Datasheet
SLUS517C − DECEMBER 2002 − REVISED SEPTEMBER 2005
13
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APPLICATION INFORMATION
selection of controller options
The UCC2851x is optimized for the most common combination of PFC/PWM stages, which is a boost PFC stage
cascaded by a buck-derived PWM stage. Other topology combinations can be used with this controller, as well.
The programmable PWM duty ratio limit feature is especially useful when using two-transistor forward and
flyback topologies for the PWM stage. The PFC boost stage is typically designed for continuous conduction
mode (CCM) of operation at full rated load in order to minimize line filter requirements. The PWM stage can be
designed for either continuous or discontinuous mode operation, as necessary.
Eight different options are available for the UCC2851x. This device is available in two under voltage lock out
(UVLO) turn-on thresholds, two PWM UVLO hysteresis levels and, two combinations of PFC/PWM switching
frequencies as shown in Table 1.
Table 1. Available Options
UVLO TURN-ON
THRESHOLD
(V)
PWM
HYSTERESIS
(V)
PFC:PWM
FREQUENCY RATIO
THRESHOLD
(V)
HYSTERESIS
(V)
1:1 1:2
16
1.45
UCC28510 UCC28514
10.2
1.45
UCC28511 UCC28515
16
3.20
UCC28512 UCC28516
10.2
3.20
UCC28513 UCC28517
Select the UVLO option first, based on biasing topology. Then, select the PFC versus PWM switching frequency
based on the allowable switching loss of the intended PWM stage. Last, select the PWM UVLO option based
on bulk ripple voltage and load transients.
The UVLO turn-on threshold is selected based on line range, bias supply topology and gate drive voltage
requirements. The 16-V turn-on options are intended for applications where the bias voltage is self-generated
from an auxiliary winding, with little or no regulation. The 10.2-V turn-on / 0.5-V hysteresis options are intended
for applications where the bias voltage is derived from an auxiliary supply source and is regulated.
The PWM UVLO hysteresis level option is selected based on the desired operational range of the energy
storage capacitor voltage. A narrow range permits a highly optimized PWM stage. However, the wider range
permits larger energy storage capacitor voltage ripple and load transients.
Two options are available for the PFC:PWM switching frequency, 1:1 and 1:2. Both versions are synchronized
as LEM/TEM oscillators. The best minimization of the energy storage capacitor ripple current occurs with the
1:1 option. However, the diode in the PFC stage often has high reverse recovery currents that restrict the
switching frequency of the PFC stage. Situations where the switching losses of the PWM stage permit higher
switching frequencies can benefit from the 1:2 option. For example, the 1:2 option would be a good choice for
PWM stages that have Schottky diode output rectifiers. The energy storage capacitor ripple current for a system
that is controlled by the 1:2 option will be larger than if it were controlled by a 1:1 option. However the capacitor
current of the 1:2 option is less than a system that is TEM/TEM modulated.