User's Guide
SLUU178A – November 2003 – Revised December 2006
Using the UCC2891 Active Clamp Current Mode PWM Controller
1Introduction
The UCC2891EVM 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 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-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 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.
2Description
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. It 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'sunique TrueDrive™ Bipolar/CMOS output. To the user, this 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 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 –November 2003 –Revised December 2006 | Using the UCC2891 Active Clamp Current Mode PWM Controller | 3 |
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