Low Voltage Transistor LED Driver Timing Control
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Solution Overview
Problem
Conventional LED drivers require high-breakdown voltage FETs to handle the full driving voltage, which are expensive and less efficient, especially when controlling multiple LEDs in series.
Innovation Solution
The proposed solution involves disabling the high voltage regulator shortly before turning off the transistor and enabling it shortly after turning it back on, using delay circuits or feedback techniques to ensure the transistor is not subjected to the high voltage during PWM control, allowing the use of low-voltage FETs that can withstand the switching cycles without the need for high breakdown voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a high voltage regulator is used to drive multiple LEDs in series, then the required driving voltage is achieved, but the FET must withstand high breakdown voltage which increases cost and reduces efficiency
Solution Approach 1:
The high voltage regulator is disabled before the FET turns off and enabled after the FET turns on, ensuring the FET never experiences high voltage stress. This preliminary timing action allows using low breakdown voltage FETs while still achieving the required high driving voltage for the LED string.
Solution Approach 2:
The control circuit acts as an intermediary between the high voltage regulator and the FET, coordinating their operation timing. The control circuit ensures the regulator and FET are never simultaneously active, thereby protecting the FET from high voltage while maintaining the ability to drive high voltage LEDs.
2Ease of operation
If the FET is turned off to control LED current, then brightness control is achieved, but the FET is subjected to full drive voltage which requires high breakdown voltage components
Solution Approach 1:
The high voltage regulator is disabled in advance before the FET turns off, so when the FET is off and would normally block the full drive voltage, the regulator is already not producing high voltage. This eliminates voltage stress on the FET while preserving brightness control capability through PWM.
Solution Approach 2:
The system applies preliminary anti-action by disabling the high voltage regulator before the FET needs to block voltage. This preventive measure counteracts the potential harmful voltage stress before it can occur, allowing the FET to operate safely at low voltage while still enabling brightness control.
3Ease of manufacture
If delay circuits are used to coordinate regulator and FET timing, then low voltage FETs can be used, but device complexity increases
Solution Approach 1:
The system uses periodic PWM signals to control both the FET and the high voltage regulator timing. By synchronizing both components to the same PWM period with appropriate phase shifts, delay circuits are minimized and the control logic becomes more regular and predictable, reducing overall system complexity.
Data Source
AI summary
Various circuits are described herein where a series transistor used to control current through a string of LEDs, driven by a high voltage, is not subjected to the high voltage when the transistor is turned off pursuant to a PWM signal. To avoid the transistor experiencing the high voltage, the HV regulator is disabled shortly before the transistor is turned off and is enabled shortly after the transistor has turned back on. Control circuits for controlling the regulator and transistor include delay circuits and/or voltage sensing circuits to ensure that the transistor is always on prior to the voltage regulator being enabled pursuant to the incoming PWM signal, and the voltage regulator is always disabled when the first transistor is off pursuant to the incoming PWM signal.