AN1889 datasheet(8/33 Pages) STMICROELECTRONICS

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Application description

AN1889

8/33

2.1

Pre-design requisite

Figure 5 below reported shows the simplified fly-back schematic plus the voltage qualitative

behavior (not in scale) that the switch has to sustain during turn-off operation (point T in the

figure).

Figure 5.

Simplified fly-back schematic diagram

VIN is the maximum input rectified voltage that in 3-phase auxiliary power supplies can be

as high as 750 V.

Vfl is the reflected fly-back voltage; this is one of the most important parameter in the

transformer design since the input-to-output turn ratio strongly depends on it. The voltage

ringing across VIN is generated by the primary inductance that resonates with the equivalent

capacitance at the point T in the figure. This capacitance is mainly due to parasitic

capacitances of the transformer and main switch.

Vspike is the voltage generated by the leakage inductance of the transformer at primary side.

It must be noted that the energy generated by the mutual inductance at the primary of the

transformer cannot be transferred to the secondary until the leakage inductance is fully

demagnetized. It is the clamping circuit that fixes the value of the voltage at which the

demagnetization process occurs. The voltage ringing in the figure across Vfl is generated by

the leakage inductance that resonates with the equivalent capacitance at point T mentioned

before.

The highest overvoltage peak is due to the delay of the diode in the clamping circuit. A good

safety margin must take care of this overvoltage and some transient spikes at start-up.

Transformer design is the major part in a power supply project. A few degrees of freedom

are available to the designer starting from the target data, and most of them are linked each

other so that the final project is often achieved through a re-iteration process.

The table below lists the converter specification data and the first parameters that a

designer has to fix from scratch when a new project of a fly-back converter is going to be

approached. In the last column the values chosen for the present board are reported.

Clamping circuit

Main switch

V IIN

V ffll

V SSppiikkee

giin

Offff

olltta

riin

giin

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