LM5027A Datasheet, PDF (1/28 Pages) National Semiconductor (TI) – Evaluation Board evaluation board is designed to provide

LM5027A Evaluation Board
National Semiconductor
Application Note 2067
Terry Allinder
June 15, 2010
Introduction
The LM5027A evaluation board is designed to provide the
design engineer with a fully functional power converter based
on the Active Clamp Forward topology to evaluate the
LM5027A controller. The evaluation board is provided in an
industry standard quarter-brick footprint.
The performance of the evaluation board is as follows:
Input Operating Range: 36 to 78V (100V peak)
Output Voltage: 3.3V
Output Current: 0 to 30A
Measured Efficiency: 90.5% @ 30A, 92.5% @ 15A
Frequency of Operation: 250 kHz
Board Size: 2.3 X 1.45 x 0.5 inches
Load Regulation: 1%
Line Regulation: 0.1%
Line UVLO, Hiccup Current Limit
A 70% Maximum Duty Cycle
The printed circuit board consists of 6 layers of 2 ounce cop-
per on FR4 material with a total thickness of 0.050 inches.
The unit is designed for continuous operation at rated load at
<40°C and a minimum airflow of 200 CFM.
Theory of Operation
Power converters based on the Forward topology offer high
efficiency and good power handling capability in applications
up to several hundred Watts. The operation of the transformer
in a forward topology does not inherently self-reset each pow-
er switching cycle; a mechanism to reset the transformer is
required. The active clamp reset mechanism is presently find-
ing extensive use in medium level power converters in the 50
to 200W range.
The Forward converter is derived from the Buck topology
family, employing a single modulating power switch. The main
difference between the topologies are, the Forward topology
employs a transformer to provide input / output ground isola-
tion and a step down or step up function.
Each cycle, the main primary switch turns on and applies the
input voltage across the primary winding, which has 12 turns.
The transformer secondary has 2 turns, leading to a 6:1 step-
down of the input voltage. For an output voltage of 3.3V the
required duty cycle (D) of the main switch must vary from ap-
proximately 60% (low line) to 25% (high line). The LM5027A
limits the PWM duty cycle output to a maximum of 70% (typ-
ical). The maximum duty cycle limits the voltage stress on the
Active Clamp Forward converter MOSFETs. The clamp ca-
pacitor along with the reset switch reverse biases the trans-
former primary each cycle when the main switch turns off.
This reverse voltage resets the transformer. The clamp ca-
pacitor voltage is Vin / (1-D).
Feedback from the output is processed by an amplifier and
reference, generating an error voltage, which is coupled back
to the primary side control through an optocoupler. The
LM5027A voltage mode controller pulse width modulates the
error signal with a ramp signal derived from the input voltage.
Deriving the ramp signal slope from the input voltage provides
line feed-forward, which improves line transient rejection. The
LM5027A also provides a controlled delay necessary for the
reset switch. The evaluation board can be synchronized to an
external clock with a recommended frequency range of 275
to 300 kHz.
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