Kepco BOP 1000M Troubleshooting Application

The GL series is a model optimized for extremely low ripple and noise based on 1KW, especially suitable for driving large inductive loads such as magnets and motor windings. Its voltage and current ripple are only 0.02% and 0.01% (rms), respectively, far lower than the MG series. Currently, three models are available:

BOP 20-50GL：±20V/±50A

BOP 36-28GL：±36V/±28A

BOP 50-20GL：±50V/±20A

The output impedance characteristics of the GL series enable it to maintain stability without oscillation when driving inductors up to 1H in current mode. For particle beam correction magnets or MRI gradient coils that require extremely high stability, GL is the preferred choice.

Troubleshooting experience: When the GL series experiences low-frequency oscillations (about several Hz to tens of Hz) with inductive loads in current mode, it is usually due to the load inductance exceeding the design range or additional parasitic capacitance introduced by excessively long connecting cables. Solution: Connect a damping resistor that matches the load inductance in parallel at the BOP output (usually 0.1~0.5 times the load inductance), or use the bandwidth correction capacitor of the rear panel programming interface (see Chapter 4 for details).

2.3 Brief comparison of other BOP sub series

BOP 40W High Voltage Linear Series (BOP 500M/1000M): Output voltage up to ± 500V/± 80mA or ± 1000V/± 40mA, using linear amplification stage, extremely fast response speed, but maximum load capacitance is only 0.01 μ F, inductance is only 0.5mH. It is mainly used for low-power and high-voltage applications such as photomultiplier tubes, piezoelectric actuators, and electron beam deflection.

BOP 200W/400W Linear Series (Standard Type): A classic linear four quadrant power supply with a source to absorption power ratio of 100%/50% and a bandwidth of over 10kHz. Suitable for audio power amplifier testing, high-precision analog circuit power supply, etc.

ML series (inductance optimized type): Based on a 200W/400W linear platform, optimized for large inductance loads within 1H, with reduced bandwidth but still stable in current mode.

MC series (capacitor optimized type): Optimized for large capacitor loads within 10mF, internal compensation capacitors can be used for bandwidth adjustment in voltage mode.

Chapter 3: Bandwidth Correction Techniques for Inductive Load Optimization (ML Model)

3.1 Key specifications of ML series

For testing inductive loads such as motors, solenoid valves, audio speakers, or magnet coils, standard BOPs may oscillate in current mode due to phase lag caused by load inductance. Kepco offers an ML (Inductive Load) version, which has a lower bandwidth in current mode compared to the standard model, but with sufficient phase margin. Taking BOP 20-10ML as an example:

Bandwidth under resistive load: 11.2kHz

Bandwidth under 2mH inductive load: 4.1kHz

Rise/fall time (10% -90%): 35 μ s (resistive)/220 μ s (inductive)

Load effect (load adjustment rate): 12ppm/Hz (non-linear increase)

From the data, it can be seen that the bandwidth decreases to about one-third of its original value after being loaded with inductive load, but still remains stable.

3.2 Selection of bandwidth correction capacitors

The ML series allows users to further reduce bandwidth through external capacitors between pins 16 and 18 of the rear programming connector (PC12) to match loads with extremely high inductance. The relationship between the calibration capacitance value and bandwidth is shown in the following table (taking BOP 20-10ML as an example):

External capacitor bandwidth (kHz)

No capacitor 4.1

0.01μF 2.3

0.02μF 1.0

0.05μF 0.57

0.1μF 0.27

0.2μF 0.11

0.5μF 0.06

1.0μF 0.06

Practical application: When driving an inductor up to 0.5H, it is recommended to start from 0.1 μ F and use an oscilloscope to monitor the output current waveform until high-frequency oscillation is eliminated. Note that capacitors must be selected with high stability C0G or thin film capacitors, with a withstand voltage of not less than 50V.

3.3 Common faults: Current overshoot under inductive load

Fault phenomenon: When the current mode setting changes step by step, the output current exceeds the set value by more than 20% and slowly falls back.

Reason: The back electromotive force of the load inductance interacts with the internal compensation network of the BOP, resulting in instantaneous overshoot.

Exclusion steps:

Confirm that the load inductance does not exceed the maximum specification of the model (ML series maximum 1H).

Check if the current feedback cable is a twisted pair shielded wire and if the shielding layer is grounded at one end.

Connect appropriate correction capacitors in parallel on the programming connector (see table above) to reduce bandwidth.

If it still doesn't work, connecting a power resistor in parallel at both ends of the load (resistance=value of load inductance at the highest operating frequency) can significantly reduce transient overshoot.

Chapter 4: Stability and Compensation of Capacitor Load Optimization (MC Model)

4.1 Key specifications of MC series

For capacitive loads such as piezoelectric ceramics, solar panels, and supercapacitors, standard BOPs may oscillate in voltage mode due to additional poles caused by load capacitance. The MC (Capacitive Load) version has been optimized for capacitive loads up to 10mF, with typical specifications such as BOP 36-6MC: