Many people ask how to design the driving circuit of laser diode. Do you want APC or ACC circuit? Let's take a look at it from the perspective of Xiaobian.

Why do laser diodes need special driving?

Laser diode is also a kind of diode in essence, but it is much more delicate than LED.

Its threshold current is very narrow, once it exceeds the rated current, it is almost instantaneous breakdown. Coupled with its strong temperature characteristics, the threshold current will increase when the temperature rises, but the light output will decrease, which means that it can not be driven by ordinary constant voltage power supply.

Common driving circuit structure

Driving circuits can be divided into two types: open-loop constant current type and closed-loop optical feedback type.

1. Open-loop constant current drive (simple type)

This circuit is the easiest to understand, that is, using an operational amplifier to control a power transistor to keep the output current stable.

Common design ideas are:

Detecting the output current with a sampling resistor;

The operational amplifier compares the voltage difference and adjusts the conductivity of the transistor;

Achieve constant current output.

This kind of circuit has simple structure and low cost, and is suitable for those red or infrared LD modules with low output requirements.

However, the disadvantage is also obvious temperature drift, laser power will change with temperature, and its stability is limited.

2. Optical feedback closed-loop drive (PD feedback type)

Laser diode usually has a monitoring photodiode (PD) inside.

PD will detect the laser output power in real time and feed it back to the driving circuit through the current signal.

The driver then automatically adjusts the output current according to this feedback, thus realizing constant power output.

The advantage of this design is that the laser output is stable and not afraid of temperature drift.

Especially in precision measurement or communication equipment, power stability is required, and almost all of them are PD closed-loop control.

Several key points to be paid attention to in designing laser driving circuit

Starting current overshoot

Many novices are most likely to overlook this point when designing.

Laser diodes are most afraid of the surge current at the moment of starting. At the moment of power-on, capacitor charging will lead to the peak current far exceeding the rated value, which will shorten the service life and directly burn out.

Power ripple

Laser current is particularly sensitive to ripple, even a noise of tens of millivolts may make the output light jitter.

Therefore, the power filter of the driving circuit must be clean, and it is best to add LC filter and voltage stabilizing chip.

temperature compensation

When the LD temperature increases, the threshold current increases and the output power decreases.

If the ambient temperature changes greatly, you can add a thermistor (NTC) to compensate, or directly connect it to the temperature control module (TEC control).

High-end laser modules will basically add a temperature control closed loop.

The laser diode driving circuit seems to be a "constant current circuit", but its essence is to find a balance among electricity, light and temperature.

When designing, we should not only consider current control, but also give consideration to optical feedback and prevent thermal runaway.

If only driven by constant current source, the structure is simple but easy to drift; Coupled with optical feedback, it can be truly stable and reliable.