AN3400 datasheet(22/78 Pages) STMICROELECTRONICS

22 / 78 page

Steady-state stage-wise circuit analysis

AN3400

22/78

Doc ID 018840 Rev 1

where ICCBn is the charging current of the NPN bipolar B-C junction capacitance and ICBCp

is the discharging current of the PNP bipolar C-B junction capacitance.

●

Stage 5: t6<t<t8: (re-circulating phase before the PNP conduction time:

-VCpnpon-char -VEBsatpnp< V(t)<0 with

)

As done for the re-circulating phase before the NPN conduction time (t0<t<t2), this phase, in

which both Q1-Q2 switches are inactive, can be subdivided into a further two sub-phases

distinguished by the following:

–

First re-circulating phase before the PNP conduction time (t6<t<t7): both Q1-Q2

switches are turned off and the load inductor residual magnetization current ILP

circulates freely through the PNP transistor anti-parallel diode D2 and the directly

biased C-B junction of the PNP bipolar Q2. Voltage across the filter capacitor C

increases in the opposite polarity than the NPN turn-on phase and charges the

capacitor Cp in series to the not yet forward biased base of the PNP, assuming the

same polarity of the relevant VEBonpnp, while the capacitor Cn begins to decrease

its charge through the breakdown resistance Rb. In this re-circulating stage,

currents and voltages on the driving network are represented by the following

Figure 13

Figure 13.

Fifth stage: first re-circulating phase before the PNP conduction time

According to the agreement of the previous image, the following equations are valid in the

driving section during the first re-circulating stage for t6<t<t7:

()

() 0

AM09804v1

First re-circulating phase

before PNP conduction

VRs

IRs

VCn

VCp

ICn

ICp

IRb

VRb

ID2

ILP

VDD

VCs

GND

PNP collector-base

diode conduction

PNP collector-base

diode conduction

ICBp

VD2

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