HYBRID SWITCHER IC
For our final illustration of switch-mode power regulator operation, Fig. 11.9 shows one example of many types of fully encapsulated power switching/regulator ICs, typically the size of a matchbox or larger, and bolted to a large heat sink.
Mains voltage is full-wave rectified in bridge D801–4 and developed across reservoir C805 for application to chopper transformer T802 primary. Meanwhile a starting current for the IC is drawn through R802 and smoothed by C812, passing into the chip on V-in pin 9. Chopper drive pulses pass out of the IC on pin 8 to re-enter at pin 3, the base of the power switching transistor; its emitter is grounded inside the IC, and its collector (pin 1) pulses current through wining 1–4 of the chopper transistor. Now T802 pin 6 perks up and delivers energy via D806 into C812 to provide a proper operating voltage for the device. Inside the chip is an oscillator whose duty-cycle is varied to match the load conditions by a sensing circuit working from IC pin 7: it samples the voltage developed in T802 winding 5–7, and narrows the drive pulses in proportion to achieve regulation. Pin 6 of the IC ensures that no energy is fed into chopper transformer T802 until its magnetic field has decayed to zero on each cycle (quasi- resonant operation) in similar fashion to the operation of the TDA4600 family described on page 227. This chip also shares with
CAUTION LIVE MAINS
the TDA4600 series the proportional drive system in which chopper base current is regulated to suit the load conditions for optimum ‘lifestyle’ of the internal power-switching transistor.
Overcurrent protection is catered for at IC pin 5, which goes negative of the ‘ground’ point at pin 2 to a point determined by the cur- rent in sampling resistor R804. When the level reaches –1 V the protection is invoked. In the event of overvoltage the charge on C812 becomes excessive (>11 V) and this is detected at IC pin 9 to trigger an internal latch circuit: its effect is to ‘pump’ the system, with fluctuations of voltage from about 5 V to 8 V at chip pin 9. The same latch circuit is brought into operation if and when the substrate of the IC becomes overheated to the point where the internal TSD (Thermal Shut Down) device comes into operation. Once triggered, the latch circuit stays on until the voltage at IC pin 9 falls below 3.3 V, in practice after the mains supply has been switched off.
DEGAUSSING
At switch-on the magnetic shield and shadowmask of the colour picture-tube must be demagnetised to ensure correct beam landing and good display-colour purity. To this end a large coil is wound around the magnetic shield and given a decaying burst of 50 Hz a.c. mains current when the TV’s power switch is closed. Its feed arrange- ment is shown at the left side of Fig. 11.9. When both sections of posistor R801 are cold their resistance is low and a high current flows through section B and the coils via plug 802 to create a strong alternating magnetic field. It quickly decays to virtually zero as the resistance of posistor section B rises very high with the temperature of the device, subsequently sustained by the warmth of section A (in close thermal contact with B) which passes a continual mains cur- rent. These double posistors are notoriously unreliable.