Theory Of Operation - TDK-Lambda CM4 Application Notes

1. Theory of Operation

The diagram below outlines the topology and major internal components of a fully assembled CM4 configurable power supply. Four output slots
are provided and can be populated by any combination of output modules. The remaining components in the block diagram are housed in the
input and transformer modules.
The input module is responsible for receiving the AC mains line voltage and converting it to an appropriate DC voltage whilst providing protection
from AC line disturbances and preventing excessive EMI emissions and current harmonics. The integrated EMI filter attenuates high frequency
current emissions to levels below EN55032 class B. It also provides dual pole fusing, one fuse in each conductor and protection from line
disturbances as outlined in EN61000.
Inrush current is controlled by a resistive element upon initial connection to the AC line. Once the internal capacitances have been charged, the
resistive element is bypassed to reduce losses.
Active Power Factor Correction (PFC) is used to ensure an accurate input current waveform with extremely low harmonic content, exceeding the
requirements of EN61000. This stage also provides active input current limiting which prevents overloading of the input stage while maintaining
high power factor.
The output of the PFC stage charges the hold-up electrolytic capacitors which store enough energy to allow the CM4 configured product to
continue operating during minor line disturbances. Long lifetime and high temperature capacitors are used which ensures extended lifetime and
product reliability.
A highly efficient zero voltage switching circuit is used to drive the isolated transformer from the hold-up capacitors. The output modules connect
to the transformer secondary and provide safe isolated power to a high performance synchronous rectifier power converter. This power converter
is controlled using the latest analog control technology to produce superior output performance in a miniature size.
CM4 App Note 260424 issue 2.1.docx Document No. 260424 Page 3 of 29