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One of the problems with the push-pull converter is that the flux in the two sections of the center-tapped transformer primary and secondary windings can become unbalanced and cause heating problems. Another problem is that each transistor must block twice the amount of voltage than other converters block. The half-bridge converter provides several advantages over the push-pull converter. Fig. 1 shows the electronic diagram for the half-bridge converter. In this schematic, notice that the more expensive center-tapped transformer is replaced with a traditional transformer. This circuit still uses two transistors and two sets of diodes like the push-pull circuit. The main difference of the half-bridge converter is that it utilizes two large bulk capacitors (C1 and C2). These capacitors are connected so that each one is in series with one of the transistors. This means that power can be transferred to the output during the on-time for each transistor, which increases efficiencies to the 90% range. Since center-tapped transformers are not used, the problem with flux unbalance is also eliminated. These advantages also allow this type of converter to be utilized in power supplies up to 1000 Watts.
|The Push-Pull Converter
|The Full-Bridge Converter