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Circuit breaker provides overcurrent and precise overvoltage protection

Requiring only a handful of inexpensive components, the circuit breaker in Figure 1 responds to both overcurrent- and overvoltagefault conditions. At the heart of the circuit, D₂—an adjustable, precision, shunt-voltage regulator—provides a voltage reference, comparator, and open-collector output, all integratedinto a three-pin package.

Figure 2 shows a simplified view of the ZR431, D₁. The voltage appearing at the reference input is compared with the internal voltage reference, VREF, nominally 2.5V. In the off state, when the reference voltage is 0V, the output transistor is off, and the cathode current is less than 0.1 A. As the reference voltage approaches VREF, the cathode current increases slightly; when the reference voltage exceeds the 2.5V threshold, the device fully switches on, and the cathode voltage falls to approximately 2V. In this condition, the impedance between the cathode and the supply voltage determines the cathode current; the latter can range from50 A to 100 mA.

Under normal operating conditions, D₂’s output transistor is off, and the gate of P-channel MOSFET Q₄ goes through R₉. Consequently, the MOSFET is fully enhanced, allowing the load current, ILOAD, to flow from the supply voltage, VS, through R₆ into the load. Q2 and current-sense resistor R₆ monitor the magnitude of ILOAD, where Q₂’s base-emitter voltage, VBE, is ILOADR₆. Fornormal values of ILOAD, VBE is less than the 0.6V necessary tobias Q₂ on, such that the transistor hasno effect on the voltage at the junctionof R3 and R4. Because the inputcurrent at D2’s reference input is lessthan 1 A, the voltage drop across R₅ isnegligible, and the reference voltage iseffectively equal to the voltage on R4.

In the event of an overload when ILOAD exceeds its maximum permissible value, the increase in voltage across R6 results in sufficient base-emitter voltage to turn on Q₂. The voltage on R₄ and, hence, the reference voltage now pull up toward VS, causing D₂’s cathode voltage to fall to approximately 2V. D₂’s output transistor now sinks current through R7 and R8, thus biasing Q₃ on. Q₄’s gate voltage now effectively clamps to the supply voltage through Q₃, and the MOSFET turns off. At thesame instant, Q3 sources current into R₄ through D₁, thereby pulling the voltageon R₄ to a diode drop below the supplyvoltage. Consequently, no load currentflows through R₆ because Q₂, whosebase-emitter voltage is now 0V, hasturned off. As a result, no load currentflows through R₆, D₂’s output transistorlatches on, and the circuit remains inits tripped state, in which the load currentis 0A. When choosing a value forR₆, ensure that Q₂’s base-emitter voltageis less than approximately 0.5V atthe maximum permissible value of theload current.

As well as responding to overcurrent conditions, the circuit breaker also reacts to an abnormally large value of the supply voltage. When the load current lies within its normal range and Q₂ is off, the magnitude of the supply voltage and the values of R3 and R4, which form a potential divider across the supply rails, determine the voltage at the reference input. In the event of an overvoltage at the supply voltage, the voltage on R4 exceeds the 2.5V reference level, and D₂’s output transistor turns on. Once again, Q₃ turnson, MOSFET Q₄ switches off, and the load becomes effectively isolated fromthe dangerous transient.

The circuit now remains in its tripped state until reset. Under these conditions, Q₃ clamps Q₄’s gate-source voltage to roughly 0V, thereby protecting the MOSFET itself from excessive gate-source voltages. Ignoring the negligibly small voltage across R5, you can see that the reference voltage is VSR4/(R3R4) in volts. Because D₂’s output turns on when the reference voltage exceeds 2.5V, you can rearrange the equation as R₃[(VST/2.5)1]R₄ in ohms, where VST is the required supply- voltage trip level. For example, if R₄ has a value of 10 k
, a trip voltage of 18V would require R₃ to have a value of 62 k
. When choosing values for R3 and R4 to set the desired trip voltage, ensure that they are large enough so that the potential divider will not excessively load the supply. Similarly, avoid values that could result in errorsdue to the reference-input current.

