Synchronous converter
In power electronics, a synchronous converter (occasionally also DC voltage transformer ) is a DC voltage converter that has a storage choke and two switches that always close alternately. One of the switch replaces the diode of the buck converter or boost converter - so it is just such with synchronous rectifier , which gives the converter the name.
The output voltage always has the same sign as the input voltage, which is why the synchronous converter is included in the group of non-inverting DC voltage converters.
A main feature of the synchronous converter is that the energy flow can go in both directions: the choice of the duty cycle of the synchronous converter enables a current flow both into and from the load back to the source. The level of the output voltage can therefore be higher or lower than the input voltage, depending on the choice of input and output.
construction
The synchronous converter is made up of an active energy store, an inductance , which is cyclically charged with energy and discharged by means of a semiconductor switch . Only components that can conduct currents in both directions are used as semiconductor switches, e.g. B. MOSFETs .
Basically, the synchronous converter can be viewed as a combination of step-down converter and step- up converter . The circuit of the synchronous converter corresponds to that of a step-down converter or step-up converter in which the diode has been replaced by a semiconductor switch that can conduct current bidirectionally.
Depending on the definition of the input and output of the synchronous converter, it works as a down converter or as an up converter.
function
As with every DC / DC converter, the output voltage is determined by the switch-on time and the switch-off time of the semiconductor switch. In the case of a step-down converter or step-up converter, it is sufficient to control the one actively switchable semiconductor switch. The second switch, the diode, switches automatically, depending on which direction the current has through it.
Since the synchronous converter is made up of two switchable semiconductor switches, the second must be operated in push-pull to the first. If the first switch becomes conductive, the second opens, and vice versa. A switch is therefore conductive at all times - apart from a small dead time in order to reliably avoid short circuits due to finite switching times.
Voltage source and load
If the voltage U A is defined as an input and a voltage source is applied and the voltage U B is defined as an output and a load is applied, the synchronous converter works as a step-down converter, whereby the output voltage can be set in continuous operation as with this converter.
If the voltage U A is defined as an output and a load is connected and the voltage U B is defined as an input and a voltage source is connected, the synchronous converter works as an up-converter. The behavior corresponds to that of a conventional step-up converter, which is always in continuous operation. The tax law for non-discontinuous electricity applies in any case.
Operation with two voltage sources
If a voltage source is connected to the input as well as to the output - whereby the voltage U A must be higher than the voltage U B - then caution is required, because destructive currents can flow. This is e.g. B. the case when the synchronous converter is used to charge a battery.
If the pulse width ratio is equal to the ratio U B / U A , the mean voltage across the inductance is zero and the mean current flow decays. In the case of deviating pulse width ratios, the inductance has an average voltage in one direction or the other and the current increases. It is limited by internal resistances or better by regulating the pulse width ratio, which can then also be used to determine the direction of the energy flow, upwards or downwards. In practice, the current is regulated by the inductance. This must be measured by a current sensor and fed to the control algorithm.
The synchronous converter works for DC voltages in a similar way to an AC voltage transformer for AC voltage , with the pulse width ratio as the transformation ratio , and for this reason is sometimes referred to as a DC voltage transformer .
Application and advantages
The synchronous converter is primarily used where bidirectional energy transmission is required. An example of this is an electric drive using a DC motor and accumulator . The inductance required for the converter is formed by the leakage inductance of the motor, the buffer capacitance C UB is not required. The motor is therefore connected directly to the half-bridge and acts as a load or energy source for the accumulator depending on the ratio of speed, i.e. induced voltage, and pulse width ratio . The synchronous converter thus acts as a two-quadrant controller .
However, the synchronous converter is also used when no bidirectional current flow is required. Since a diode always causes a voltage drop in the forward direction , there are significant losses . Modern power transistors, on the other hand, have significantly lower losses, which is why the implementation of a step-up or step-down converter as a synchronous converter can improve efficiency .
Another advantage of the synchronous converter is the lack of discontinuous (intermittent) operation. In a conventional step-down converter, if the mean value of the output current is low, the current through the inductance periodically becomes zero and the diode blocks. In this discontinuous operating range, the output voltage no longer depends linearly on the input voltage and the pulse width ratio, which makes regulation more difficult. With a synchronous converter, on the other hand, the current rises negatively after its zero crossing due to the reverse conductivity of the semiconductor switch, energy flows from the output capacitance back to the input, linearity and thus easy controllability are given regardless of the load. However, if the operating range with oscillating energy makes up a large proportion of the operating time, higher losses than with a step-down converter with a diode can result, see also H5 topology .
Multi-parallel converter
At higher powers, at the latest when the current carrying capacity (active surface) of the power semiconductors is reflected almost linearly in the costs, one should consider operating several synchronous converters in parallel and controlling them cyclically and offset. The distributed energy absorption and output of the separate inductors creates a margin that can be used for the given requirement on the voltage ripple
- Reduction of the capacitance and current load of the common buffer capacitors at the input and output or
- Increase in the fluctuation amplitude of the individual currents and magnetizations - scope for reduction
- the core volume and weight of the inductors or
- the clock frequency and thus the switching losses .
See also
literature
- Ulrich Schlienz: Switching power supplies and their peripherals. 3. Edition. Vieweg & Sohn Verlag, Wiesbaden 2007, ISBN 978-3-8348-0239-2 .