ES2C6 – Power Electronics 6 - DC/DC Converters
DC/DC Converters
In electromechanical systems it is common to require different DC voltages. An example is powering
car accessories, where the alternator supplies high voltage AC but the accessories need low voltage
DC.
Solid state technology – transistors and diodes – can be used to convert DC voltages to different DC
voltages, as using a transformer would not work, but would cause a short-circuit instead.
When a transistor is on, we aim to get an ideal short, meaning there is no resistance. When it is off,
we want it to act as an open circuit.
Buck Converter
A buck converter steps down voltage whilst stepping up current.
When the transistor is in the ‘on’ position it acts as a short-circuit:
Applying KVL, the voltages can be related by:
𝑉𝐿 = 𝑉𝑖𝑛 – 𝑉𝑜𝑢𝑡
When it is ‘off’ it acts as an open circuit, so cuts off the voltage source. The inductor releases the
energy stored in its magnetic field as current and the diode acts as an open circuit as the current
flows in its forward direction:
The current in this circuit then free-wheels around the loop connecting the inductor and resistor,
assuming that the capacitor was fully charged and 𝑉𝑜𝑢𝑡 is constant. The voltages can now be related
through:
𝑉𝐿 = −𝑉𝑜𝑢𝑡
26
DC/DC Converters
In electromechanical systems it is common to require different DC voltages. An example is powering
car accessories, where the alternator supplies high voltage AC but the accessories need low voltage
DC.
Solid state technology – transistors and diodes – can be used to convert DC voltages to different DC
voltages, as using a transformer would not work, but would cause a short-circuit instead.
When a transistor is on, we aim to get an ideal short, meaning there is no resistance. When it is off,
we want it to act as an open circuit.
Buck Converter
A buck converter steps down voltage whilst stepping up current.
When the transistor is in the ‘on’ position it acts as a short-circuit:
Applying KVL, the voltages can be related by:
𝑉𝐿 = 𝑉𝑖𝑛 – 𝑉𝑜𝑢𝑡
When it is ‘off’ it acts as an open circuit, so cuts off the voltage source. The inductor releases the
energy stored in its magnetic field as current and the diode acts as an open circuit as the current
flows in its forward direction:
The current in this circuit then free-wheels around the loop connecting the inductor and resistor,
assuming that the capacitor was fully charged and 𝑉𝑜𝑢𝑡 is constant. The voltages can now be related
through:
𝑉𝐿 = −𝑉𝑜𝑢𝑡
26
