Soft-switching resonant d.c.–d.c. converters
The power supply of a PDP TV employs three types of d.c.-to-d.c. converters:
● linear regulated converter,
● switched mode power supply (SMPS),
● soft-switching resonant converter.
The linear regulators and the SMPSs have been discussed in detail in Chapter 15. The type used here is normally the flyback d.c.–d.c. converter. SMPSs suffer from a number of limitations: high EMI, high stress levels on the switching devices and limited switching speeds to less than 100 kHz. These limitations are mainly a result of the switching taking place at a high current and/or voltage levels. Hence these converters came to be known as hard-switching converters. When switching at high frequencies, these converters are associated with high power dissipation and high EM interference caused by high-frequency harmonic components associated with their quasi-square switching current and/or voltage waveforms.
To overcome the limitations of the traditional SMPS, a third-generation power converters known as soft-switching (resonant) converters were intro- duced in the late eighties.
Like SMPSs, soft-switching power units are d.c.–d.c. converters. D.c. from a rectifier is first converted to a.c. through a switching element. This a.c. is fed into the primary of a transformer with a number of secondary windings for multi-outputs. The secondary outputs are then rectified in the normal way to produce the various d.c. voltage levels. The new ele- ment that resonant converters bring is switching takes place when the voltage across the switching element is zero, known as zero voltage switch- ing (ZVS) or when the current through the switching element is zero, known as zero current switching (ZCS). ZVS and ZCS are produced by using resonant circuits; hence these converters are also known as resonant or quasi-resonant converters. The result is low switching power dissipa- tion and reduced component stress. These in turn result in increased power efficiency, reduced size and weight, faster responses and reduced EMI problems. The reduction in losses due to zero voltage or ZCS makes it possible to utilise much higher switching frequencies in 100 s kHz or even few MHz. And since the size and weight of the magnetic components (inductors and transformers) and capacitors are inversely proportional to the switching frequency, the higher the latter, the smaller the size and weight of the power supply improving its power density. A further advan- tage of soft-switching resonant converters is, because of their switching frequencies, they can utilise transformer and switching elements’ leakage inductance and capacitance respectively as part of the resonant circuit.