When you first apply power to the circuit, you’ll find that capacitive, bulb-filament, motor, and similar loads having large inrush currents can trip the circuit breaker, even though their normal, steady-state operating current is below the trip level that R₆ sets. One way to eliminate this problem is to add capacitor C₂, which slows the rate of change of the voltage at the reference input. However, although simple, this approach has a serious disadvantage in that it slows the circuit’s response time to agenuine overcurrent-fault condition.

Components C₁, R₁, R₂, and Q₁ provide an alternative solution. On power-up, C1 initially discharges, causing Q₁ to turn on, thereby clamping the reference input to 0V and preventing the inrush current from tripping the circuit. C₁ then charges through R₁ and R₂ until Q₁ eventually turns off, releasing the clamp at the reference input and allowing the circuit to respond rapidly to overcurrent transients. With the values of C₁, R₁, and R₂, the circuit allows approximately400 msec for the inrush current to subside. Selecting other values allowsthe circuit to accommodate anyduration of inrush current you applyto a load. Once you trip the circuitbreaker, you can reset it either by cyclingthe power or by pressing S1, thereset switch, which connects acrossC₁. If your application requires no inrushprotection, simply omit C₁, R₁, R₂,and Q₁ and connect S₁ between thereference input and 0V.

When choosing components, make sure that all parts are properly rated for the voltage and current levels they will encounter. The bipolar transistors have no special requirements, although these transistors, especially Q₂ and Q₃, should have high current gain, Q₄ should have low on-resistance, and Q₄’s maximum drain-to-source and gate-tosource voltages must be commensurate with the maximum value of supply voltage. You can use almost any smallsignal diode for D₁. As a precaution, it may be necessary to fit zener diodes D₃ and D₄ to protect D₂ if extremely largetransient voltages are likely.

Although this circuit uses the 431 device—which is widely available from different manufacturers—for D₂, not all of these parts behave in exactly the same way. For example, tests on a Texas Instruments (www.ti.com) TL431CLP and a Zetex (www.zetex. com) ZR431CL reveal that the cathode current is 0A for both devices when the reference voltage is 0V.However, gradually increasing the reference voltage from 2.2 to 2.45Vproduces a change in cathode current,which increases from 220 to 380 Afor the TL431CLP and 23 to 28 Afor the ZR431CL—roughly a factor of10 difference between the two devices.You must take this difference in themagnitude of the cathode current intoaccount when selecting values for R7and R8.

The type of device you use for D2 and the values you select for R7 and R8 can also have an effect on response time. A test circuit with a TL431CLP, in which R7 is 1 kΩ and R₈ is 4.7 kΩ, responds within 550 nsec to an overcurrent transient. Replacing the TL431CLP with a ZR431CL results in a response time of approximately 1 µsec. Increasing R₇ and R₈ by an order of magnitude to 10 and 47 kΩ, respectively, produces a response time of 2.8 µsec. Note that the relatively large cathode current of the TL431CLP requires correspondinglysmall values of R₇ and R₈.

To set the overvoltage-trip level at 18V, R₃ and R₄ must have values of 62 and 10 k
, respectively. The test circuit then produces the following results: Using a TL431CLP for D2, the circuit trips at 17.94V, and, using a ZR431CL for D ₂, the trip level is 18.01V. Depending on Q₂’s base-emitter voltage, the overcurrent-detection mechanism is less precise than the overvoltage function. However, the overcurrent-detection accuracy greatly improves by replacing R₆ and Q₂ with a high-side current-sense amplifier that generates a ground-referred current proportional to load current. These devices are available from Linear Technology (www.linear.com), Maxim (www.maxim-ic.com), TexasInstruments, Zetex, and others.

The circuit breaker should prove useful in applications such as automotive systems that require overcurrent detection to protect against faulty loads and that also need overvoltage protection to shield sensitive circuitry from high-energy-load-dump transients. Other than the small current flowing in R₃ and R₄ and the current in D₂’s cathode, the circuit draws no current from the supply in its normal,untripped state.