The Circuit Designer's Companion - diagramas.diagram...

Power supplies 247Z2Z1Z3Figure 7.18 Transient suppression in a linear supplyenergy rating to withstand the maximum expected surge withoutdestruction, and it will have a fairly high ratio of clamped voltage to normalrunning voltage. In effect, voltage surges up to about twice the peakoperating voltage will be let through.• Z2: this is a more effective position as it still protects the vulnerablerectifiers, but is itself protected by the additional source impedance of thetransformer. It can therefore be a smaller component but still have a goodratio of clamped to peak operating voltage. It has no effect on spikes whichmay have been converted from differential to common mode by theinterwinding capacitance of the transformer.• Z3: this protects the regulator and subsequent circuitry but not the rectifiers.It is something of a “belt-and-braces” position, but it does suppress inputcommon mode spikes that the previous positions would have let through. Itshould be sized so that its peak clamping voltage is just less than theabsolute maximum input voltage of the regulator. Smaller surges then relyon the transient response of the regulator to contain them.7.3.4 Overvoltage protectionIf the circuit that your power supply is driving is very expensive and susceptible toovervoltages − for instance it may include a £100 microprocessor which must not besubject to more than 7V − then it is worth including extra circuitry at the power supplyoutput for overvoltage protection. The first time that it operates, it will have saved theextra expense of designing it in.This might be as simple as a 6.2 or 6.8V zener diode across the output of a 5Vsupply. See section 4.1.8 for a discussion of how to size such a zener. This does notoffer foolproof protection, because if the overvoltage is sustained and derives from alow source impedance − perhaps the series-pass element has failed − then the zener islikely to fail itself, and may fail open-circuit, in which case it has been wasted.Something more drastic is called for, and the conventional solution is a crowbar.This gets its name from the time-honoured method of ensuring that no voltage ispresent between two live terminals, by the simple expedient of putting a crowbar −which is assumed to be able to carry any likely short-circuit current − across them. Inits more sophisticated version in electronic power supplies, the crowbar takes the formof a triggered thyristor. The thyristor is permanently in place across the output, or insome designs across the reservoir, but it is only triggered when a supervisory circuitdetects the presence of an overvoltage. It then stays triggered, holding the outputvoltage to V H , until the current through it is interrupted by external circumstances suchas a power supply reset. Although this current may be high, the voltage across it is not,so its dissipation is fairly low. Obviously the power supply itself must be protected