Datasheet

User's Guide

SLUU407A–June 2010–Revised May 2013

48-V to 3.3-V Forward Converter

With Active-Clamp Reset Using the UCC2891 Active-

Clamp Current-Mode PWM Controller

1 Introduction

The UCC2891EVM-520 evaluation module (EVM) is a forward converter providing a 3.3-V regulated

output at 30 A of load current, operating from a 48-V input. The EVM operates over the full 36-V to 75-V

telecom-input range, and is able to fully regulate down to zero-load current. The module uses the

UCC2891 current-mode active-clamp PWM controller for effectively demonstrating the active-clamp

transformer-reset technique.

Benefits of the active clamp include a control-driven transformer-reset scheme allowing zero-voltage

switching (ZVS) to increase overall efficiency, lower drain-to-source voltage stress, extended duty cycle

beyond 50%, and reduced electromagnetic radiated emissions. Combined with synchronous rectification,

this EVM is configured to operate at 300 kHz and exhibits a peak efficiency of just over 92%, with a full

load efficiency of 90%. The EVM displays many features that might be typical of a more complex design,

yet the compact board layout and low component count make it elegantly simple.

This EVM features active-current limiting on the output. Because the unit continues to operate into a short

circuit or overload condition the operator must shut the unit off before the thermal limitations of the

components are exceeded.

2 Description

The UCC2891 controller family provides advanced active-clamp control features such as programmable

maximum duty-cycle clamp, programmable dead time between the two primary switches and the ability to

drive either a P-channel, or N-channel MOSFET in either a high-side or low-side active-clamp

configuration. The UCC2891 also allows the ability to start-up directly from the 48-V telecom-bus voltage,

eliminating the need for external start-up circuitry. The EVM includes programmable soft start, internal

slope compensation for peak current-mode control, internal low-line voltage sensing, internal

syncronizable-clock input, cycle-by-cycle current limiting, and a robust 2-A sink/source TrueDrive™

internal gate-drive circuit. The result is a highly efficient design loaded with features, requiring very few

external components.

The TrueDrive™ hybrid output architecture used in the UCC2891 uses TI's unique TrueDrive Bipolar-

CMOS output. To the user, TrueDrive simply means ultra-fast rise and fall times by providing the highest

possible drive current where it is needed most, at the MOSFET Miller plateau region.

The UCC2891/2/3/4 is available in either a 16-pin SOIC or 16-pin TSSOP package for applications where

absolute minimal board space is required.

The UCC2891EVM-520 highlights the many benefits of using the UCC2891 active-clamp current-mode

PWM controller. This user's guide provides the schematic, component list, assembly drawing, artwork, and

test setup necessary to evaluate the UCC2891 in a typical telecom application. More detailed design

information is listed in the Section 10 section.

48-V to 3.3-V Forward Converter With Active-Clamp Reset Using the SLUU407A–June 2010–Revised May 2013

UCC2891 Active-Clamp Current-Mode PWM Controller

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