UCC28740_15 Datasheet, PDF (14/37 Pages) Texas Instruments – UCC28740 Constant-Voltage Constant-Current Flyback Controller Using Opto-Coupled Feedback

UCC28740
SLUSBF3B – JULY 2013 – REVISED DECEMBER 2014
www.ti.com
Device Functional Modes (continued)
The UCC28740 VS-signal sampler includes signal-discrimination methods to ensure an accurate sample of the
output voltage from the auxiliary winding. Controlling some details of the auxiliary-winding signal to ensure
reliable operation is necessary; specifically, the reset time of the leakage inductance and the duration of any
subsequent leakage-inductance ringing. See Figure 14 for a detailed illustration of waveform criteria to ensure a
reliable sample on the VS pin.
The first detail to examine is the duration of the leakage-inductance reset pedestal, t_{LK_RESET}, in Figure 14.
Because t_{LK_RESET} mimics the waveform of the secondary-current decay, followed by a sharp downslope,
t_{LK_RESET} is internally blanked for a duration which scales with the peak primary current. Keeping the leakage-
reset time to less than 600 ns for I_PP(min), and less than 2.2 µs for I_PP(max) is important.
The second detail is the amplitude of ringing on the V_AUX waveform following t_{LK_RESET}. The peak-to-peak voltage
variation at the VS pin must be less than 100 mVp-p for at least 200 ns before the end of the demagnetization
time (t_DM). A concern with excessive ringing usually occurs during light or no-load conditions, when t_DM is at the
minimum. The tolerable ripple on VS is scaled up to the auxiliary-winding voltage by R_S1 and R_S2, and is equal to
100 mV × (R_S1 + R_S2) / R_S2 .
Figure 14. Auxiliary-Winding Waveform Details
During voltage regulation, the controller operates in frequency-modulation mode and amplitude-modulation
mode, as shown in Figure 15. The internal operating-frequency limits of the device are 100 kHz and f_SW(min). The
maximum operating frequency of the converter at full-load is generally chosen to be slightly lower than 100 kHz
to allow for tolerances, or significantly lower due to switching-loss considerations. The maximum operating
frequency and primary peak current chosen determine the transformer primary inductance of the converter. The
shunt-regulator bias power, output preload resistor (if any), and low-power conversion efficiency determine the
minimum-operating frequency of the converter. Voltage-loop stability compensation is applied at the shunt-
regulator which drives the opto-coupled feedback signal. The tolerances chosen for the shunt-regulator reference
and the sense resistors determines the regulation accuracy.
14
Submit Documentation Feedback
Product Folder Links: UCC28740
Copyright © 2013–2014, Texas Instruments Incorporated