What is a Bridge Converter?
A Bridge Converter is a DC to DC converter topology (configuration) employing four active switching components in a bridge configuration across a power transformer. This has lower losses than a bridge rectifier.
A full bridge converter is one of the commonly used configurations that offer isolation in addition to stepping up or down the input voltage. Other functions may include reversing the polarity and providing multiple output voltages simultaneously.
Bridge converter has three main stages:
- The Square wave generator
- Energy transfer network
- Rectifier network
The basic circuit consists of four active switching devices such as MOSFETs, IGBTs, Bipolar transistors or thyristors, and their associated drive circuits. In the isolated bridge converter, a transformer is used to provide isolation in addition to stepping up or down of the voltage. MOSFETs are widely used for the high power bridge converters due to their simple and low power gate drive circuits as compared to the bipolar transistors.
The basic operation involves switching one pair of transistors at a time for one half cycle of the control waveform and the other pair during the other part of the waveform. The two control strategies use the pulse width modulation signal to either switch the transistors on each leg simultaneously as a pair or switch them independently in series.
The switching waveform is either square or sinusoidal and while square wave is simpler to implement, it has more harmonic content. However, it is still used at high frequencies above 20 kHz and usually used for loads such as RF heating and SMPSs which can be tuned to minimize the effects of the harmonics. Sinusoidal switching waveforms are used for low frequencies such as in ac motor drives where high harmonics would require expensive and bulky filter components in addition to causing higher losses.
Fig 1: Full-bridge buck-derived converter – (Image Source)
The two pairs of MOSFETS are used to drive the transformer primary symmetrically. Q1 and Q4 conduct in first switching period and the other pair of transistors Q2 and Q3 conduct during the next one. As one pair conducts, the other one remains in the off state.
The output voltage is determined by the transistor duty cycle, the transformer’s turn’s ratio, input voltage. As full bridge output is controlled by unipolar switching using four switching devices, care must be taken to avoid direct current in the transformer since these increase losses and can also saturate the transformer.
- Available in several standard sizes
- Handling of a wide range of input and output voltage levels
- High power density
- Low power losses
- Losses due to freewheeling currents
- Transformer performance losses based on the transformer parameters
- Expensive as compared to other converters since it uses more components
The bridge converter circuits are flexible and widely used in power supplies above 150W. Typical applications include the power supply units, dc servo motor drives, generating ac voltage for ac motor drives, RF heating and isolated switched mode power supplies.
Isolated DC-DC bridge converters with bi-directional power flow are used in applications such as hybrid electric vehicles, rail guided shuttles and automatic industrial applications.