SNYCHRONOUS RECTIFICATION

Prepared by:-

1) Hardik Sonaiya

2) Ketan Gohil

Guidance by:-

Prof. Rakesh Patel

CONTENT Introduction of Fly Back

ConverterIntroduction of Synchronous

rectificationHigh efficiency Fly Back

converter using synchronous rectification

Advantage of synchronous rectification

Conclusion

Flyback ConverterFlyback converters are very commonly used

in applications at low power leveis below 50 W.

The Flyback converter in Fig.(1) consists of two mutually-coupled coils, where the coil orientations are such that at the instant when the transistor is turned-off, the current switches to the second coil to maintain the same flux in the core.

Therefore, the dots on coils are as shown in Fig. 8-2b where the current into the dot of either coil produces core flux in the same direction.

Circuit Diagram

Waveforms

Introduction of Synchronous Rectification

In flyback converter with synchronous rectification topology ,the primary side gate signal has transfer to the secondary side of transformer with minimum delay .

By using synchronous rectification technology is implemented in a 15W ,3.3V@ 4.5A DC-DC converter, with efficiency reaching 90% at full load.

CIRCUIT DIAGRAM

Waveform

Circuit Operation

In figure (1) is represented the simplified schematic key waveforms for continuous mode flyback with synchronous rectification.

operation of synchronous rectification in flyback converter explain in four stage.

Stage -1(t0 – t1) :-At the moment of t0 , primary power

switch SW is turned off and magnetizing current will flow into the secondary winding , initially through the body diode , Dsr of the synchronous rectifier .

During conduction time of the body diode the voltage drop on the rectifier is large and as result during this time interval there is higher power dissipation .

One of the design goals is to minimize the t0 - t1 time interval.

Stage 2 (t1 – t2):-

At the moment of t1 , the synchronous rectifier is turned on and voltage drop will decrease as depicted by Vsr(t).

By design the voltage drop on the synchronous rectifier has to be much lower than the voltage drop on Schottky rectifier.

During this time interval the power is processed very efficiently.

Stage 2 (t2 – t3):-At moment t2 , synchronous rectifier

is turned off.The secondary will continue to flow

through the body diode of SR.During the conduction time of the

body diode the voltage drop on the rectifier is larger and as a result the is higher power dissipation .

One of the designs goals is to minimize also the t2 – t3 time interval.

Stage 2 (t3 – t4):-At the moment t3 , the power switch

is turned on.Through the synchronous rectifier

has been turned off at t2, the current continues to flow through the body diode.

At the moment of t3 , when the primary switch turns on there are still carriers in the junction of body diode.

Advantages of synchronous rectification

Eliminates the need for an additional magnetic element for information transfer from primary to secondary , which translate in higher power density and lower cost.

Improve the utilization of power transformer by offering an additional energy or (signal)transfer path without interference with main power processing path.

Using the large cross section of the power transformer, the leakage inductance of the signal transfer magnetic element is very low (due to reduced number of turns) and as result there is very low delay time signal transfer.

The additional winding complexity in transformer does not add additional cost due to the use of multilayer PCB implementation.

ConclusionOverall this method offers a

simple and low cost solution for the implementation of flyback topology with synchronous rectification

REFERNCES

Ionel Dan Jitaru ,“High Efficiency Flyback Converter using Synchronous Rectification” IEEE- 2012.

Pedro Alou, Jose A. Cobos, Member, IEEE, Oscar García, Member, IEEE, Roberto Prieto, Member, IEEE, and Javier Uceda, Senior Member, IEEE “A New Driving Scheme for Synchronous Rectifiers: Single Winding Self-Driven Synchronous Rectification.”IEEE-2011

THANK YOU