To reduce the size of a power converter, the transformer can be used in two quadrants instead of one, as is the case with a flyback converter. In this way, the efficiency of a given transformer size can be doubled. The efficiency can be increased even more if the transformer is used as a direct converter component and not as a storage component, i. e. as a direct power supply to the output. By increasing the frequency, the number of turns can be reduced without the risk of transformer saturation. In order to increase the switching frequency, switching losses must be reduced, which can be achieved by a resonant or soft-switching topology.
A half bridge, as shown in Figure 2, allows both to increase operation on two quadrants (the current can flow in both directions) and to use the transformer as a direct conversion component, i. e. the load is supplied directly from the primary side without the need for energy storage. Lp in Figure 2 represents the primary winding of the transformer. When the Q1 switch is closed, current flows from left to right through Lp, charging capacitor C2 and discharge capacitor C1. After opening switch Q1, the residual energy in the transformer will cause a natural transformation. After that, the Q2 switch can be closed with low losses and the current flows in the opposite direction. They can be replaced by a single capacitor, resulting in an asymmetric half bridge with a negligible effect on symmetrical balance and electromagnetic emissions.
If the frequency of the half-bridge converter is adjusted near the resonance frequency of the primary LC circuit (Lp + C1/C2), a reduction of switching losses can be achieved. The opening and closing of the primary switches (Q1 and Q2) can then take place in the vicinity of zero voltage or zero current (ZVS or ZCS), which leads to minimal switching losses. The transformer still recognizes a square wave voltage, but the current flowing through the transformer and feeding the output is sinusoidal. This sinusoidal current is the main driving force for increasing efficiency and reducing electromagnetic emissions, since the harmonics of the switching frequency contain less energy compared to hard-switching technologies.
Flyback converter versus Chibi LLC converter
Figure 3 shows a 60 W ChiBi LLC converter for wall mounting and a normal 60 W flyback converter for desktop use. In addition to the fact that the ChiBi converter is around 30% smaller, its reduced weight also allows wall mounting, which further reduces the total cost as no AC cable is required.
The standard ChiBi converters are cost-optimized and therefore feature only minor improvements in terms of efficiency and electromagnetic emissions, but are very competitive with a size reduction of 30 % and a price reduction of more than 10 %.