Diode Driver Bias Circuit for Noise Rejection and Constant Current
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Solution Overview
Problem
Switching power supplies for LEDs and laser diodes generate voltage fluctuations that cause current fluctuations, leading to light intensity noise affecting light detectors and devices that incorporate these diodes, with parasitic capacitors exacerbating the issue.
Innovation Solution
A driver circuit configuration using transistor switches with adaptive biasing to maintain a constant voltage and current, minimizing the impact of parasitic capacitance by operating transistors in saturation or ohmic regions based on load conditions, and employing a gate bias circuit with an operational amplifier to stabilize current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If switching power supplies are used to drive LEDs and laser diodes, then power conversion efficiency is improved, but voltage fluctuations cause current fluctuations and light intensity noise
Solution Approach 1:
A capacitor is introduced as an intermediary element connected in parallel with the LED or laser diode. This capacitor acts as a local energy storage device that smooths voltage fluctuations from the switching power supply, preventing them from directly causing current fluctuations through the diode. The capacitor absorbs voltage spikes and releases energy during voltage dips, thereby reducing light intensity noise while maintaining the efficiency benefits of switching power supply operation.
Solution Approach 2:
The patent modifies the electrical parameters by adding a capacitor that changes the voltage waveform characteristics. The capacitor alters the voltage parameter across the LED or laser diode from a fluctuating waveform to a more stable waveform, thereby changing the current parameter and reducing light intensity variations. This parameter transformation resolves the contradiction by maintaining power conversion efficiency while eliminating harmful voltage fluctuations.
2Ease of operation
If transistors are used for switching LEDs and laser diodes, then switching control capability is improved, but parasitic capacitors induce extra current and supply noise
Solution Approach 1:
The patent acknowledges the parasitic capacitance of the transistor switch as an unavoidable harmful element, but converts its effect from harmful to beneficial by intentionally adding a capacitor of comparable or larger value in parallel with the LED or laser diode. This additional capacitor dominates the parasitic capacitance effect, transforming the situation where parasitic capacitance causes noise into a scenario where the intentionally added capacitor filters voltage fluctuations. The harmful parasitic capacitance is effectively masked and its negative effects are converted into a beneficial filtering action.
3Speed
If higher frequency pulses are used for turning on/off LEDs and laser diodes, then switching speed is improved, but parasitic capacitance effects increase causing more supply noise
Solution Approach 1:
The capacitor is connected in advance (before switching operations) and is already charged to the appropriate voltage level. When high-frequency switching occurs, the capacitor is immediately available to supply or absorb current, preemptively counteracting the effects of parasitic capacitance. This preliminary preparation of the capacitor allows the system to handle high-frequency switching without experiencing increased noise from parasitic capacitance, as the capacitor is already positioned to neutralize its effects.
Data Source
AI summary
A device includes a first circuit, a ground, a reference voltage source that provides a reference voltage, and a first transistor that includes a first drain, a first source, and a first gate. The first circuit is coupled between the first source and the ground. The device has a second transistor that includes a second source and a second gate. The second transistor is biased as a source follower with the second source of the second transistor being set at the reference voltage. The first gate of the first transistor is coupled to the second gate of the second transistor, the first source has equal voltage as the second source, and the first circuit is coupled between the first source having the reference voltage and the ground to draw a constant current from the first source and to bias the first transistor in the saturation region to reduce parasitic capacitance.