UCC28070-Q1
SLUSA71A –JULY 2010–REVISED JUNE 2011
www.ti.com
Advanced Design Techniques
Current Loop Feedback Configuration
(Sizing of the Current Transformer Turns Ratio and Sense Resistor (RS)
A current-sense transformer (CT) is typically used in high-power applications to sense inductor current while
avoiding significant losses in the sensing resistor. For average current-mode control, the entire inductor current
waveform is required; however low-frequency CTs are obviously impracticable. Normally, two high-frequency
CTs are used, one in the switching leg to obtain the up-slope current and one in the diode leg to obtain the
down-slope current. These two current signals are summed together to form the entire inductor current, but this
is not the case for the UCC28070.
A major advantage of the UCC28070 design is the current synthesis function, which internally recreates the
inductor current down-slope during the switching period off-time. This eliminates the need for the diode-leg CT in
each phase, significantly reducing space, cost and complexity. A single resistor programs the synthesizer down
slope, as previously discussed in the Current Synthesizer section.
A number of trade-offs must be made in the selection of the CT. Various internal and external factors influence
the size, cost, performance, and distortion contribution of the CT.
These factors include, but are not limited to:
•
•
•
•
•
•
•
•
•
Turns-ratio (NCT)
Magnetizing inductance (LM)
Leakage inductance (LLK)
Volt-microsecond product (Vμs)
Distributed capacitance (Cd)
Series resistance (RSER
External diode drop (VD)
External current sense resistor (RS)
External reset network
)
Traditionally, the turns-ratio and the current sense resistor are selected first. Some iterations may be needed to
refine the selection once the other considerations are included.
32
Copyright © 2010–2011, Texas Instruments Incorporated
TI [ TEXAS INSTRUMENTS ]
