Topologies of DC/DC Converter

Reading time: approx. 5 minutes – In this first of a total of two articles on DC/DC converters, you will learn interesting facts about the different modes of operation and topologies of DC/DC converters.

DC/DC converters are part of many electronic devices or are necessary for the operation of almost all electrical products. DC/DC converters are electrical circuits that are capable of converting a voltage applied to the input into a lower, a higher or an inverted output voltage. They are therefore always used when an available supply voltage does not match the input voltage of the subsequently used electronic components. Accordingly, many different versions of DC/DC converters with different specific characteristics are available on the market.

Functionality of DC/DC converters

The basic function of a DC/DC converter is that the DC voltage at the input is transformed into a rectangular AC voltage by opening and closing a switching element, for example a power transistor. Subsequent filtering, usually using a combination of inductance and capacitance, converts this AC voltage back to a DC voltage with a voltage level different from the input. The pulse width factor, defined as the division of the duration of the controlled power transistor divided by the duration of the blocked power transistor, regulates the ratio of the input voltage to the output voltage.

The placement of the reactances, i.e. the inductances and the capacitances in the circuit, as well as the active components that serve as switches, for example transistors and diodes, determines the topology of the DC/DC converter. In contrast to a linear voltage regulator, which works like an electronically variable resistor, DC/DC converters can achieve a significantly better efficiency due to their switching operation and the subsequent filtering by low-loss reactive components.

Topologies of DC/DC converters

Basically, a distinction is made between a DC/DC converter with and without galvanic isolation. Galvanically isolated means that the input and output of the DC/DC converter are connected to each other by non-conductive coupling elements and thus there is a potential separation. Depending on the application, this may be necessary for safety or measurement reasons. Galvanic isolation by means of a transformer or a coupled storage choke additionally offers the possibility of relatively freely selecting the magnitude and polarity of the converted voltage.

Examples of DC/DC converter topologies without galvanic isolation

The buck converter uses a storage choke to filter the square-wave voltage chopped by the switching transistor, whereby the input voltage can be greater than or equal to the output voltage. One of its basic features is the transfer of energy to the output side during the transistor’s turn-on phase.

The boost converter also uses a storage choke for filtering, but produces an output voltage that is higher or equal with respect to the input voltage. It is also characterised by the fact that the energy transfer to the output occurs during the switch-off phase of the switching element.

The buck-boost converter, also with the aid of a storage choke, supplies an output voltage that is inverted relative to the input voltage and whose magnitude can be either higher or lower than the input voltage. Here, too, the energy transfer takes place during the opening phase of the circuit breaker.

Examples of DC/DC converters with galvanic isolation

The fly-back converter, which is derived from the buck-boost converter, is mainly used in applications with a power of less than 300 watts. The energy is transferred via two coupled storage chokes, which ensure galvanic isolation.

The forward converter, which belongs to the buck family, transmits the energy by means of a transformer. To filter the output voltage, this topology requires a storage choke in addition to the transformer. It is typically used in power ranges smaller than 500 watts. Depending on the requirements, a single-transistor flux converter with one switching transistor or a two-transistor flux converter with two switching transistors can be selected. The two-transistor variant of the flux converter offers the advantage of better utilisation of the transformer and can typically be used sensibly up to about 1 kW power.

Depending on the design, the push-pull converter is used in the power range between 1 kW and over 10 kW. It also belongs to the Buck family and requires a storage choke on the output side for filtering. The energy is transmitted via a transformer. Here, a distinction can be made between half-bridge push-pull converters with two transistors, or full-bridge push-pull converters with four switching transistors. The difference between this type of circuit and the previous ones is that the energy transfer to the secondary side can take place during all switching phases. This increases the achievable power density enormously, at the cost of higher circuit complexity.

A whole family of topologies is referred to as resonant converters, the common characteristic of which is to manage energy transfer with the aid of a series or parallel resonant circuit. The aim of this type of operation is, among other things, to reduce or, if possible, completely avoid the power losses when switching the power transistors by avoiding high voltages and/or currents in the switching moment. A great advantage of such DC/DC converters is the possibility, based on the properties of the resonant network, to design self-oscillating circuits that do not require external control or regulating electronics and are thus inexpensive and very easy to build.

Different resonant converters are used in applications with a wide range of power, from a few watts to over 10 kW, depending on the circuit and structure.

All topologies have specific properties, particular advantages, but also always disadvantages, and are used in practice depending on the requirements. The challenge is to select the topology used in such a way that its properties are ideally suited to the application and thus the requirements can be fulfilled in the best possible way.