.. _principle-of-operation:

Principle of Operation
----------------------

This section provides a general overview of the technology and power processing stages in a |product_full|. :numref:`fig-block-diagram-ui` and :numref:`fig-block-diagram-3phase` provides an visual overview of the power supply's main power and control stages.

.. _fig-block-diagram-ui:

.. figure:: _images/block_diagram_slx_ui.*
    :align: center

    |product_full| functional block diagram for SLx Series models with UI/UI2 1-phase AC input

.. _fig-block-diagram-3phase:

.. figure:: _images/block_diagram_slx_3_phase.*
    :align: center

    |product_full| functional block diagram for SLx Series models with 3-phase AC input


Power is fed through fuses, an EMI filter, rectifiers, and an inrush limiter. The inrush-limiting resistors are always active. After a timed delay, these resistors are bypassed using a contactor (creating a low-resistance path between the AC line mains and the bus voltage). The delay begins when the power rocker switch on the display board is turned on.

The fuses and EMI filter, located at the input, reduce both common-mode and differential-mode noise emanating from the supply. The line detector, part of the input stage, monitors input phasing and amplitude. It will prevent operation if the input voltage does not meet base quality requirements, such as during brownouts, missing phases, or phase imbalances. Upon detecting a voltage issue, the main control shuts down the PWM throughout the system.

The auxiliary power supply manages the contactor, fans, and provides power to additional assemblies. It is powered from a line-to-line phase for three-phase (3Φ) inputs and a line-to-neutral phase for single-phase (1Φ) inputs. It includes onboard fuses for protection and adjusts fan speed based on heatsink temperatures and output current. The auxiliary power supply will prevent operation of the |product_type| if any of the fan rotors are locked.

Output power is controlled through a polyphase chopper. For the 4, 6, 8, and 10 kW |product_full|, two choppers phased 180° apart supply a current source to the current-fed inverter. For the 1.5 and 2.6 kW models, a single chopper is used. The choppers employ current-mode PWM, providing fast transient response and effective harmonic filtering on the DC bus. Chopper current is measured for balancing and overload protection. The polyphase chopper minimizes harmonic components, reducing circulating currents in the power supply.

The polyphase chopper produces a controlled DC bus, which connects to DC link inductors, the IGBT current-fed inverter, and the power transformer. The transformer provides ohmic isolation between the AC input and DC output, reducing the total volume needed for power conversion.

The current-fed inverter operates at a 50% duty cycle with switching frequencies above 25 kHz, ensuring its operation is transparent to the power supply's performance.

The transformer output is converted to DC via rectifiers. Low-voltage versions of the |product_full| use midpoint diode configurations, while higher-voltage versions use full-bridge rectifiers.

The DC output voltage is filtered using a PIE filter, which, in combination with the DC link inductors, forms a double-stage inductive-capacitive (LC) filter to smooth the output.

The main control generates synchronized PWM signals, monitors system performance, and provides control for the polyphase chopper and current-fed inverter. PWM signals are sent to gate drivers, which provide isolated power and signals to the IGBTs. The main control is earth-ground referenced, allowing the DC output to float up to ±2000 Vdc above ground while maintaining safe operation.

The display board includes a processor for managing user inputs via keypad buttons and a knob. It displays operating conditions using LEDs and a vacuum fluorescent display (VFD). Diagnostic LEDs at the rear of the product provide low-level status information for troubleshooting.