Texas Instruments UCC2891 User Manual
307.97 Kb

48-Vto3.3-VForward Converter with Active Clamp Reset Using the UCC2891 Active Clamp Current Mode PWM Controller

User's Guide

December 2006

Power Supply MAN


48-Vto3.3-VForward Converter with Active Clamp Reset Using the UCC2891 Active Clamp Current Mode PWM Controller

User's Guide

Literature Number: SLUU178A

November 2003 –RevisedDecember 2006

User's Guide

SLUU178A – November 2003 – Revised December 2006

Using the UCC2891 Active Clamp Current Mode PWM



The UCC2891EVM evaluation module (EVM) is a forward converter providing a 3.3-Vregulated output at 30 A of load current, operating from a48-Vinput. The EVM operates over the full 36 V to 72 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-sourcevoltage 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 89%. The EVM displays many features that might be typical of a more complex design, yet its compact board layout and low component count make it elegantly simple.


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,orN-channelMOSFET in either ahigh-sideorlow-sideactive clamp configuration. The UCC2891 also allows the ability tostart-updirectly from the48-Vtelecom bus voltage, eliminating the need for externalstart-upcircuitry. It includes programmable soft start, internal slope compensation for peak current mode control, internallow-linevoltage sensing, internal syncronizable clock input,cycle-by-cyclecurrent limiting, and a robust2-Asink/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'sunique TrueDrive™ Bipolar/CMOS output. To the user, this simply means ultra-fastrise 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-pinSOIC or16-pinTSSOP package for applications where absolute minimal board space is required.

The UCC2891EVM highlights the many benefits of using the UCC2891 active clamp current mode PWM controller. This user'sguide provides the schematic, component list, assembly drawing, artwork and test set up necessary to evaluate the UCC2891 in a typical telecom application. More detailed design information can be found listed in the References section.

SLUU178A –November2003–RevisedDecember 2006

Using the UCC2891 Active Clamp Current Mode PWM Controller


Submit Documentation Feedback




The UCC2891 is suited for use in isolated telecom 48-Vinput systems requiringhigh-efficiencyandhigh-powerdensity for verylow-outputvoltage,high-currentconverter applications, including:

Server Systems





The UCC2891EVM features include:

ZVS transformer reset using active clamp technique in forward converter

All surface mount components, double sided half brick (2.2 × 2.28× 0.5) inches

Complementary auxilliary drive for active clamp with programmable dead time for ZVS

Current mode control with synchronization function

Internal PWM slope compensation

Start-updirectly from telecom input voltage

Synchronous rectifier output stage allows high-efficiencyoperation

Programmable soft-start

Up to 30-Adc output current

Regulation to zero load current

Non-latching,output overcurrent and short circuit protection

Non-latching,Input undervoltage protection

1500-Visolation primary to secondary


Using the UCC2891 Active Clamp Current Mode PWM Controller

SLUU178A –November2003–RevisedDecember 2006

Submit Documentation Feedback