Push-pull flux converter
As a push-pull converter , push-pull forward converter or push-pull converter ( English push-pull converter ) is known in the electronics an electronic circuit the one electric voltage can transform into another electrical DC voltage. Since the main part of the voltage conversion in the push-pull flux converter is carried out by a high-frequency transformer , the output voltage can assume almost any size, as it is therefore not limited by the converter topology, as is the case, for example, with the step- up converter or step-down converter .
Due to the potential-free output voltage due to the transformer , the push-pull flux converter is counted to the group of galvanically isolated DC voltage converters .
Structure and motivation
The main element of the push-pull flux converter is a high-frequency transformer without an air gap , which is operated with an alternating voltage by means of semiconductor switches . Depending on the circuit variant, the polarity of the primary winding is reversed cyclically or it is switched between two primary windings. In any case, the transformer experiences alternating magnetic flux (alternating flux ), whereby the magnetic circuit of the transformer, in contrast to the single-ended flux converter , is used in both directions - i.e. by a positive and a negative flux - for energy transfer . Accordingly, the transformer of the push-pull flux converter does not need a demagnetization winding , since this task is taken over by reversing the polarity of the flow. The transformer is thus used much better than with the single-ended flux converter.
The output voltage of the high-frequency transformer is rectified and fed to an LC filter , which thus works like a down converter. The rectification can be done either by means of a bridge rectifier or by two diodes and a secondary winding with a center tap as a midpoint rectifier .
Embodiments
Push-pull flux converter in parallel feed
In the case of a push-pull flux converter fed in parallel, the primary winding of the high-frequency transformer is divided in the middle. The center tap of the transformer is connected to the supply voltage potential and the two winding ends are connected to ground in a push-pull cycle using semiconductor switches. Since the other (and opposite) winding conducts current after each switchover, there is an alternating flow in the transformer.
The respective switch-on times of the transistors must last exactly the same length, otherwise a constant field will develop in the transformer and drive the core into saturation . Overlapping switching of the transistors should also be avoided, as this would cancel out the field in the transformer and result in a short circuit.
With the parallel-fed design of the push-pull flux converter, each partial winding on the primary side must be designed for the full supply voltage, and the transistors for double the supply voltage.
Push-pull flux converter with half-bridge control
When controlling a push-pull flux converter using a half bridge, the supply voltage is halved in terms of AC voltage with the help of two capacitors and fed to one end of the primary winding. So that half the supply voltage is applied to the capacitors even in the idle state (no current), two high-value resistors are connected in parallel to halve the DC voltage potential. The other end of the winding is now switched cyclically in push-pull to the supply voltage potential or ground by means of transistors in order to alternately reverse the polarity of the winding.
Different switch-on times of the transistors do not have any particular effect, since a direct current - and thus a direct field in the transformer - is excluded due to the capacitors. Only the originally symmetrical voltage distribution on the capacitors is shifted.
With the half-bridge version of the push-pull flux converter, the primary winding must be designed for half the supply voltage and the transistors for the full supply voltage.
Push-pull flux converter with full bridge control
When a push-pull flux converter is controlled using a full bridge (H bridge), the primary winding of the transformer is located between two half bridges and can therefore be connected to the supply voltage in both directions. For this purpose, switches S2 and S3 or S1 and S4 are always switched on at the same time. By cyclically changing these two switching states, this version also ensures that the transformer is operated with an alternating flow.
As the circuit already shows, it must be ruled out in any case that switches S1 and S3 or S2 and S4 are switched on at the same time, since this short-circuits the supply voltage.
In the full-bridge version of the push-pull flux converter, both the primary winding and the transistors must be designed for the full supply voltage.
function
The function of the push-pull flux converter is to be discussed here on the basis of the push-pull flux converter with full bridge control, whereby all components are regarded as ideal. The descriptions can also be transferred to the other circuit variants.
To control the push-pull flux converter, each switching cycle is divided into four time intervals, which are carried out in sequence. In each switching cycle, the first switching position ( S2 and S3 are conductive) is output for the time t on , which is followed by a timeout t off during which no switch is conductive. After these first two time intervals the push-pull follows, in which again the second switch position ( S1 and S4 conduct) is output for exactly the same time span t on and again concluded with a time-out t off , whereby a switching cycle is run through.
