High Current Power Supply Systems
In order for the power supply system to be able to provide high output currents for short periods of time, the reservoir capacitor, C3 in Figure 5.1(b), must be large and have a low ESR value. Ideally, the rectifier diodes used in the power supplies should have a low conducting resistance, the mains transformer should have low resistance windings and low leakage inductance, and all the associated wiring, including any PCB tracks, should have the lowest practicable path resistance. The output current drawn from the transformer secondary winding, to replace the charge lost from the reservoir capacitor during the previous half cycle of discharge, occurs in brief, high current bursts in the intervals between the points on the input voltage waveform labeled 1 and 2, 3 and 4, 5 and 6, and so on, shown in Figure 5.1(c). This leads to an output ripple pattern of the kind shown in Figure 5.1(d). Unfortunately, all of the measures that the designer can adopt to increase the peak DC output current capability of the power supply unit will reduce the interval of time during which the reservoir capacitor is able to recharge. This will increase the peak rectifier/reservoir capacitor recharge current and will shorten the duration of these high current pulses. This increases the transformer core losses, both the transformer winding and the lamination noise, and also the stray magnetic field radiated from the transformer windings. All of these factors increase the mains hum background, both electrical and acoustic, of the power supply unless steps are taken—in respect of the physical layout, and the placing of interconnections—to minimize it. The main action that can be taken is to provide a very large mains transformer, apparently excessively generously rated in relation to the output power it has to supply, in order that it can cope with the very high peak secondary current demand without mechanical hum or excessive electromagnetic radiation. Needless to say, the mains transformer should be mounted as far away as possible from regions of low signal level circuitry; its orientation should be chosen so that its stray magnetic field will be at right angles to the plane of the amplifier PCB.