Application Report

SLUA312 ? May 2004

1

200-W Interleaved
Forward Converter Design Review

Using TI’s UCC28221
PWM Controller

Michael O’Loughlin
System Power

ABSTRACT

Interleaved buck converters are widely
used in the personal computer industry in voltage

regulation module (VRM) applications to
power central processing units, CPUs, like the

Pentium 4? and Athlon?. This topology is widely used due to the reduced input and
output

capacitor ripple current that is gained
by interleaving the converters as compared to a single

buck power stage. The reduction in
input and output capacitor RMS currents allows the

designer to reduce the input and output
capacitor banks that are required for the design. The

same benefits that are gained from
interleaving buck converters can be gained from

interleaving forward converters. In
high current applications such as telecom dc-to-dc

converters reducing the input and
output capacitor banks can reduce the size and cost of the

design. This application note reviews
the design of a telecom converter that converts a

telecom input range of 36 V to 75 V dc
down to a regulated 12-V, 200-W dc output. The design

information in this application note is
used in the design of the UCC28221 Evaluation Module

(EVM) HPA035 [5].

Table 1. Applicable
Devices

MIN TYP MAX UNITS

Input voltage (VIN) 36 75 V

Output voltage (VOUT) 11.4 12 12.6 V

Output power (POUT) 50 200 W

Switching frequency (fS) 500 kHz

Efficiency at maximum output power (ç) 85%

Maximum duty cycle (DMAX) 50%

Input voltage ripple (VIN(ripple)) 1 V

Output voltage ripple (VOUT(ripple)) 200
mV

NOTES:This design was based on typical
design values.

SLUA312 ? May 2004

2 200-W Interleaved Forward Converter
Design Review Using TI’s UCC28221 PWM Controller

1 Schematic

+

+

+

1 N/C

2

INA

3 GND

4

INB

5 OUTB

6 VDD

7 OUTA

8 N/C

UCC27324D

U2

+ +

Figure 1. Typical
Application Diagram

SLUA312 ? May 2004

200-W Interleaved
Forward Converter Design Review Using TI’s UCC28221 PWM Controller 3

2 Power Stage Design

2.1 Transformer Turns Ratio (T3 and T4)

The first step is to calculate the
required transformer turns ratio required for the design. The

following equations are used to
calculate the approximate transformer turns ratio. With a

maximum duty cycle of 0.5 at minimum
input voltage (VIN(min)) the calculated turns ratio for the

design is roughly 1.4.

a _

NP

NS

a _ DMAX

VIN(min)_1 V

VOUT_1 V _ 1.4

2.2 Output Inductor Selection (L2 and
L3)

The output inductor was sized based on
the worst case ripple current that occurs at minimum

duty cycle, DMIN, and a maximum output
power, POUT(max), of 200 W. The output
capacitor ripple

current cancellation of interleaving
two forward converters allowed the output inductors LOUT to

be designed for a ripple current that
was roughly 60% of the maximum load current. For this

design we used 3.2-?H low profile inductors from Vishay, part number

IHLP?5050FD?RZ?3R3?M?01.