A Half Bridge Buck Boost Converter with high side N-type MOSFET

The buck-boost converter is of value where the output and/or input voltages vary to such an extent that on occasions a buck converter is needed, while on other occasions a boost converter is needed. An example (that will be described further in another project) is that of a battery charger for a number of different battery types, voltages and capa

cities and powered by a 12V source. The simplest buck-boost circuit uses a single inductor, MOSFET, catch diode and filter capacitor. This circuit is non-isolating and inverting. Other non-isolating buck-boost circuits include the †uk, which is inverting, and the Sepic, which is non-inverting. These circuits require two inductors which need not be coupled, although coupled inductors can provide much lower output ripple. A disadvantage of these circuits is that they require a coupling capacitor which must be able to support high ripple current. The H-Bridge circuit provides a buck-boost function using a single inductor and without inversion. This is essentially a buck converter followed by a boost circuit. Since the former has its inductor is at the output while the latter has the inductor at the input, they may be replaced by a single inductor. A filter capacitor is not present in the buck part of the circuit. The control of this circuit involves separate control of each section, and as such tends to be somewhat complicated. A number of companies produce controller chips for this type of circuit, including Linear Technology (eg LTC3780, LTM4607). The H-Bridge is so called because the MOSFETs form the sides of an H with the inductor as the crossbar. In this form the buck and boost parts of the circuit are operated in synchronous mode (second diagram below). It is also possible to operate the circuit non-synchronously using...

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