1.10. Calibration

The SLx Series MagnaDC power supply stores the calibration of its measurement and programming circuits in nonvolatile memory. These values are set during factory testing and seldom need adjustment. When recalibration is necessary, it is performed entirely with MagnaCTRL from the Calibration side menu – no other software tools are required to read or write the values.

MagnaCTRL groups the calibration values into Control Gains, Feedback Sensors, Analog Inputs, and Miscellaneous sections, shown in Fig. 1.16. Select the MagnaDC power supply – and, in a master-slave system, the individual module – from the selectors at the top of the menu. The increment and decrement buttons step a value one count at a time and write it to the product immediately, so each step takes effect at once. A value typed directly is held until committed with the Save button that appears beside it; Save All appears at the top of the menu when typed values are pending, writing them all at once.

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Fig. 1.16 The Calibration menu in MagnaCTRL, with values grouped into Control Gains, Feedback Sensors, Analog Inputs, and Miscellaneous sections

Caution

Calibration should only be performed in strict adherence to Magna-Power Electronics’s documented calibration procedure. Failure to calibrate the product properly, or manipulation of these values, can result in undesired performance and damage to attached products.

Each measurement is corrected by an offset and a gain. As shown in Fig. 1.17, an offset error shifts the reported value by the same amount at every output level, while a gain error grows in proportion to the output. The procedures below correct each in turn: the offset is adjusted at zero output, then the gain is adjusted near 90% of the rating, marked on the figure.

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Fig. 1.17 Offset and gain measurement errors relative to an ideal reading, with the zero and 90% adjustment points marked

1.10.1. Terminal Voltage

The Voltage Offset (ID 126 - offset1) and Voltage Gain (ID 117 [0]) in the Feedback Sensors section, shown in Fig. 1.18, calibrate the MagnaDC power supply’s voltage measurement so that the voltage reported in MagnaCTRL matches a reference meter. On models rated 1000 V and below, the remote-sense path is calibrated the same way with the Remote Sense Voltage Offset (ID 127 - offset2) and Remote Sense Voltage Gain (ID 117 [2]).

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Fig. 1.18 Feedback Sensors section with the terminal voltage and current gains and offsets

Equipment

  • A calibrated precision multimeter as the voltage reference.

  • A resistive load (loading a MagnaDC power supply) or a DC source (supplying a MagnaLOAD electronic load), sized near the model’s rating.

Warning

Check the multimeter’s maximum input voltage rating before connecting it to the output; most multimeters are rated for 1000 V or less. On models rated above 1000 V, measure through a calibrated voltage attenuator rated for the product’s full output voltage and apply its division ratio to every reading. Connecting a multimeter above its input rating is a shock hazard and can damage the meter, and an uncalibrated attenuator adds its ratio error directly to the calibration.

Procedure (local sense)

  1. With the output off (0 V), adjust the Voltage Offset until the voltage reported in MagnaCTRL reads zero, then save.

  2. Set the voltage to about 90% of rated, enable the output, and let it settle. Adjust the Voltage Gain until the reported voltage matches the reference meter, then save.

Remote sense (models rated 1000 V and below)

  1. Return the output to standby and jumper the plus and minus remote-sense leads to the plus and minus output terminals.

  2. Repeat the offset and gain steps with the Remote Sense Voltage Offset and Remote Sense Voltage Gain.

Adjust each value until the reported voltage matches the reference within the MagnaDC power supply’s digital measurement accuracy, ± 0.08% of rated voltage.

1.10.2. Terminal Current

The Current Offset (ID 128 - offset3) and Current Gain (ID 117 [1]) in the Feedback Sensors section (Fig. 1.18) calibrate the MagnaDC power supply’s current measurement so that the current reported in MagnaCTRL matches a reference.

Equipment

  • A calibrated DC current sensor as the current reference, with its output read by a precision meter set to a long integration time (for example, INTEG 100 PLC) for a stable reading.

  • A resistive load (loading a MagnaDC power supply) or a DC source (supplying a MagnaLOAD electronic load), sized so the MagnaDC power supply can conduct about 90% of rated current below its rated voltage.

Procedure

  1. With the output off (0 A), adjust the Current Offset until the current reported in MagnaCTRL reads zero, then save.

  2. Set the current to about 90% of rated, enable the output, and let it settle. Adjust the Current Gain until the reported current matches the reference meter, then save.

Adjust each value until the reported current matches the reference within the MagnaDC power supply’s digital measurement accuracy, ± 0.08% of rated current.

1.10.3. Analog Input Offsets

The Analog Inputs section, shown in Fig. 1.19, holds two offsets: the Analog Input Offset (ID 136 - offset11) for the standard analog inputs and the High-Speed Analog Input Offset (ID 135 - offset10) for the high-speed analog input. Each offset is calibrated so that zero voltage applied at the input produces a zero set point: an offset that is too low clips small input voltages to zero, while an offset that is too high commands a positive set point with the input grounded.

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Fig. 1.19 Analog Inputs section with the standard and high-speed input offsets

No reference meter is required and the output remains off; the applied reference is 0 V.

Procedure

  1. Assign the analog input being calibrated to the voltage set point and set the set-point source to external control.

  2. Tie the analog input to its ground reference on the external user I/O connector.

  3. Increase the offset until the voltage set point reported in MagnaCTRL first reads above zero, then decrease it until the set point just returns to zero, and save.

  4. Repeat for the other analog input offset, then restore the set-point source and pin assignments.

1.10.4. Master-Slave Capture Gain

In a master-slave system, the connected units share the total load current. Each unit measures its own output current with its internal shunt, already calibrated by the Terminal Current procedure, and the slave’s Capture Gain (ID 124 [1], in the Miscellaneous section, shown in Fig. 1.20) scales that measurement so the slave carries an equal share of the load. No external reference meter is required – the adjustment balances the units against each other. The capture gain is calibrated on each slave with its Current Sense cable disconnected.

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Fig. 1.20 Capture Gain in the Miscellaneous section, showing the selected module’s own measured current and voltage

The Module selector at the top of the menu points the Calibration values at one product at a time: every value displayed is read from, and saved to, the selected module’s nonvolatile memory. The Current and Voltage readings shown with the Capture Gain are likewise the selected module’s own internal measurements – not the system total reported by the master – which is what allows each slave to be balanced individually with Current Sense disconnected.

Recalibrate the capture gain after replacing a slave or its control board, after a hard Factory Restore, or whenever the units share current unevenly.

Setup

  • Disconnect the Current Sense cable on every unit.

  • Wire all product outputs in parallel into a shared resistive load bank (for MagnaDC power supplies) or a shared DC source (for MagnaLOAD electronic loads), rated for the full system current.

Procedure

  1. Connect MagnaCTRL to the master and open the Calibration side menu.

  2. Command 95% of rated current and enable the output; the load holds the system in Auto-Crossover current regulation, below rated voltage.

  3. For each slave, select it with the Module selector. In the Miscellaneous section the Capture Gain control shows that module’s live measured current; adjust Capture Gain until the slave carries an equal share of the system current, within 0.5%, then save the value.

  4. Disable the output, reconnect the Current Sense cables, and power-cycle the system.

After recalibration, confirm that current is shared evenly across all units.