High-frequency chokes
The switching action of the SMPS, which may involve large currents, can introduce interference known as mains pollution in the form of sharp transients, spikes or glitches superimposed upon the mains waveform. This is overcome by the introduction of high-frequency chokes or decoupling capacitors at the input terminals; they prevent high-frequency pulses from going back into the mains supply.
Start-up and soft start
The switching element and the control unit require a d.c. supply before they can begin to function. This voltage may be obtained from the nominal 12 V regulated supply for the control unit or other regulated or unregulated d.c. rails. It is normal that, once the control chip is brought into operation, the SMPS itself is used to provide the required regulated volt- age to the chip. A slow or soft start is desirable for the switching element to prevent it from overworking at switch-on when the output voltage of the SMPS is zero. Soft start also ensures that the auto-degaussing is completed before the tube starts to be scanned.
Standby supply
The standby supply to a TV receiver is a separate power supply. It is separated from the main power supply of the receiver in order to reduce power consumption when the set is in the standby mode as required by legislation. The standby power supply is fed with a.c. from the input mains filter. It is therefore always operational when the mains input is present regardless of the position of the power switch.
The circuit diagram of a standby power unit is shown in Figure 15.13. Rectified mains is applied to the SMPS control TNY256 chip. This IC contains a control circuitry as well as a MOSFET transistor. The power supply provides 9, 5, 3.3 and 25 V (HOT). Feedback is provided by sensing the secondary voltage using a reference zener and an opto-coulper for mains
isolation. When the output voltage rises above the reference voltage of the diode, its current increases and with it that of the opto-coulper. When this current goes above a threshold of 50 J.A, the MOSFET will switch off. When the current falls below 40 J.A, the MOSFET will switch on.