Short pulse laser driver KiCad project

Introduction

This project is for a laser diode driver that produce short laser pulses from a laser diode. There are many applications for pulsed diodes lasers. Laser range finding is one obvious application. The relevant pulse widths are from 1n to 100ns. With these short pulses and low (<5%) duty cycle the heating of the laser diode is not a problem, and they can be driven to high peak current and resulting peak power. The figure show and example of a laser diode designed for high peak power at low duty cycle.

It has a plastic package which can not dissipate a high power. The total dissipated power is P=U*I. The typical operation voltage is 4.3 V, so at IF=30A that is a total power of 129W. Of this 25W is emitted as optical power and the rest 104W is dissipated as heat. Such peak power is only possible in a small plastic package at very low duty cycle. The specification is max tp = 100 ns and f = 1 kHz, 100ns/1ms is 0.001% duty cycle. In that operation the average heat power is only 1mW.
In applications that requiter higher pulse repetition frequency the peak power has to be reduces. There are also laser packages with higher power dissipation capability. For example in fig. 2 is a laser diode designed for CW operation at 50mW.

To drive diode lasers in pulsed mode we need a special driver. The simplified schematic shows the principle of a circuit which can drive current pulses through such laser. The laser diode (LD) is forward biased with the laser cathode connected to a capacitor, so there is no DC current flowing. The other terminal of the capacitor is connected to the drain of a MOSFET (Q). This net is charged through the resistor R when the MOSFET is OFF. If a step signal is applied to the gate of Q then it turns ON and the capacitor will pass the derivative and a pulse current will flow through the laser diode. The value of R is picked so it is much larger than the on resistance of the MOSFET. When the gate is pulled low again the resistor will recharge the drain side of the capacitor, ready for the next pulse.

SPICE simulation

Fig. 4 below shows a KiCad schematic for running a ngspice simulation of the pulse drive circuit. There are some more components compared to the simplified schematic. V1 is a simulation DC voltage source and V2 is a simulation pulse source. These represent the real world power supply and signal sources, the actual PCB design will have connectors for those inputs.

The laser diode is modeled by the subcircuit shown below in fig. 5. The circuit elements represent the unwanted parasitic inductance, capacitance, and resistance which exist in the laser diode module. The inductance is mainly from electrical connections and vary depending on the technology using, wire bonding, ribbon bonding or strip lines have reducing inductance order. The diode model can be adjusted to give the laser diode forward voltage drop. The junction capacitance dominates the capacitance. We expect a larger capacitance for a large high power laser diode than for a small low power laser diode.

A transient simulation result is shown in fig. 6 . The signal source is a SPICE pulse source with y1=0 y2=12 td=20n tr=2n tf=2n tw=30n per=1u. KiCad uses the ngspice simulator.
The input pulse is shown in red (V(IN)). V(Capacitor) is the voltage on the MOSFET side of the capacitor C1 in fig. 4. As expected the voltage drops abruptly when the Q2 is turned on by the input signal on its gate. The transient is coupled by the capacitor and draw a pulse current through the lase diode LD1 and R7. The voltage on the capacitor recharges through D2 and R6 and reach steady state voltage in 1us. This sets the maximum pulse repetition frequency with these values of D2 and R6 to around 1MHz. At higher frequency the peak current will drop since the steady state voltage is not reached.

D1 and R3 gives protection against reverse polarization of the laser diode (LD1). Because the LD module has serial inductance the cathode side can swing positive when the MOSFET is switched OFF. The diode D1 allow this voltage to return to the supply.

The current pulse is a nice Gaussian like shape that returns to zero without ringing. This is the desired behavior for a pulse driver. The shape and peak current can be selected by changing the values of R1 and C1. The peak current (Imax) depends on the supply voltage V1. Below is a plot of the peak current for different values of V1. The 3dB pulse width does not depend on the voltage.

The simulation results are encouraging. It is a pulse driver circuit that can give high peak current pulses at moderate duty cycle. It is a pulse range that can not easily be reached with an analog driver like the Modulation laser diode driver.
We will use this pulse driver in the PCB design.

PCB design

The MOSFET used in the simulation is selected for the application. It is a standard power mosfet with low gate charge. In the simulation the gate was driven directly with the signal source. For the actual PCB it will help to use a gate driver to relax the requirements on the signal source. A gate driver IC like the onsemi NCP81074BDR2G has logic input and a high peak current push-pull output driver. Using this driver makes the rise time independent on the control signal.

he full schematic for the PCB is shown in fig.10. It is based on the simulation schematic in fig.4. The gate driver U1 is added, and the sources are replaced with connectors. The connector J2 has separate connection for the gate driver supply +12V and VDC for the laser diode. This is to allow different voltages for the gate driver and the laser diode, so the peak current can be adjusted by means of the VDC. A resistor R12 allow a low biasing current through the laser diode. A laser diode has a threshold current below which there is no lasing output. If needed this resistor can allow biasing of the laser diode closer to the threshold. The default value of 100K gives no effective biasing. 

The PCB layout is shown in fig. 11. It is a 36mm x 60mm two layer board. The critical connections are the gate driver output to the MOSFET gate through R2 and R5. This connection is kept short and stay on the top layer. The fast high current path from decoupling capacitors C4 and C6 is also kept short and low impedance by using wide tracks.

KiCad project files

The KiCad project and design files are share as open source hardware. Feel free to download explore and fabricate or modify to tailor for your applications.