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

96 The Circuit Designer’s Companionstable time periods are needed it is normally better to divide down a high frequencyusing a digital divider chain than it is to use large value capacitors, or to expect stabilityfrom an electrolytic.3.3.7 Series capacitors and dc leakageCapacitor voltage ratings can hide pitfalls. As discussed earlier, it is always better tounder-run the working voltage of a capacitor for reasons of reliability. If a particularworking voltage is just too high for the wanted capacitor type, it may seem reasonableto simply put two or more capacitors in series and add up the overall voltage ratingaccordingly, always taking into account the reduced total capacitance.This is certainly possible but more is required than just multiple capacitors. Thecapacitor equivalent circuit (Figure 3.11) also includes the dc leakage resistance R dc ofeach capacitor, as shown in Figure 3.15. The dc working voltage impressed across theR dcR dcCCFigure 3.15 DC leakage resistanceterminals is divided between the capacitors not by the ratio of capacitance, but by theratio of the two values of R dc . These are undefined (except for a minimum) and can varygreatly even between two nominally identical components. Because R dc is usually high,of the order of tens to thousands of megohms, other leakage resistance factors −particularly pc board leakage (see section 2.4) − will also have an effect. The result isthat the actual voltage across each capacitor is unpredictable and could be greater thanthe rated voltage. The problem is at its worst with electrolytics whose leakage currentis large and varies with temperature and time.The situation is to a certain extent self correcting because an overvoltage will inmost cases result in increased leakage which will in turn reduce the overvoltage. Themajor consequence is that the actual capacitor working voltage will be unpredictableand therefore reliability of the combination will suffer. Once one component goesshort-circuit the other (or others) will be immediately over-stressed and rapid failure ofall will follow.Adding bleed resistorsThe solution is simple and consists of placing resistors across each capacitor to swampout the dc leakage resistance (Figure 3.16). The resistors are sized to be comfortablybelow the minimum specified leakage resistance so that variations in working voltageare kept below the rated maximum for each capacitor. Naturally this increases theleakage current of the combination but this is often an acceptable price, particularly ifthe application is for a high-voltage reservoir where some extra drain current isavailable.Indeed, it is often necessary for safety reasons to have a defined “bleed” resistanceacross a high-voltage reservoir capacitor. If the load resistance is very high, it can takeseconds or even minutes for the capacitor voltage to discharge to a safe level after poweris removed, with a consequent risk of shock to repair or test technicians. A bleed resistor