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Self-resetting circuit breaker needs few parts

28 Jan 2016  | Anthony Smith

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Many of us are familiar with the current-limiting circuit in figure 1, in which the load current, IL, is limited to a value of IL≈VBE/RS, where VBE is the base-to-emitter voltage and RS is the sense resistance. Under normal conditions, in which the base-to-emitter voltage is too small to bias Q1 on, P-channel MOSFET Q2's gate resistor, RG, biases Q2 fully on, and only the load resistance, RL, and the load voltage, VL, determine the load current. However, if the load current increases to a point at which the base-to-emitter voltage is approximately 0.7V, Q1 starts to conduct and reduces Q2's gate-to-source voltage, VGS, to a level that holds the load current roughly constant at a value you derive from ILMAXIMUM≈0.7V/RS.

Figure 1: A conventional two-transistor current limiter prevents excessive current from reaching the load.

This linear current limiter is effective for applications in which the maximum load current, the supply voltage, or both are relatively small. However, the power that the circuit's pass transistor, Q2, dissipates limits the circuit's applicability. For example, if the maximum load current is 200 mA and the supply voltage, VS, is 24V, a short circuit across the load would dissipate almost 5W into Q2. Q2 must handle this power with adequate margin, and additional heat-sinking may be necessary to keep its junction temperature at a safe level. Using larger values of maximum load current, supply voltage, or both exacerbates this problem. In many applications, the cost, size, and weight of the components necessary to handle the short-circuit power dissipation may be prohibitive.

Figure 2: Adding a few components turns the current-limiting circuit into a pulser that reduces heat in the pass transistor, Q2 (a). The circuit's waveforms show the relationship between the input voltage and the load voltage (b).

However, by adding a few inexpensive components, you can adapt the circuit to provide effective current limiting with none of the power-dissipation headaches. The resulting circuit functions as a self-resetting circuit breaker (figure 2a). Again, Q1 and RS provide a current-monitoring function in which the sense voltage VSENSE=IL×RS. In this circuit, however, Q2 is either fully on or fully off and never biases into its linear region. Because Q1's base current is normally small, the voltage drop across base resistor RB is also small, such that the base-to-emitter voltage is approximately equal to the sense voltage.

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