AN3400 datasheet(13/78 Pages) STMICROELECTRONICS

13 / 78 page

AN3400

Principle of self-oscillating operation

Doc ID 018840 Rev 1

13/78

Figure 5.

Steady-state waveforms: filter capacitor voltage signal (V(t)),

NPN base-emitter voltage (VBEnpn) and PNP emitter-base voltage(VEBpnp)

Figure 6.

Steady-state waveforms: filter capacitor voltage signal (V(t)), NPN base

series capacitor voltage (VCnpn) and PNP base series capacitor voltage

(VCpnp)

In Figure 2 the filter capacitor voltage V(t) and the base currents Ibnpn and Ibpnp of the two

bipolars during a period of the voltage signal filtered are shown. Two different signal

thresholds fix respectively the turn-on and turn-off instants of the devices during their

working operation period. In particular, in the previous figure it is evident that, starting from

the driving source voltage, the turn-on of the NPN transistor is possible only when the V(t) is

higher than the VCnpnon-char+VBEsatnpn value and similarly the turn-on of the PNP transistor

is possible only when the V(t) is lower than the -VCpnpon-char-VEBsatpnp value. On the

contrary, the turn-off of the NPN transistor is possible only when the V(t) is lower than the

VCnpnon-dischar+VBEsatnpn value and, similarly, the turn-off of the PNP transistor is possible

only when the V(t) is higher than the -VCpnpon-dischar-VEBsatpnp value.

Figure 3 shows the filter capacitor voltage V(t) and the collector currents ICnpn and ICpnp of

the two bipolars during a period of the voltage signal filtered. It can be seen as the collector

currents pass through the zero value at the end of the re-circulating phases in which the

whole residual external demagnetization current, imposed by the load inductor, has

AM09796v1

AM09797v1

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