Input-protection scheme tops other approaches

Kannan Natarajan, Mediatronix Private Limited, Kerala, India -- 1/6/2000

You typically accomplish overvoltage or surge protection at circuit inputs by connecting diodes to the supply rails, connecting zener diodes to ground, or connecting transzorbs to ground. Unfortunately, for high-energy surges at the inputs, connecting diodes to the supply rails results in surges in supply lines and affects other components because of the inductance of supply rails, regulator shutdown, and so on. Zener diodes have limited surge capability, and transzorbs have large capacitance and are therefore suited only for low-bandwidth applications.

The circuit in Figure 1 has many advantages over these approaches: wide bandwidth and low capacitance; high surge-energy handling because the diodes can carry 50A peak; 1A continuous current; and fast response. Also, the circuit doesn't affect the supply rails and is suitable for protecting multiple I/O lines because the lines can share the bias voltage. You can further improve the response time by using faster diodes; a ground plane; low-inductance, short connections; and close, high-frequency decoupling.

The circuit reverse-biases D₁ and D₄ to bias voltages of ±1.2V, respectively. R₁ and R₂ bias two pairs of diodes, D₂/D₃ and D₅/D₆, respectively, to generate the ±1.2V. R₁ and R₂ prevent input surges from reaching the supply rails. The surge shunt path consists of D₁, D₂, and D₃ to ground or D₄, D₅, and D₆ to ground, depending on the surge's polarity. Because of the ±12V bias-voltage settings, the circuit works with maximum input signals of ±1V. Above this level, D₁ and D₄ start to leak and distort the signal. The circuit was tested using a 100-kWF/50V test capacitor charged to 30V and then discharged to the input. A DSO captured the results (Figure 2). In Figure 2a, with R_S=100kW, the peak is approximately 3.5V, and settling to around 2V occurs within 15 nsec. Figure 2b shows the same response as Figure 2a but with a horizontal scale of 1 msec/div. Figure 2c is also the response under the same conditions but shows the long-term response and the coupling-capacitor recovery. If you let R_S=0, the peak rises to 10V and settles within 500 nsec. Thus, some small resistance, such as 100W, is necessary for R. (DI #2463)

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