Cascode Current Drive Circuit for Stable Series LED Dimming
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Solution Overview
Problem
Current drive circuits for LEDs face challenges in maintaining accurate current output while minimizing voltage fluctuations, which can affect color rendering and dimming, especially when driving multiple LEDs in series with high output voltage requirements, and may generate unpleasant sounds due to pulsed current frequencies within the human audible range.
Innovation Solution
A current drive circuit configuration utilizing a cascode connection of transistors with varying sizes and current sources to maintain a constant current output, incorporating feedback loops and startup assist circuits to stabilize voltage and reduce startup time, while ensuring the circuit operates efficiently even with large output voltage fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pulse width modulation is used to control LED current for accurate color rendering and dimming, then current control accuracy is improved, but voltage fluctuations at the LED connection terminal increase
Solution Approach 1:
The patent introduces an intermediary circuit stage between the pulse width modulation control and the LED connection terminal. This intermediary circuit absorbs and isolates the voltage fluctuations generated by PWM switching, preventing them from propagating to the LED terminal while preserving the accurate current control capability. The intermediary acts as a buffer that decouples the control signal from the voltage instability.
2Illumination intensity
If multiple LEDs are driven in series to obtain required luminous intensity, then light output is improved, but drive power output voltage requirements increase to several tens of volts or more
Solution Approach 1:
The patent segments the high-voltage drive circuit into multiple lower-voltage transistor stages connected in series. Each transistor stage handles a portion of the total voltage requirement, allowing the circuit to achieve several tens of volts output capability while using individual transistors with lower breakdown voltage ratings. This segmentation enables high luminous intensity through series LED configuration without requiring a single high-voltage component.
3Adaptability or versatility
If pulsed current frequency is set within human audible range for practical power supply operation, then power supply compatibility is improved, but unpleasant sounds are generated in the power supply circuit
Solution Approach 1:
The patent changes the operating frequency parameter of the pulsed current from the human audible range (20 Hz - 20 kHz) to the ultrasonic range (>20 kHz). This parameter change maintains power supply compatibility while eliminating the generation of unpleasant audible sounds. The circuit is designed to operate efficiently at ultrasonic frequencies, achieving both adaptability to standard power supplies and elimination of harmful acoustic emissions.
Data Source
AI summary
A current drive circuit has a first transistor that outputs a current, a second transistor connected to the first transistor by cascode connection, a third transistor connected to the second transistor by cascode connection, a first current source that supplies a current to the third transistor and the second transistor, a fourth transistor that shares a gate with the third transistor, a fifth transistor that is connected to the fourth transistor by cascode connection and shares a gate with the second transistor, a second current source that supplies a current to the fourth transistor and the fifth transistor, a sixth transistor that shares a gate with the third transistor and the fourth transistor and controls a gate voltage of the first transistor, and a third current source that supplies a drain current of the sixth transistor.