BLOCKING-OSCILLATOR PSU
One form of self-contained SMPSU is illustrated in Fig. 11.3. The primary power source is mains energy, converted to 290 V d.c. in a
full-wave bridge rectifier. This unstabilised voltage enters the regulator circuit at XQ53 and XQ51, the latter point (+ve) being connected to receiver metal chassis (ground). RN04 act as surge-limiter and CN03 as reservoir to render a negative supply line at choke LN01. The switch is TN03, a high-power transistor, and the secondary windings of chopper autotransformer LN03 feed diode/capacitor sets to render ±250 V for RGB output amplifiers; +160 V for the line output stage; +34 V for field timebase; and +22 V for the audio power amplifier. Other operating potentials are derived indirectly from these, either from the l.o.p.t. or by separate series regulator circuits.
The emitter of TN03 is connected directly to the –290 V rail via sampling resistor RN09 (1 Ω) and 1 A fuse FN01. LN03 primary (pins 2–12) completes the circuit between TN03 collector and ground, so that when the transistor turns on TN03 primary winding is con- nected directly across the 290 V rail. A secondary winding between pins 5 and 7 provides positive feedback to TN03 base, so that once started by a positive ‘kick’ pulse from the mains rectifier via RN11 and CN09, self-oscillation takes place in TN03, LN03 and CN12. Each time TN03 cuts off, the collapsing magnetic field in LN03 induces voltages at the output taps for rectification and storage in reservoirs CN13–CN16. It remains to regulate these output voltages by varying the duty-cycle of TN03.
Regulation
For the purpose of the regulation circuit, the ‘ground’ line may be taken as that connected to TN03 emitter. With respect to this ground line transformer winding 1–3, DN03 and CN04 set up a sample volt- age of about +22 V. This is used to control the regulator via sampling transistor TN01. DN01 and RN07 set up a steady voltage on its emitter for comparison with the ‘potted down’ (RN01/2, PN01) sample applied to its base. TN01 collector is connected to the mid-point of potential divider RN05/RN06, the latter fed by a –5.5 V potential from DN04 and CN05. TN01 collector, then, takes up a potential negative of ‘ground’ depending on the sample voltage across CN04. This is applied to the gate of thyristor TN02.
Sampling resistor RN09 is in series with the switch, and its upper end develops a negative sawtooth voltage corresponding to LN03 cur- rent. TN02 cathode is connected here, and as the negative ramp reaches and passes the standing gate voltage the thyristor conducts to ground TN03 base via CN08 – TN03 switches off as a result. When TN03 goes off, CN08 is charged through DN02. The regulating action, then, is based on turning off TN03 at an earlier point in each cycle than it would if left to free-run.
A rise in output voltage increases the voltage across CN04 and increases the current in TN01, causing the negative potential at TN02 gate to fall. TN02 conducts earlier in its cathode ramp cycle; TN03 conduction is terminated earlier and the secondary voltages reduce to cancel the original rise. The converse is also true. The stabilised output voltages depend on the d.c. operating point of TN02 gate, set up by adjustment of PN01.
Two characteristics of this PSU system are a variable free-running frequency – which ranges between 22 kHz and 42 kHz with load requirements; and an inherent immunity to overload in the form of heavy damping of LN03, i.e. in the event of one of the secondary rectifiers shorting, or a short-circuit elsewhere in the receiver. Under these circumstances the chopper frequency falls to a very low rate, limiting current to safe levels and calling attention to the problem by a characteristic purring sound. Late variants of self-oscillating SM- PSUs have different arrangements: some use a purpose-designed power-switching/regulating IC; some use switching transistors in place of the thyristor control element. In all cases the switch principle remains the same.
Such other protection as is required by the circuit of Fig. 11.3 is provided by fusible resistor RN14 to break heavy fault currents in the line output stage; FN02 protecting the audio power stage on the 22 V rail; and the 1 A fuse FN01 which blows in the event of a short or leakage in TN03.