C2473 datasheet(9/18 Pages) ETC2

9 / 18 page

C2472, C2473 and C2474

Datasheet

RDFC Controllers for Offline Applications

Product data

Page 9 of 18

DS-1423-0709C

26-Sep-2007

Optimised Base Drive

To minimize losses in the primary switching transistor (Q1) its base current is carefully controlled. To

minimize turn-on losses, the base current is initially forced to a maximum value IBASMAX for a time TFON (the

force-on or “FON” pulse). For the remainder of the on-time the base current is reduced to a lower value such

that the on-state collector voltage is maintained at a preset target voltage, thereby minimizing turn-off time

and consequent losses. During this period, TPBD, the so called “proportional base drive” (PBD) current is

referred to as IBASPBD.

Figure 7: Primary Switch (Q1) Base Drive

The BAS pin (see Figure 7) is driven by two transistors, Qon and Qoff. Qon provides I BASMAX during TFON and

IBASPBD during the remainder of the on-period. Transistor Qoff provides a low-resistance (RBASCLAMP) path to

GND during the off-period to ensure rapid turn-off of the primary switch, Q1. IBASPBD is set by the PBD system

within the controller but IBASMAX is determined by the external resistor Raux and the Aux Supply voltage.

Figure 8: Base Driver Current Waveforms

IBAS

VCE

TFON

TPBD

IBASMAX

AUX

BAS

Raux

GND

Qon

Qoff

FON

PBD

Aux Supply

Bypass transistor

TPBD

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