2.7. Principle of Operation

The ML Series MagnaDC power supply processes power in two stages. An input stage rectifies the three-phase AC mains and regulates a current-controlled DC link, and an output stage inverts that link at medium frequency, isolates it through a power transformer, and rectifies and filters it to produce the DC output. This current-fed arrangement makes the supply tolerant of abusive loads, because the DC link inductors limit the rate of change of current into a fault.

Each ML Series system is built from 250 kW power modules operating in parallel within the cabinet: two modules in a 500 kW system and four in a 1000 kW system. The modules share feedback loops so that the cabinet behaves as a single supply, and no user configuration is required to balance them. Fig. 2.9 shows the functional arrangement of one module and the elements common to the cabinet.

Where a system requires more power than one cabinet delivers, multiple cabinets are operated in master-slave parallel using the UID47 Universal Interface Device, described in Parallel and Series.

Fig. 2.9 ML Series MagnaDC power supply functional block diagram

2.7.1. Power Processing

Three-phase AC power enters through an integrated AC breaker rated for the full power of the supply. Unlike the smaller MagnaDC products, which protect the power stage with input fuses, the ML Series uses this breaker as both the overcurrent protective device and a means of isolating the unit for service. The breaker is housed in the harmonic neutralizer section of the system, which is where the user AC connection is made.

Incoming power is routed first to a passive harmonic neutralizer, which reduces the characteristic input current harmonics of the rectifier and improves input power quality. 500 kW systems use a 12-pulse neutralizer and 1000 kW systems use a 24-pulse neutralizer; in both cases the phase-shifted outputs are distributed to the input sections of the individual power modules. The neutralizer operates upstream of the module input protection and control circuitry and does not alter output regulation. See Harmonic Neutralization for details.

Within each module, a three-phase contactor makes and breaks the mains under control of the control board, and an inrush limiter charges the DC link through a resistive path before the contactor closes across it, so the supply draws no in-rush current beyond its steady-state rating.

The rectified mains feed a polyphase chopper operating under current-mode pulse-width modulation. The chopper regulates current into the DC link inductors, which present a stiff current source to the following stage. Chopper output current is monitored both for phase balance and for overload conditions. A current-fed inverter switches this link at medium frequency into the power transformer, which provides galvanic isolation and the voltage transformation appropriate to the model’s rating. Operating the isolation transformer at medium frequency rather than at the line frequency is what keeps the magnetics small enough to fit a 250 kW module into a fraction of a floor-standing cabinet.

Output rectifiers and a pie-section filter, forming a double-stage LC network, convert the transformer secondary back to DC and reduce output ripple to the value published for the model. A midpoint diode arrangement is used for low-voltage models and a bridge arrangement for high-voltage models.

2.7.2. Thermal Management

Water cooling is standard on every ML Series model and is what allows a 250 kW module to occupy the volume it does. Each power electronics stage is mounted on a dedicated aluminum chill plate with press-fit copper tubes, and the stages slide into the cabinet on rails where they are interconnected by a wire harness. The magnetic components carry chill plates around the transformer core. A tuned water distribution manifold divides flow between the stages so that each receives the flow it requires.

An internal solenoid holds coolant flow off until internal temperatures reach a set value, which prevents condensation forming inside the cabinet when the supply is idle or lightly loaded in a humid environment. Thermal switches mounted directly to the heatsinks assert the thermal (THL) diagnostic if a stage overheats, including in the absence of coolant. Fans move air from the bottom of the cabinet to the top to prevent local hotspots; approximately 80% of the supply’s losses leave through the coolant and 20% through this airflow.

The coolant path is built from copper pipe, PEX tubing, and brass solenoids and fittings. Deionized water must not be used. Coolant requirements, water quality, the solenoid bypass, and maintenance of the water path are covered in Water Cooling.

2.7.3. Control and Monitoring

The gate driver board generates the synchronized modulation for both the polyphase chopper and the current-fed inverter, and reports fault conditions back to the control board. A phase detector monitors the incoming three-phase mains and asserts the phase loss (PHL) diagnostic when a phase is absent.

The control board closes the voltage and current regulation loops using optically isolated feedback amplifiers. Because the feedback path is optically isolated, all user interface circuitry is referenced to earth ground rather than to the negative output terminal, which allows external circuitry to be connected without concern for ground loops or voltage breakdown. The control board also evaluates the protection and diagnostic conditions described in Protection and Diagnostics, and latches a diagnostic condition into memory while disconnecting main power through the three-phase contactor.

Two further boards hang off the control board. The display board carries the front panel meters and indicators for the D Version front panel. The interface assembly carries the 37-pin external user I/O connector, the RS232 connection, and the optional IEEE-488 GPIB or LXI TCP/IP Ethernet connection.