Datasheet

UCC28070

www.ti.com

SLUS794E –NOVEMBER 2007–REVISED APRIL 2011

Advanced Design Techniques

Current Loop Feedback Configuration

(Sizing of the Current Transformer Turns Ratio and Sense Resistor (R

)

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 (N

)

• Magnetizing inductance (L

)

• Leakage inductance (L

)

• Volt-microsecond product (Vμs)

• Distributed capacitance (C

)

• Series resistance (R

SER

)

• External diode drop (V

)

• External current sense resistor (R

)

• 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.