Push–pull converter

A push-pull forward converter, a push-pull forward converter or push-pull converter (English push-pull converter ) is known in the electronics, a discrete electronic circuit that can convert an electrical DC voltage into another DC electric voltage. Since the main part of the voltage conversion is carried out in a push-pull forward converter with a high frequency transformer, the output voltage can be nearly any size, because it is thus not limited by the topology of the converter, as is the case for example, when up-converter or down-converter.

Due to the consequently floating through the transformer output voltage of the push-pull forward converter is included in the group of galvanically isolated DC-DC converter.

• 4.1 Current - Doubler

Design and motivation

Main element of the push-pull forward converter is a high frequency transformer without a gap, which is operated by means of semiconductor switches with an AC voltage. Depending on the arrangement, the primary winding is reversed cyclic or switched between two primary windings. In each case the transformer experiences a changing magnetic flux ( flux change ), whereby the magnetic circuit of the transformer, as opposed to Eintaktflusswandler in both directions - is used for power transmission - that is, by a positive and a negative flux. Accordingly, the transformer of the push-pull forward converter requires no demagnetizing coil, since this task is taken over by the respective reversal of the flux. The transformer is thus exploited much better than the Eintaktflusswandler.

The output voltage of the high-frequency transformer is rectified and supplied to a LC filter, which thus operates as a buck converter. The rectification can be carried out either with a bridge rectifier or by two diodes and a secondary winding with a center as center-point rectifier.

Embodiments

Push-pull forward converter in parallel feed

In a parallel -fed push-pull forward converter, the primary winding of the high frequency transformer is divided in the middle. The center tap of the transformer is cyclically by using semiconductor switches with power supply potential, and the two winding ends are connected to ground in opposite phase. Because every time you switch the other ( and opposing ) leads winding current is thus produced in an AC transformer flux.

The respective switch-on of the transistors must continue exactly the same length, otherwise the transformer forms a dc field and drives the core into saturation. Next an overlapping switching of the transistors should be avoided, as this pick up the field in the transformer and would lead to a short circuit.

In the execution of the parallel -fed push-pull forward converter, each winding part of the primary side must be set to the full, the transistors each designed to twice the supply voltage.

Push-pull forward converter with half-bridge control

During the actuation of a push-pull forward converter by means of the half bridge supply voltage by means of two capacitors alternating voltage is halved and supplied to a winding end of the primary winding. Thus also in the quiescent state ( no current ) to the capacitors in each case half the supply voltage is applied, are connected in parallel to these two high-value resistors in order to halve the DC potential well. The other coil end is now switched by transistors in push-pull cycle of supply voltage or ground potential to reverse the polarity of the winding alternately.

Different turn-on of the transistors act, this is not particularly, since a DC - and thus a constant field in the transformer - because of the capacitors is excluded. Only the originally symmetrical voltage distribution across the capacitors is shifting.

In the half-bridge implementation of the push-pull forward converter, the primary winding needs to half, the transistors in each case designed for the full supply voltage.

Push-pull forward converter with full-bridge control

During the actuation of a push-pull forward converter by means of a full-bridge ( H bridge ) is the primary winding of the transformer between the two half bridges and is thus connected to the supply voltage to both directions. This is always the switches S2 and S3 or S1 and S4 are simultaneously turned on. By cyclically changing these two states is also achieved in this embodiment that the transformer is operated with an alternating flux.

As the circuit suggests, must in any case be ruled out that the switches S1 and S3 or S2 and S4 are turned on simultaneously, as a result, the supply voltage is short-circuited.

In the full-bridge implementation of the push-pull forward converter, both the primary winding and the transistors must be designed for the full supply voltage.

Function

The function of the push-pull forward converter will be discussed with reference to the push-pull forward converter with a full-bridge control here, all components are considered to be ideal. The descriptions can be also transferred to the other circuit variations.

