UCC28510_13 Datasheet, PDF (15/42 Pages) Texas Instruments – ADVANCED PFC/PWM COMBINATION CONTROLLERS

UCC28510, UCC28511, UCC28512, UCC28513,
UCC28514, UCC28515, UCC28516, UCC28517
SLUS517C − DECEMBER 2002 − REVISED SEPTEMBER 2005
APPLICATION INFORMATION
Select the regulation constant, k_1R, of the energy storage voltage, as described by equation 2.
ƪ ƫ k_1R ^
0.29 for UCC28510, UCC28511, UCC28514, UCC28515
0.53 for UCC28512, UCC28513, UCC28516, UCC28517
(2)
There are effectively two options for k_1R that directly relate to the two PWM hysteresis options, k_1R = 0.29 and
k_1R = 0.53. Select the large PWM hysteresis option if the system load has large, sudden step changes during
steady state operation. Select the small PWM hysteresis option if the system load has moderate step changes
or slow load changes during steady state operation. The PWM stage can be optimized best with the small PWM
hysteresis range because the maximum primary current of transformer T1 (which occurs at minimum V_C1) is
smallest with the small PWM hysteresis range.
Select an approximate switching frequency for the PFC stage. A good starting frequency for a MOSFET based
PFC stage is in the range of 100 kHz to 200 kHz, depending on maximum line voltage and maximum line current.
Adjustments in switching frequency may result from meeting switching loss requirements in Q1 and D3, or in
order to optimize the design of L1.
Select an appropriate topology for the PWM stage using the information about the power requirements and the
magnitude of V_C1. For simplicity, the typical application in Figure 1 shows a flyback converter in the PWM stage.
In most cases, the PWM stage topology must have transformer isolation and the topology must require only one
pulse-width signal. Topologies that have these features include:
• single-transistor forward converter
• single-transistor flyback converter
• two-transistor forward converter
• two-transistor flyback converter
Estimate the nominal and the maximum duty ratios of the PWM stage (D_2(nom) , D_2(max) and the associated peak
Q2 drain current, i_Q2(peak)), based on the topology, PWM hysteresis option and output voltage requirements of
the PWM stage. Also estimate whether or not it is appropriate to operate the PWM stage at the same switching
frequency as the PFC stage or if the PWM stage can operate at twice the switching frequency of the PFC stage.
Base the estimation for the switching frequency of the PWM stage on the maximum voltages and currents of
the power MOSFETs and power diodes. Program the oscillator frequency of the PWM stage with the value of
R20.
ǒ Ǔ R20 +
31
1W
10^*12
1 Hz
f_S(pwm)
*
2.0
10^*7
(3)
Most applications require that the PWM stage regulates at the minimum energy storage capacitance voltage.
Maximum duty ratio D_2(max) and i_Q2(peak) should be calculated for the minimum energy storage voltage in order
to estimate the peak current stresses for transformer T1 and any other inductive element in the PWM stage.
V_C1(min) + ^ǒ1 * k_1R^Ǔ V_C1(nom)
(4)
At this point, enough information is available to estimate which member of the UCC28510 family should be
selected.
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