Split pi converter

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As a split-pi topology ( English Split-Pi ) is referred to in the electronics an electronic circuit , an electrical DC voltage into another DC electric power may change.

The level of the output voltage of the DC / DC converter can be greater or less than the original input voltage , as with the cascaded step-down / step-up converter . The output voltage always has the same sign as the input voltage, which is why the split-pi converter is included in the group of non-inverting DC voltage converters .

The main feature of the split-pi converter is the inserted intermediate circuit capacitance, which on the one hand can act as an energy store and on the other hand buffers periodic power interruptions. In contrast, the input and output of this converter always carry uninterrupted currents, which offers advantages in terms of EMC properties and freedom from ripple.

Origin of name and structure

Basic circuit diagram of the Split-Pi converter

The name of the Split-Pi converter is similar to the circuit of a pi filter , which in the circuit of the converter using semiconductor switches and the intermediate circuit capacitor divided ( English Split ) is. The cascaded step-down-step-up converter, on the other hand, only has a storage choke and no intermediate circuit capacitor, so it is similar to a non-split pi filter.

The split-pi converter is a chain connection of two synchronous converters , the first synchronous converter being used in reverse - i.e. mirrored - and the intermediate voltage of the converter being supported by an additional capacitor . The converter thus consists of two inductors which, as an active energy store, are charged and discharged cyclically with energy using four bidirectional semiconductor switches. The two outer capacitors serve - as with every DC / DC converter - as buffer capacitors and smooth the respective voltage.

Since there is inductance in the input line and output line, both the input current and the output current are continuous.

The central capacitor is an essential feature of the split pi converter. If only one converter part is active, it only serves as a filter capacitor. If both converters are active, it must always have a higher voltage than the highest input and output voltage. The voltage applied to the capacitor can be freely selected via the pulse duty factor of the synchronous converter and is only limited by the components. A double-layer capacitor can therefore be used here for intermediate energy storage, for example.

function

The split-pi converter can in principle also work with just one of the synchronous converters. Typical applications, however, use both converters, which means that the voltage of the intermediate circuit can and must be controlled.

Operation of only one synchronous converter

For a better understanding, the operation of only one of the synchronous converters is assumed.

Operation with unidirectional energy flow

If a voltage source is connected to one connection U A and a load is connected to the other connection U B , the voltage at the load can be controlled as required by selecting the active and passive synchronous converter.

If the voltage at the load is to be lower than the voltage of the voltage source, the synchronous converter A is operated with a voltage transformation ratio of 1 (upper switch remains closed) and the synchronous converter B is clocked with the desired pulse width ratio. The entire split-pi converter thus works as a synchronous step-down converter, the output voltage of which depends on the pulse width ratio of the synchronous converter B.

If the voltage load is to be greater than the voltage of the voltage source, synchronous converter B is operated with a voltage transformation ratio of 1 (the upper switch remains closed) and synchronous converter A is clocked with the desired pulse width ratio. The split-pi converter thus works as a synchronous step-up converter, the output voltage of which now depends on the pulse width ratio of the synchronous converter A.

Operation with bidirectional energy flow direction

Since the split-pi converter is symmetrical and the direction of energy flow is bidirectional, it does not matter which connection is defined as input and which as output.

If a voltage source is connected to each connection of the split-pi converter, then - as with the simple synchronous converter - the direction of current flow can be determined by selecting the pulse width ratio. In contrast to the simple synchronous converter, however, with this converter it is irrelevant what level the respective voltage has. The active and passive synchronous converter of the split-pi converter is specified depending on the level of the respective voltages of the voltage sources.

If U A is higher than U B , synchronous converter A must work passively (upper switch remains closed) and the desired direction of energy flow and its amount is specified by the pulse width ratio of synchronous converter B, which is actively working.

If U A is smaller than U B , the synchronous converters must work exactly the other way around.

Function of the intermediate circuit capacitor

A DC link buffer is always required, regardless of the converter operating mode. If a synchronous converter switches off the upper switch, a connection of the inductance of the other synchronous converter would possibly remain open without an intermediate circuit capacitor. However, since the current impressed in the inductance must continue to flow, the voltage would rise or fall sharply without a capacitor. This would destroy the semiconductor switches and energy would be lost. The intermediate circuit capacitor cannot be omitted, but is rather a characteristic of the topology. It allows energy to be temporarily stored, as its charge can be controlled using the two synchronous converters. It is therefore possible to charge or discharge the capacitor while the energy is being transferred. This means that the converter can, for example, at times output significantly more power than it consumes. This is used to advantage if, for example, you need high power for a short time, but do not have a high current available for this.

The DC link capacitor maintains the voltage in the event of any periodic power interruption in the synchronous converter. The external inductances, on the other hand, carry current continuously, which keeps switching interference away from the neighboring assemblies. The intermediate circuit capacitor acts as a buffer capacitor and serves as a current source or sink.

Application and advantages

The advantage of this DC / DC converter, in addition to the bidirectional energy flow direction and the independence of the voltage limits, is the intermediate circuit capacitor, which buffers interference and can temporarily store energy. The converter is thus ideally suited for battery-operated electric drives , since the EMF of the electric motor or its supply voltage can be both smaller and larger than the battery voltage for a desired speed. The bidirectional direction of energy flow means that braking ( recuperation ) of the electric motor is also possible at both low and high speeds. However, all of this could also be solved with a simple pi converter.

The main advantage of the Split-Pi converter in this application is rather the possibility of energy storage if a double-layer capacitor is used as the intermediate circuit capacitor . Double-layer capacitors can deliver significantly higher instantaneous power per storage energy than accumulators. On the other hand, due to their size, they are not able to store large amounts of energy, which in turn can be achieved by the rechargeable battery or the traction battery. Traction batteries, on the other hand, suffer when high currents are drawn. Using the split-pi converter, both components can complement each other in the vehicle. When starting and braking, the double-layer capacitor briefly supplies or stores energy at high power, which is gradually taken from or fed into the battery.

Typical applications are also buses in city traffic in regular service, which have a hybrid drive and recuperation .

See also

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