AN1889 datasheet(7/33 Pages) STMICROELECTRONICS

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AN1889

Application description

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Application description

An auxiliary power supply can be schematized in its main functional parts as illustrated in

the simplified block schematic diagram shown in Figure 4.

Figure 4.

SMPS simplified block schematic diagram

Basically the input voltage is rectified and filtered first, then transferred to the primary side of

the transformer directly through the input circuit of the transformer. This part of the power

supply fixes the voltage clamping at the desired value, the start-up network is also present in

this block to ensure the functionality of the converter during the first pulses. The blocks

power switch and driver, by converting the waveforms from continuous state into alternated

high frequency, allow the transformer, generally operating in discontinuous fly-back mode, to

transfer the stored energy to its secondary side and then to the load through the output

circuit. This basically consists in an ultrafast p-n diode (or Shottky diode) + bulk output

capacitor fixing the maximum acceptable output ripple. When necessary, for a more precise

and lower ripple level, an additional LRC block, working as output post filter network, can be

added.

Power regulation can be achieved through the blocks input feedback (primary regulation) or

output feedback (secondary regulation). By giving to the driver information on the load

requirement, they adapt the input power varying the duty cycle accordingly.

This application note illustrates all design steps needed for the project of a 50W 3-phase fly-

back converter in discontinuous mode using an ESBT as primary switch.

In this application note the bridge diode + filtering will not be analyzed while the output

circuit will be just mentioned. Most of the attention has been focused on the transformer

design and on the optimization of the driving circuit of the ESBT to allow better performance

in practical applications.

Theoretical results had been validated through a realization of a demo board, its electrical

parameters and component will be shown step by step in the next paragraphs and summed

up at the end of the present note.

INPUT

TRANSFORMER + INPUT/OUTPUT CIRCUIT

Bridge

filter

Input

Output

Power

switch

Driver

Output

Feedback

Input

Feedback

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