Current shaping

A common method to reduce the rise and the fall time is current shaping (Lee, 1975; Zucker, 1978; Saul et al., 1985). The diagram of a current-shaping circuit is shown in Fig. 24.5 (a). The current-shaping circuit is essentially a capacitor and a resistor in series with the LED. The capacitor creates a current transient when the LED is switched on or off, as shown in Fig. 24.5 (b). During the switch-on transient, the excess current flowing through the capacitor helps in reaching the steady-state carrier concentration in the active region within a time shorter than the spontaneous lifetime. During the switch-off period (V = 0), the capacitor biases the diode in the reverse direction, thus aiding the current sweep-out of the active region. Parameters entering the current transient include the power supply internal resistance, the resistor and capacitor of the current-shaping circuit, and the diode series resistance.

 (a)

 LTL

 Source Source resistance Current-shaping circuit LED

 (b)

 with current-shaping circuit without current- shaping circuit

 m*s и
 /,v Fig. 24.5. (a) Illustration of an RC “current-shaping circuit” used to decrease the rise time of LEDs, (b) Diode current pulse versus time in the limit of small diode capacitance (solid line) and effect of current-shaping circuit on diode current (dashed line).

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A reasonable design criterion for the current-shaping circuit is that the RC time constant of the circuit introduced by the RC current-shaping circuit should be equal to the spontaneous recombination lifetime and that the initial current at the beginning of the voltage pulse should be twice the magnitude of the steady-state current. In this case, the value of the resistor R of the current-shaping circuit is chosen to equal the differential resistance of the diode. The capacitance of the circuit can be chosen so that the RC time constant of the entire circuit coincides with the LED rise time (Schubert et al. 1996). As a result of the current-shaping circuit, the 3 dB bandwidth increases.

Note that the diode series resistance is strongly voltage-dependent, and therefore linear circuit theory can only provide estimates rather than accurate values. Therefore, experimental or numerical methods are required to optimize the RC current-shaping circuit.

The current-shaping circuit requires an increased operating voltage and this reduces the overall efficiency of the drive circuit. However, the power efficiency is usually of little relevance

due to the low overall power consumption of communication LEDs.

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