James Zannis
EDN
Expensive semiconductor laser diodes have no tolerance for fast voltage or current transients. To minimize the risk of damage, a standard JFET-clamp circuit shorts the laser when there is no supply voltage, thus protecting it against such transients (Figure 1). When the negative supply rail comes up, the JFET turns off. This circuit is effective for low-power laser diodes but may not be so for diodes with power dissipation greater than 150 mA. The maximum cutoff current of the JFET sets this limit. If it becomes necessary in an emergency to clamp the laser during normal operation, the selected JFET might not adequately shunt the current. Higher-current JFETs are available but are more expensive and difficult to procure.
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| Figure 1. | This circuit protects low-power laser diodes but is not suitable for higher-power laser diodes. |
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The circuit in Figure 2 avoids these deficiencies. It is similar to the standard JFET circuit but has a supplementary bipolar transistor that shunts most negative-going currents when the JFET is on. R2 prevents the gate of Q1 from floating, and R3 ensures rapid turn-off of Q2. The 1N914 diode bypasses any positive-going transients. The RC circuit ensures an adequately slow response; therefore, the transition between on and off is smooth.
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| Figure 2. | Adding a bipolar transistor to the circuit in Figure 1 allows the circuit to protect higher-power laser diodes. |
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