FORMENTI F19 CHASSIS
Service Manual

controlled in width by regulating voltage of pin 1. When switching on this corresponds to a short-circuit event, i.e. V 1 = 0. Hence the IC starts up with "short-circuit impulses" to assume a width depending on the regulating voltage feedback (the IC operates in the overload range). The maximum pulse width is reached at time point t 2 (V 2 = V 2 max ). The IC operates at the overload point. Thereafter the peak values ot V 2 decrease rapidly, as the IC is operating within the regulation range. The regulating loop has built up. If voltage V 6 falls below the switch-off threshold V 6 min before the reversal point is reached, the starting attempt is aborted (pin 5 is switched to low). As the IC remains switched on, V 6 further decreases to V 6 . The IC switches off; V 6 can rise again (time point 14) and a new start-up attempt begins at time point t 1 . If the rectified alternating line voltage (primary voltage) collapses during load, V 3 can fall below V 3A , as is happening at time point t 3 (switch-on attempt when voltage is too low). The primary voltage monitor then clamps V 3 to V 3S until the IC switches off (V 6 < V 6A ). Then a new start-up attempt begins at time point t

Regulation, Overload and No-Load Behaviour When the IC has started up, it is operating in the regulation range. The potential at pin 1 typically is 400 mV. If the output is loaded, the regulation amplifier allows broader impulses (V 5 = H). The peak voltage value at pin 2 increases up to V 2S max . If the secondary load is further increased, the overload amplifier begins to regulate the pulse width downward. This point is referred to as the overload point of the power supply. As the IC supply voltage V 6 is directly proportional to the secondary voltage, it goes down in accordance with the overload regulation behaviour. If V 6 falls below the value V 6 min , the IC goes into burst operation. As the time constant of the half-wave charge-up is relatively large, the short-circuit power remains small. The overload amplifier cuts back to the pulse width t pk . This pulse width must remain possible, in order to permit the IC to start-up without problems from the virtual short circuit, which every switching on with V 1 = 0 represents. If the secondary side is unloaded, the loading impulses (V 5 = H) become shorter. The frequency increases up to the resonance frequency of the system. If the load is further reduced, the secondary voltages and V 6 increase. When V 6 = V 6 max , the logic is blocked. The IC converts to burst operation. This renders the circuit absolutely safe under no-load conditions.

TDA4605 SMPS CONTRO I.C. e.g.tda4605R.doc

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