In each of the two active (switch conduct) time intervals per switching cycle, the primary winding of the transformer is connected to the supply voltage. Since the transformer does not have an air gap and the energy is transferred immediately (energy flows "through"), the voltage that is higher (or lower) by the winding ratio (transmission ratio ü ) is applied to the secondary side of the transformer . This voltage is rectified with the help of a bridge rectifier, which generates a pulse-width modulated direct voltage of double frequency . This pulsating voltage is now smoothed by the LC filter at the output and is available as a pure DC voltage at the output. This LC filter can also be viewed as a down converter, whereby the output voltage reaches different levels depending on the pulse width of the pulse width modulated voltage. During the period t off , the current I L , caused by the coil , continues to flow through the rectifier diodes in the output circuit.
The level of the output voltage of the push-pull flux converter thus depends primarily (apart from the input voltage) on the winding ratio of the transformer and can also be varied by the ratio of the time periods t on and t off . Depending on the definition, the pulse duty factor for the push-pull flux converter can be 1, with the two switch positions then being output alternately without a pause. Occasionally, the duty cycle of the push-pull flux converter is also described as the ratio between the switch-on duration of a switch position and the cycle time, resulting in a maximum duty cycle of 0.5.
The output voltage depending on the input voltage, the transformation ratio and the duty cycle can thus be specified as follows:
The duty cycle d is defined as
and can be approximately 1.
Secondary side
The secondary side of the push-pull flux converter can alternatively be designed as a midpoint circuit (midpoint rectifier). However, this changes the function of the converter only minimally, since the freewheeling current of the output coil now flows bifilarly through the two rectifier diodes and additionally through the secondary winding in the time period t off .
The coil (L), a smoothing choke in the secondary circuit, is used to compensate for winding-related deviations due to magnetic leakage flux and the magnetic coupling between the two secondary-side winding halves and the associated different mean currents in the winding halves. With a sufficiently good magnetic coupling of the two secondary-side winding halves, this choke can be omitted entirely - the smoothing of the output voltage is then only ensured via the output capacitor C.
Current doubler
A so-called current doubler on the secondary side, as shown in the adjacent figure, can be advantageous, especially with higher output currents of a few tens of amps and lower output voltages in the range of a few volts . The center tapping on the transformer, which is difficult to construct in the case of higher currents, is avoided, and this also eliminates the need for the best possible magnetic coupling of the two secondary-side coil halves.
The secondary side of the transformer is designed for half the output current and twice the output voltage, which reduces the cross-section of the winding wire. Two coils, L 1 and L 2 , are also required for this, which work simultaneously as smoothing chokes as in the example above. In the steady state, the currents through the two coils L 1 and L 2 are almost constant, and there is also a small residual ripple ( ripple ) in the current. The current of a choke and the current through the secondary-side transformer winding add up, alternating in each half-wave, to the output current through the respective conductive diode, the amount of the voltage on the coil approximately corresponding to the output voltage.
application
The push-pull flux converter with full bridge control is especially suitable for DC / DC converters in the higher performance class up to several kilowatts . Because of the better utilization of the transformer and the resulting higher degree of efficiency , the topology of the push-pull flux converter is clearly preferable for a few hundred watts.
The push-pull flux converter is mainly used in switched-mode power supplies , where it converts the rectified mains voltage (maximum 325 V, less under load) to a low voltage level (for example 24 V).
Conversely, the push-pull flux converter is also used in inverters with a higher output. The converter transforms the small DC voltage (for example 12 V of a starter battery ) into the intermediate circuit voltage of about 325 V required for the inverter , which is then modulated trapezoidally or sinusoidally to form the mains voltage using a high-voltage bridge.
See also
literature
- Ulrich Schlienz: Switching power supplies and their peripherals . 3. Edition. Vieweg & Sohn, GWV Fachverlage GmbH, Wiesbaden 2007, ISBN 978-3-8348-0239-2 .
- Ralf Kories, Heinz Schmidt-Walter: Pocket book of electrical engineering: Basics and electronics . 8th edition. Scientific publishing house Harri Deutsch GmbH, Frankfurt am Main 2008, ISBN 978-3-8171-1830-4 .
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, Berlin, Heidelberg, New York 2002, ISBN 3-540-42849-6 .
Individual evidence
- ↑ Laszlo Balogh: The Current-Doubler Rectifier: An Alternative Rectification Technique For Push-Pull And Bridge Converters . Unitrode Design Note DN-63, 1994 ( Online [PDF]).