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Application Report
SNVA686–March 2013
LM5023-2EVM Notebook Adapter
Terry Allinder...................................................................................................................................
ABSTRACT
Contents
1 Introduction .................................................................................................................. 1
2 Theory of Operation ........................................................................................................ 1
3 Electrical Performance Specifications .................................................................................... 3
4 Schematic .................................................................................................................... 4
5 Test Setup ................................................................................................................... 4
6 Powering Up ................................................................................................................. 4
7 Performance Characteristics .............................................................................................. 5
7.1 Over Current Protection ........................................................................................... 5
8 Performance Data and Typical Characteristic Curves ................................................................. 5
8.1 Efficiency ............................................................................................................ 5
8.2 Startup at 115Vac Input ........................................................................................... 7
8.3 QR Switching Waveforms ......................................................................................... 8
8.4 Skip Cycle ........................................................................................................... 8
8.5 Output Voltage Ripple ............................................................................................. 9
8.6 Transient Response ............................................................................................... 9
8.7 Implementing Synchronous Rectification ...................................................................... 10
9 BILL OF MATERIALS ..................................................................................................... 13
9.1 PWB Layout ....................................................................................................... 16
1 Introduction
The LM5023-2 is a 65 W evaluation board for off-line Notebook Adapter applications. The LM5023-2EVM
is intended for evaluation purposes only and not intended to be an end product. The LM5023-2EVM takes
the 90-264 Vac input and converters it to an isolated 19 V DC output with a maximum load current of 3.43
A.
2 Theory of Operation
A Quasi-Resonant Controlled Flyback converter operates by storing energy in the transformers primary
during the MOSFETs on-time. During the on-time (ton) VIN is applied across the primary of the
transformer. The primary current starts out at zero and ramps towards Ipeak. When the peak primary
current reaches the feedback compensation voltage the PWM comparator resets the output drive, turning-
off the MOSFET. Due to the phasing of the transformer, the output diode is reversed biased.
During the MOSFETs off time the output diode is forward biased and the stored energy in the transformer
primary inductor is transferred to the output. The voltage seen on the secondary inductor is V
OUT
plus the
output diodes forward voltage drop, V
F
. The current in the output inductor linearly decreases from Ipeak •
Ns/Np to zero.
When the current in the secondary reaches zero, the transformer is demagnetized, and there is an open
circuit on the secondary, and with the primary MOSFET turned-off, there is an open on the primary. A
resonant circuit is formed between the transformers primary inductance and the MOSFET output
capacitance. The resonant frequency is calculated by:
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SNVA686–March 2013 LM5023-2EVM Notebook Adapter
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