Switched mode power supply (SMPS): Working Principle of SMPS  ITI’s 50V – 2000A Power Plant?

General description of the operation

The Switched mode power supply (SMPS) 48V-5600W rectifier is a state-of-the-art switch-mode power supply. It is composed of 3 identical single-phase sub-modules (R, S, and T) as shown in the block diagram :

3-identical-single-phase-sub-modules 4

The sub-modules are connected between neutral and one of the phases (R, S or T) on the input, and in parallel on the output. All ‘-‘ wires are protected by circuit breakers, which are mechanically coupled.

The interface card IFC 52 provides:

  1. all reference voltages and protections of the sub-modules.
  2. Signalization and manual interface (adjustment potentiometers and test jacks) for the whole unit, and
  3. Communication with power system controller ‘ITI’.

Each of the sub-modules consists of two cascaded power converters performing over factor correction and dc/dc conversion. The power stages are synchronized and working with a constant switching frequency of ~100 kHz. The rectified ac mains voltage is processed first in the power factor corrector circuit, which is based on a boost topology. The boost converter has the inherent advantage of continuous input current waveform which relaxes the input filter requirements.

The performance of the basic boost cell is improved by a proprietary snubber circuit which reduces the switching losses of the power semiconductors due to non-zero switching times. Furthermore, the snubber circuit also decreases the electromagnetic interference generated primarily during the turn-off process of the boost diode. The output of the boost converter is a stabilized 400 Vdc voltage.

Further conversion of the stabilized high voltage output of the power factor corrector circuit is necessary to generate the isolated low voltage output and to provide the required protection functions for telecommunication applications. These tasks are achieved in the dc/dc converter circuit which is based on a full-bridge topology.

The full-bridge circuit is operated by phase-shift pulse-width modulation with current-mode control. This control method provides zero voltage switching conditions for all primary side power semiconductors, effectively reducing switching losses and electromagnetic interference. An advanced solution reduces the stresses on the output rectifier diodes.

Proper operation of the power converters is managed by individual control circuits and supervised by housekeeping electronics.

Remote commanding and monitoring of the modules are possible through a power system controller.

Block diagram of a single sub-module R, S or T

This chapter gives more detailed information about the technical merit of the single sub-module based on the functional blocks shown in the diagram below (R, S, and T).

Block-diagram-of-a-single-sub-module-R-S-or-T 5

Block 1 of the drawing presented above is the input EMI filter of the rectifier. The fixed frequency, synchronized operation of the different circuits allowed to optimize the filter’s performance. It has only one differential and one common mode filter stage.

Block 2 represents the Inrush Current Limiter circuit, which consists of a series combination of surge-rated power resistors and fuse. The circuit limits the input current of the rectifier during the initial charging of the energy storage capacitors connected to the output of the boost power factor corrector circuit. In normal operation, the current limiting components are by-passed through relay which is controlled by the housekeeping electronics.

A general-purpose full-wave Bridge Rectifier circuit forms Block 3. It is directly mounted on the heat sink.

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