For the actuation of the push-pull forward converter of each switching cycle is divided into four time intervals, which are executed in sequence. In each switching cycle, thus the first shift position (S2 and S3 are conducted ) is output for the period ton, followed by a break follows toff in which conducts no switch. After these first two time intervals following the push-pull, in which, in turn, for exactly the same amount of time ton the second switch position (S1 and S4 conduct ) issued and is in turn completed toff with a time-out, whereby a switching cycle is passed.

At each of the two active ( switch lead ) time intervals per switching cycle is the primary winding of the transformer to the supply voltage. Since the transformer has no air gap and the energy transfers immediately ( energy flows " through " ), located at the secondary side of the transformer respectively to the turns ratio ( transmission ratio u ) larger (or smaller ) voltage. This voltage is rectified using a bridge rectifier, whereby a pulse-width modulated DC voltage is twice the frequency generated. This pulsating voltage is then smoothed by the LC filter at the output and is available as a pure DC voltage at the output will be available. This LC filter can be considered as down-converter, whereby the output voltage reaches different heights depending on the pulse width of the pulse width modulated voltage. Here, the current IL flows in the toff period, caused by the coil through the rectifying diodes in the output circuit on.

The height of the output voltage of the push-pull forward converter thus depends primarily (apart from the input voltage) from the winding ratio of the transformer, and can in addition by the ratio of the time periods ton and toff will vary. The duty cycle can therefore be in a push-pull forward converter, depending on definition 1, and then the two switch positions are output alternately without pausing. Occasionally, the duty cycle of the push-pull forward converter will now be described as the ratio between the duty cycle of switch positions and the cycle time, resulting in a maximum duty cycle of 0.5 results.

The output voltage as a function of the input voltage, the transmission ratio and the duty ratio can be therefore given to the following:

In this case, the duty cycle d is defined as

And can take approximately 1.

Secondary side

The secondary side of the push-pull forward converter can alternatively be carried out as a center tap ( center rectifier ). At the operation of the converter, however, this changes only minimally somewhat as the freewheeling current of the output coil in the period toff now by the two rectifier diodes and additional flow through the bifilar secondary winding.

The coil (L ), a smoothing reactor in the secondary circuit, is used to compensate developing technical deviations According magnetic leakage flux and the magnetic coupling between the two secondary-side winding halves and, consequently, different high average currents in the winding halves. At sufficiently high magnetic coupling of the two secondary-side winding halves this choke can also be omitted entirely - smoothing the output voltage is then ensured only through the output capacitor C.

Current Doubler

Particularly at higher output currents of a few amperes 10 upwards and low output voltages in the range of a few volts, a so-called current- doubler as shown in the illustration, on the secondary side can be advantageous. The structurally difficult at higher currents at the center tap of the transformer is avoided, consequently, also eliminates the need for the best possible magnetic coupling between the two secondary coil halves.

The secondary side of the transformer is in this case designed for half of the output current and the double Ausgangangsspannung, whereby the cross section of the winding wire is reduced. These are two coils L1 and L2, in addition necessary that operate at the same time as smoothing reactors as in the example above. In the steady state, the currents are approximately constant by the two coils L1 and L2, in addition there is still a small ripple component in the stream before. The current of a choke and the current through the secondary side transformer winding are added together, in each half-wave alternately to the output current through the conducting diode, respectively, wherein the magnitude of the voltage across the coil is approximately equal to the output voltage.

Application

Especially the push-pull forward converter with full-bridge drive is suitable for DC-DC converters of the higher power class up to several kilowatts. Due to the better utilization of the transformer and, consequently, higher efficiency, the topology of the push-pull forward converter is clearly preferable in this few hundred watts.

Applies the push-pull forward converter with main switching power supply, where this ( about 325 V), converts the rectified mains voltage to low voltage level (e.g., 24 V).

Conversely, is the push-pull forward converter even with inverters of higher power application. Here, the transducer transforms the small DC voltage ( for example 12 V a starter battery ) in the time required for the inverter intermediate circuit voltage of about 325 V, which is then modulated by means of high-voltage bridge trapezoidal or sinusoidal to mains voltage.