Texas Instruments AN-1387 LM5026 User Manual

1 Introduction
The performance of the evaluation board is as follows:
• Input range: 36V to 78V
• Output voltage: 3.3V
• Output current: 0 to 30A
• Measured efficiency: 90% at 30A, 92.5% at 15A
• Frequency of operation: 230kHz
• Board size: 2.3 x 2.4 x 0.5 inches
• Load Regulation: 1%
• Line Regulation: 0.1%
• Line UVLO, Hiccup Current Limit
The printed circuit board consists of 4 layers of 3 ounce copper on FR4 material with a total thickness of
0.050 inches. Soldermask has been omitted from some areas to facilitate cooling. The unit is designed for
continuous operation at rated load at < 40°C and a minimum airflow of 200 CFM.
2 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 power switching cycle, a mechanism to reset the transformer is required.
The active clamp reset mechanism is presently finding 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 isolation 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
approximately 65% (low line) to 25% (high line). The clamp capacitor along with the reset switch reverse
biases the transformer primary each cycle when the main switch turns off. This reverse voltage resets the
transformer. The clamp capacitor voltage is Vin / (1-D).
The secondary rectification employs self-driven synchronous rectification to maintain high efficiency and
ease of drive.
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 COMP input to the LM5026
greatly increases the achievable loop bandwidth. The capacitance effect (and associated pole) of the
optocoupler is greatly reduced by holding the voltage across the optocoupler constant. The LM5026
current mode controller pulse width modulates the error signal with a ramp signal derived from the
transformer primary. A standard "type II" (pole-zero-pole) is used as a compensation network. The
LM5026 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
230 to 300KHz.
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SNVA117A – September 2005 – Revised May 2013
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AN-1387 LM5026 Evaluation Board
Copyright © 2005–2013, Texas Instruments Incorporated
SNVA117A – September 2005 – Revised May 2013
AN-1387 LM5026 Evaluation Board
User's Guide
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