low effective resistance to enable fast charging of the Vcc capacitor. After start-up completes and the controller begins normal operation, Q1 transitions to cutoff or saturation, presenting high resistance to eliminate steady-state power losses. This dynamic behavior allows the system to achieve both fast start-up and low power consumption at different times.
Solution Approach 2:
The patent implements periodic action through the switching behavior of the start-up circuit. The transistor Q1 is actively controlled to provide low resistance only during the initial start-up period, then disconnects afterward. This time-limited low-resistance phase enables fast start-up charging, while the subsequent high-resistance state prevents continuous power losses during steady-state operation, achieving both goals through temporal separation.
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 significantly reduces start-up time across various dimmer settings, minimizing power losses and ensuring rapid LED luminaire activation even at low dimmer output voltages, providing a balanced start-up time and power efficiency.
Implementation Method 1
a Vcc power supply capacitor coupled to the Vcc power supply input of the SMPS controller
Implementation Method 2
a rectifier circuit for converting an alternating current (AC) power source to a direct current (DC) power source
Implementation Method 3
a SMPS transformer having a primary winding thereof coupled to the DC power source and a controller supply winding for powering a SMPS controller
Data Source
AI summary
Improving start-up time of a light emitting diode (led) driver at lower input voltage is accomplished with a quick start circuit comprising a constant current source that replaces the traditional trickle charge start-up path for charging of a Vcc capacitor supplying operating voltage to an SMPS controller. Also the constant current source will only be operational during SMPS start-up, then will turn off after the SMPS is capable of producing its own regulated power supply to the Vcc terminal of the SMPS controller, thereby minimizing E2/R power losses in the SMPS.