AN2744 datasheet(43/56 Pages) STMICROELECTRONICS

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AN2744

Board description

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The blocking diode D4 and the clamping network (R1-C1) clamp the peak of the leakage

inductance voltage spike, assuring reliable operation of the VIPer12AS-E. D4 must be not

only very fast-recovery but also very fast turn-on type to avoid additional drain overvoltage.

The clamp capacitor C1 must be low-loss type (with polypropylene or polystyrene film

dielectric) to reduce power dissipation and prevent overheating, since it is charged with high

peak currents by the energy stored in the leakage inductance.

Also a leading edge blanking (LEB) circuit for leakage inductance spikes filtering has been

implemented (Q3 - C30 - R23). It blanks the spike appearing at the leading edges of the

voltage generated by the self-supply winding, greatly improving the behavior in short-circuit.

The output rectifiers have been selected considering the maximum reverse voltage and the

RMS secondary current. A STPS1H100 Power Schottky rectifier has been chosen for this

purpose.

A LC filter has been added on the output (made of L1 and C4) in order to filter the high

frequency ripple without increasing the output capacitors size or quality.

The transformer used for this application has three windings, since one of them is needed to

supply the VIPer12AS-E. The primary inductance has been chosen at 2.7 mH and the

reflected voltage has been set to 80 V.

A layer type has been chosen, with EF12.6 or E13/7/4 core. The characteristics are listed in

Table 11

In the following pictures some significant waveforms are represented. Figure 40 and

Figure 41

show typical waveforms in both open load and full load conditions.

An important behavior in any SMPS is the protection against output short circuit. All tests

have been done by shorting the SMPS output at maximum input voltage. The results are

shown in Figure 42.

The main parameters are the drain-source voltage (VDS), the output current (IOUT) and the

supply voltage (VDD).

The output current is an important parameter to be checked during shorts. Although the

output current peaks are quite high, the mean value is very low, thus preventing component

melting for excessive dissipation. In this way, the output rectifier, transformer windings and

PCB traces won't be overstressed. This assures system reliability against long-term shorts.

Besides, in case of device overheating, the integrated thermal protection stops the device

operation until the device temperature falls.

Table 11.

SMPS transformer specifications

Parameter

Value

Core Geometry

SRW12.6ES or E13/7/4

Primary Inductance

2.7 mH±10%

Leakage Inductance

180 µH max

224 turns – 0.1mm

NAUX

39 turns – 0.1mm

NSEC

31 turns – 0.2mm (TEX-E wire)

Withstanding Voltage

4 kVRMS

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