Forward converter

The Eintaktflusswandler (English Forward Converter ) is an embodiment of an electrically isolated DC-DC converter, which is characterized in that the energy transfer takes place exclusively in the conducting phase of each switching device. Applications of this converter, for example, switching power supplies.

In contrast to flyback converters, which are particularly suitable for low power, Eintaktflusswandler up to about 500 W used for services from about 100 watts, because they have better efficiency. For higher ratings, push-pull forward converter are used.

Function

The converter is as simplified in the drawing on the essential elements, left fed the DC voltage UE. This voltage is in the scope of switching power supplies typically the rectified mains voltage of 230 V, which corresponds to after the rectification of a smoothed DC voltage of about 325 volts. Of the switch S is implemented in practical circuits as a switching transistor, which is connected to the switching frequencies of a few 10 kHz to a few 100 kHz and different load-dependent since, pulse width is controlled by a control unit, not shown.

The transformer Tr is used of the galvanic separation of the turns ratio and is corresponding to the input voltage to a secondary side voltage. The secondary-side rectifying diode D1 is made, it is followed by a circuit portion which is largely identical to the non-isolated buck converter. This circuit is designed to implement part on the final DC output voltage UA for consumers to terminals outside right connected. This down converter consists of a freewheeling diode D2, the storage inductor L and the smoothing capacitor C which is connected in parallel with the load.

The transducer is characterized by two states, which are determined by the state of a switch S (hence " single ended " ):

Conduction

When switch S is closed a current flows through the primary winding of the transformer Tr and a translated with the turns ratio of the output-side power diode D1 and the choke L. The diode D2 is connected in this state, and blocks in the reverse direction flows. The current increases linearly, as in the storage inductor builds up a magnetic field. The capacitor C is charged to the output voltage. Since the energy transport primary to happen in the conducting phase of the secondary side of this transformer, it is one of the group of the forward converter.

This state is kept for a certain time ton, and then opens the switch S and the transducer is in the lock state.

Off state

When the switch S is open blocks D1 because the secondary voltage reverses the polarity itself. The current through the storage inductor L is continuously and flows in the reverse phase to be conductive D2. Together with the capacitor C is the output voltage UA, kept constant except for a small ripple.

This state is kept for a certain time toff, then closes the switch S and the cycle begins with the conduction state of the new.

The magnetizing current must be kept as small as possible in Eintaktflusswandler and will be implemented in this highly simplified example, in the blocking phase in heat loss, as the primary-side current always flows in one direction only and so it comes to a direct current in the transformer pad. A small magnetizing current is achieved with core material with high permeability and by avoiding an air gap. The core halves of a forward converter are flat ground, which are processed on the bearing surfaces without contamination with braces compressed core halves without gap.

The magnetization energy can also be reduced by increasing the number of turns for a given operating frequency, which however, would like to avoid in the interest of lower copper loss. To improve the efficiency of the transformer of the Eintaktflusswandler is therefore usually provided with an additional demagnetizing.

Entmagnetisierungwicklung

The additionally introduced winding N2 in the adjacent circuit diagram used for demagnetization of the transformer core in the blocking phase and executes the stored energy in the transformer via the diode D1 in the supply voltage or C1 back. Have the primary winding N1 and N2 the demagnetizing the same number of turns, the ratio between ON and OFF phase ( pulse-width ratio ) is limited to a maximum of 50 %.

The demagnetizing winding N2 and D1 are for the function is not absolutely necessary, but they increase the efficiency and reduce the voltage stress of the switching transistor T1. Alternative to such a winding, the magnetizing energy of the transformer can also be degraded via a catch circuit of diode, resistor and capacitor or in the limit of thermally only with the avalanche breakdown of the MOSFET safely.

Push-pull forward converter like the half and full bridge forward converter require no Entmagnetisierungwicklung, her pulse width ratio is, however, generally limited to 50%.

The output voltage depends on the load and must be controlled by a not shown in the picture, feedback.