STEP-UP SWITCH-MODE PSU
Except where they drive very small picture-tubes, TV chassis need a main supply line voltage around 110–170 V to satisfy the requirements of the line output stage. Where operation from a mobile power source such as 12 V and 24 V vehicle and marine batteries is required a step-up converter is used. Fig. 11.8 shows the circuit of a switch- mode d.c.–d.c. converter designed by Ferguson for this purpose. This circuit (shown in simplified form) uses a T9005 V transistor TR407 in a blocking oscillator configuration. The oscillator/output transformer T401 has a preset switching arrangement in its primary winding to cater for 12 V or 24 V inputs. The voltage induced across secondary winding 2–4 is rectified and smoothed to form the 115 V stabilised output. Secondary winding 3–5 supplies positive feedback to the transistor’s base, whose operating voltage sits on a pedestal of 0.5 V set up by diodes D408 and D409 to ensure correct start-up conditions via resistors R410 and R413. R427 and C413 protect TR407 from voltage surges.
The regulating action is based on the collector-current sampling transformer T402 and a two-transistor regenerative switch TR405/ TR406. TR406 base is biased from the junction of R419 and R422 (via
T402 secondary) such that when the voltage across R425 (corresponding to TR407 collector current) reaches a predetermined level TR406 conducts, turning on TR405 and rapidly latching the pair into hard conduction. A grounding path for the positive plate of C407 is thus established. This point previously carried about 6 V, so the negative plate of C407 tries to move 6 V negative likewise. The effect is to divert TR407 base current into C407, as a result of which TR407 turns off; its base is now driven into reverse bias by T401’s feedback winding.
A further circuit (not shown) compares a sample voltage derived from secondary winding 2–7 with a zener reference potential; an error volt- age is developed for application to the junction of R419 and R422 where it influences TR407 base bias to complete a voltage stabilisation loop. Further circuit sections cater for low supply voltage and overcurrent trip functions; the latter operates on any load current exceeding 650 mA. The low supply voltage protection device shuts down the converter if the battery voltage falls below 10.9 V (21.8 V in 24 V mode). Other specifications are: Input voltage 11 V–14.4 V and 23–28 V; input cur- rent 4–5 A (depending on beam current) in 12 V mode, 2–2.5 A in 24 V mode; reverse-input voltage protection by diode and fuse.