C2471 Datasheet, PDF (9/18 Pages) List of Unclassifed Manufacturers – RDFC Controllers for Offline Applications up to 6 W

C2471 Datasheet
RDFC Controllers for Offline Applications up to 6 W
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 I_BASMAX for a time T_FON (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, T_PBD, the so called “proportional base drive” (PBD) current is
referred to as I_BASPBD.
Aux Supply
Bypass transistor
Qon
AUX
Raux
FON
PBD
BAS
Q1
T_PBD
Qoff
GND
0V
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 T_FON and
I_BASPBD during the remainder of the on-period. Transistor Qoff provides a low-resistance (R_BASCLAMP) path to
GND during the off-period to ensure rapid turn-off of the primary switch, Q1. I_BASPBD is set by the PBD system
within the controller but I_BASMAX is determined by the external resistor Raux and the Aux Supply voltage.
I_BASMAX
I_BAS
V_CE
T_FON
T_PBD
Figure 8: Base Driver Current Waveforms
Product data
© Cambridge Semiconductor Ltd 2008
Page 9 of 18
DS-1639-0805
02-May-2008