Texas Instruments UCC2891 User Manual

48-V to 3.3-V Forward Converter with
Active Clamp Reset Using the
UCC2891 Active Clamp Current Mode
PWM Controller

User's Guide

December 2006 Power Supply MAN

SLUU178A

48-V to 3.3-V Forward Converter with Active

Clamp Reset Using the UCC2891 Active

Clamp Current Mode PWM Controller

User's Guide

Literature Number: SLUU178A

November 2003 – Revised December 2006

User's Guide

SLUU178A – November 2003 – Revised December 2006

Using the UCC2891 Active Clamp Current Mode PWM

Controller

1 Introduction

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.

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. 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's unique 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's guide 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.

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Description

2.1 Applications

The UCC2891 is suited for use in isolated telecom 48-V input systems requiring high-efficiency and
high-power density for very low-output voltage, high-current converter applications, including:

• Server Systems

• Datacom

• Telecom

• DSP's, ASIC's, FPGA's

2.2 Features

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-up directly from telecom input voltage

• Synchronous rectifier output stage allows high-efficiency operation

• Programmable soft-start

• Up to 30-A dc output current

• Regulation to zero load current

• Non-latching, output overcurrent and short circuit protection

• Non-latching, Input undervoltage protection

• 1500-V isolation primary to secondary

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3 UCC2891EVM Electrical Performance Specifications

The UCC2891EVM electrical performance specifications are listed in Table 1 .

PARAMETER TEST CONDITIONS MIN TYP MAX UNITS

Input Characteristics

Input voltage range 36 48 72 V
No load intput current VIN= 36 V, I
Maximum input current VIN= 36 V, I
Input voltage ripple VIN= 72 V, I

Input voltage ripple

Output voltage 36 V ≤ VIN≤ 72 V, 0 A ≤ I
Output voltage regulation Line regulation (36 V ≤ VIN≤ 72 V, I

Output voltage ripple VIN= 48 V, I
Output load current VIN= 48 V, I
Output current limit 36 V ≤ VIN≤ 72 V 32

Output current limit

Switching frequency 275 325
Control loop bandwidth 36 V ≤ VIN≤ 72 V, I
Control loop bandwidth 36 V ≤ VIN≤ 72 V, 2 A ≤ I
Peak efficiency 92%
Full load efficiency VIN= 48 V, I

Table 1. UCC2891EVM Performance Summary

= 0 A 75 100 mA

OUT

= 30 A 3.00 3.25 A

OUT

= 30 A 1.50 1.75 V

OUT

≤ 30 A 3.25 3.30 3.35 V

OUT

Load regulation (0 A ≤ I

= 30 A 30 35 mV

OUT

= 30 A 0 30

OUT

= 30 A 89%

OUT

≤ 30 A, VIN= 48 V) 0.060%

OUT

= 10 A 5 8

OUT

≤ 30 A 30 50 ° C

OUT

UCC2891EVM Electrical Performance Specifications

= 0 A) 0.003%

OUT

P-P

P-P

A

kHz

4 Schematic

A schematic of the UCC2891EVM is shown in Figure 1 . Terminal block J1 is the 48-V input voltage source
connector and J8 is the output and return for the 3.3-V output voltage.

On the primary side, U1 is the UCC2891 shown with the necessary discrete circuitry for configuring the
controller to operate at 300 kHz with the maximum duty clamp set for 0.65. The EVM is programmed to
start at VIN=36 V, as determined by R11 and R12. To minimize power dissipation in the current sense, a
current sense transformer, T1 is used, as opposed to simply using a sense resistor between the source of
Q2 and power ground. Q2 is the primary switching MOSFET and is selected based upon VDS and low
RDS(on). Q1 is the AUX (active reset) MOSFET and is selected based upon preferred package only, with
only minor consideration given for R
referenced, Q1 must be a P-channel type MOSFET. The reason for this is further explained in application
note SLUA299[2]. C9 is the clamp capacitor used to maintain a constant dc voltage. The input voltage is
subtracted from the clamp voltage to allow transformer reset during the active clamp period.

and Qg. Since the active clamp used in this design is low-side

DS(on)

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Schematic

Figure 1. UCC2891EVM Schematic

6 Using the UCC2891 Active Clamp Current Mode PWM Controller SLUU178A – November 2003 – Revised December 2006

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