SLUS329B − JUNE 1998 − REVISED FEBRUARY 2005
UC1854B
UC2854A, UC2854B
UC3854A, UC3854B
Current Amplifier
The current amplifier for an average current PFC controller needs a low offset voltage in order to minimize AC
line current distortion. With this in mind, the UC3854A/B current amplifier has improved the input offset voltage
from
±4
mV to 0 V to
±3
mV. The negative offset of the UC3854A/B guarantees that the PWM circuit will not drive
the MOSFET is the current command is zero (both current amplifier inputs zero.) Previous designs required an
external offset cancellation network to implement this key feature. The bandwidth of the current amplifier has
been improved as well to 5 MHz typical. While this is not generally an issue at 50 Hz or 60 Hz inputs, it is
essential for 400 Hz input avionics applications
Miscellaneous
Several other important enhancements have been implemented in the UC3854A/B. A V
CC
supply voltage clamp
at 20 V allows the controller to be current fed if desired. The lower startup supply current (250 mA typical),
substantially reduces the power requirements of an offline startup resistor. The 10.5 V/10 V UVLO option
(UC3854B) enables the controller to be powered off of an auxiliary 12 V supply.
The VREF GOOD comparator guarantees that the MOSFET driver output remains low if the supply of the 7.5 V
reference are not yet up. This improvement eliminates the need for external Schottky diodes on the PKLMT and
Mult Out pins that some UC3854 designs require. The propagation delay of the disable feature has been
improved to 300 ns typical. This delay was proportional to the size of the VREF capacitor on the UC3854, and
is typically several orders of magnitude slower.
8
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