Circuit-Level Transient Suppression:Protecting Low-Voltage Supplies.

Introduction

The transient-suppression provisions built into professional and industrial equipment have improved dramatically over the past few years. This trend is driven by both the increased availability of high-quality transient-suppression devices and the increased need for clean power. Computer-based equipment is known for its intolerance of transient disturbances.

The transient-suppression applications presented in this section are intended only as examples of ways that protection can be built into equipment to increase reliability. End users should not attempt to modify existing equipment to provide increased transient-suppression capabilities. Such work is the domain of the equipment manufacturer. Transient suppression must be engineered into products during design and construction, not added on later in the field.

Protecting Low-Voltage Supplies

Figure 17.1 shows the recommended transient protection for a typical low-voltage series-regulated power supply. MOV1 to MOV3 clip transients on the incoming ac line. C1 aids in shunting turn-on/turn-off and fault-related disturbances on the secondary of T1. Resistor R1 protects diode bridge D1 by limiting the amount of current through D1 during turn-on, when capacitor C2 (the main filter) is fully discharged. MOV4 protects series regulator Q1 and the load from damage resulting from transients generated by fault conditions and load switching. The varistor is chosen so that it will conduct current when the voltage across L1 is greater than would be encountered during normal operation. Diode D2 protects Q1 from back-emf kicks from L1.

Three-terminal integrated circuit voltage regulator U1 is protected from excessive back-current because of a short circuit on its input side by diode D3. Capacitors C3 to C7 provide filtering and protect against RF pickup on the supply lines. Diode D4 protects U1 from back-emf kicks from an inductive load, and zener diode D5 protects the load from excessive voltage in case U1 fails (possibly impressing the full input voltage onto the load). D5 also protects U1 from overvoltages caused by transients generated through inductive load switching or fault conditions. D6 performs a similar function for the input side of U1.

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