Clamping circuit dissipates minimal power

Carlisle Dolland, Allied Signal Aerospace, Torrance, CA -- EDN, 3/30/2000

The circuit in Figure 1 is a quasi-linear regulator. It functions as a source follower for input voltages greater than a preset level, determined by VR₃. For input voltages lower than the preset level, the pass element, Q₂, operates as a saturated switch. The circuit comprises an oscillator, a charge pump, and a linear regulator. The linear regulator, consisting of Q₁, R₁, VR₁, and R₂, drives a charge pump comprising C₂, D₂, D₃, and C₃. The charge pump generates a voltage equal to the output of the linear regulator for input voltages greater than the breakdown voltage of VR₁. For input voltages lower than VR₁'s breakdown voltage, the output voltage is V_OUT = V_IN - I_OUTx(R_ON of Q₂). For input voltages that exceed VR₃'s breakdown voltage by approximately 3V, R₆ and VR₃ dissipate the energy the charge pump supplies. In this mode, the circuit functions as a source follower, and the output voltage is approximately 3V lower than VR₃'s breakdown voltage. The circuit dissipates minimal power. During transients, the load current determines the dissipation in Q₂. (DI #2499)

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