Audio amplifier power supply design - Part 2: External supplies, inrush current & RF emissions

The inrush resistor has to sustain a very large short-term overload, so a chunky wire-wound type is appropriate, and it is vital to ensure that it can cope with this overload many times over the life of the amplifier.

However, resistor manufacturers are noticeably reluctant to specify how their products will cope with such conditions. It is therefore a good plan to use inrush suppression in its intended final form from the very start of the development process; by the time all other design issues have been addressed you will almost certainly have put the inrush suppression through enough operating cycles to have confidence in its durability. (Using inrush suppression from the start may well be essential anyway to prevent the workbench circuit-breakers from tripping.)

The inrush current is a complex phenomenon and the resistance value and power rating of the resistor is usually determined by experience rather than protracted mathematical analysis. Here are some typical values that I have used with success:

• 10 Ω, 10 W for an 800 VA toroid;

• 10 Ω, 20 W for a 1300 VA toroid.

Wire-wound resistors come in a limited number of types for sizes above 10 W, and it is often more convenient to use two 22Ω 10 W resistors in parallel when a 20 W capability is required. If the resistor is correctly sized, after a single inrush event it should be warm rather than hot. Repeated and rapid cycling of the power, as may occur in testing, can cause it to get very hot and could eventually lead to failure. Fortunately this is not likely to occur in service.

The circuitry must be arranged so that if the power is turned off then immediately turned on again, inrush suppression still operates for the full period. This situation is called a 'hot restart'.

Many amplifiers are not simply switched on and off, but have an on/standby system where the mains switch initially applies power only to a small transformer that energizes a small amount of control circuitry. A low-current switch, which can be more cosmetically attractive than something hefty enough to control the full mains power, activates the control circuitry and causes it to close a relay that energizes the main supply.

When this function is combined with inrush protection there are usually two identical relays in the primary circuit as shown in Figure 9.5a; at switch-on RLA closes and applies power to the transformer through the inrush resistor R1. After a second or so RLB closes and shorts out the resistor; RLA is now doing nothing so it is de-energized after a very short delay to make sure that RLB is fully closed. The alternative arrangement in Figure 9.5b should be avoided as now it is necessary to keep both relays energized all the time, which is a pointless waste of perfectly good electricity.