Pulsed Laser Diode Driver Circuit with Pre-Charge Path
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Solution Overview
Problem
Designing a circuit that can generate narrow and high current pulses to drive laser diodes for applications like LIDAR and time-of-flight cameras is challenging due to limitations in FET turn-on time and in-circuit inductances, which restrict the speed of current flow and di/dt requirements.
Innovation Solution
A pulsed laser diode driver circuit with a pre-charge path and a fire path, utilizing switches M1 and M2 to pre-charge inductive elements before directing current through the diode, thereby reducing the impact of inductances and FET turn-on time limitations, allowing for faster diode turn-on and higher di/dt rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional FET switches are used to drive laser diodes, then the circuit structure is simple, but the turn-on time is slow and di/dt rate is limited due to in-circuit inductances
Solution Approach 1:
The patent applies preliminary action by pre-charging the inductive elements through a dedicated pre-charge path before the actual firing event. The pre-charge switch closes to charge the inductance Lp, and when the firing switch closes, the pre-charged inductance is already ready to deliver current rapidly to the laser diode, achieving fast turn-on without complexity in the main firing path
Solution Approach 2:
The patent segments the current path into two separate functions: a pre-charge path with its own switch and inductance for charging, and a firing path with its own switch for actual laser diode firing. This segmentation allows each path to be optimized independently - the pre-charge path handles the slow charging process while the firing path handles the rapid discharge, resolving the contradiction between speed and complexity
2Measurement precision
If in-circuit inductances are present, then the circuit is easy to implement, but the di/dt rate is restricted and measurement precision deteriorates
Solution Approach 1:
The patent extracts the inductance element Lp from the conventional simple circuit structure and places it in a dedicated pre-charge path. This extracted inductance is specifically used for pre-charging and does not interfere with the firing path's ability to deliver high di/dt rates, thereby improving measurement precision while managing complexity through functional separation
Solution Approach 2:
The pre-charge inductance Lp acts as an intermediary element that facilitates rapid current delivery to the laser diode. By pre-charging this intermediary inductance, the system can achieve high di/dt rates during firing without requiring the main firing path to have low inductance, thus improving precision without proportionally increasing overall circuit complexity
3Duration of action of moving object
If FET turn-on time is reduced, then diode turn-on is faster, but the circuit requires more complex switching control and higher cost
Solution Approach 1:
The patent employs periodic action by using a two-stage switching sequence: first the pre-charge switch closes to charge the inductance, then it opens, and finally the firing switch closes to discharge. This periodic switching pattern achieves fast diode turn-on by preparing the circuit in advance, while the switching control logic remains relatively simple and manufacturable using standard control circuits
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 circuit achieves faster diode turn-on times and higher di/dt rates, exceeding 20 Amps per nanosecond, improving measurement resolution and signal-to-noise ratio in optical systems without increasing system complexity or cost.
Implementation Method 1
a pre-charge path comprising one or more inductive elements and a fire path comprising the diode. Switches in the driver circuit are controlled with predefined states during different intervals to pre-charge current in the one or more inductive elements prior to flowing current through the fire path to pulse the diode
Data Source
AI summary
Optical systems can emit train(s) of light pulses onto objects to derive a distance between the light source and the object. Achieving meter or centimeter resolution may require very short light pulses. It is not trivial to design a circuit that can generate narrow current pulses for driving a diode that emits the light pulses. An improved driver circuit has a pre-charge path comprising one or more inductive elements and a fire path comprising the diode. Switches in the driver circuit are controlled with predefined states during different intervals to pre-charge current in the one or more inductive elements prior to flowing current through the fire path to pulse the diode.