Kepco BOP 1000M Troubleshooting Application

Kepco BOP 1000M Four Quadrant Power Supply Complete Guide

Introduction: Why Four Quadrant Power Supply is the Key to Complex Load Testing

In fields such as motor drive, battery simulation, solar panel characterization, magnet excitation, and power quality testing, engineers often face a tricky problem: the load not only draws energy from the power source, but also feeds back energy to the power source under certain operating conditions. Traditional single quadrant DC power supplies cannot absorb reverse current and can only consume this energy through parallel high-power resistors or electronic loads, which not only has low efficiency but also has complex systems and severe heating.

The Kepco BOP (Bipolar Operational Power Supply) series of four quadrant power supplies fundamentally solves this contradiction. The BOP 1000M (i.e. BOP 1KW series) adopts a switch type topology with built-in bidirectional power factor correction (PFC) circuit. It can seamlessly switch to sink in the second and fourth quadrants while outputting power as a source, and feed back the absorbed energy to the grid with an efficiency of over 65%. This article is based on the official technical information of Kepco, systematically sorting out the technical characteristics, selection points, and common troubleshooting methods of BOP 1000M and the entire BOP family, providing a complete practical manual for engineers who are evaluating or using this series of power supplies.

Chapter 1: Four quadrant working mode and energy feedback principle

1.1 The meaning of the four quadrants

Conventional DC power supplies can only operate in the first quadrant (positive voltage, positive current), providing power to the load. The BOP series can work in any of the four quadrants:

First quadrant (+V,+I): The power supply serves as the source and outputs power in the forward direction.

Third quadrant (- V, - I): The power supply serves as the source and outputs power in reverse (outputting negative voltage and current).

Second quadrant (+V, - I): voltage is positive, current is negative - the power source absorbs power, equivalent to an electronic load.

Fourth quadrant (- V,+I): Voltage is negative, current is positive - the power source also absorbs power.

This feature enables it to perfectly drive inductive or capacitive loads, such as feeding back energy during motor deceleration, battery discharge testing, or charging and discharging cycles of capacitive loads.

1.2 Bidirectional PFC and Energy Feedback

When traditional linear BOPs (200W/400W) are used as absorbers, the internal linear adjustment tubes dissipate energy in the form of heat, so their rated absorption current is usually only half of the source current (e.g. 10A source, 5A absorption). The switch type BOP 1KW (1000W series) is completely different: its input terminal adopts a patented bidirectional power factor correction circuit. When BOP is in source mode, PFC converts the AC input into a stable DC bus voltage; When the BOP is in absorption mode, the PFC circuit works in reverse, reversing the energy feedback from the load back to the AC power grid, and the power factor is still as high as 0.97. This means:

No need for external braking resistors or electronic loads;

Long term continuous power absorption (such as battery discharge testing) will not cause equipment overheating;

The overall efficiency can reach 65%~70% in source mode (slightly lower in low-voltage models), and is close to the same level in absorption mode.

Practical operation and maintenance reminder: When connecting active loads (such as batteries, generators), it is necessary to ensure that the steady-state voltage of the active load does not exceed the maximum rated voltage of the BOP. For example, the voltage range of BOP 100-10MG is ± 100V. If a 110V battery is connected, the overvoltage protection (OVP) will immediately turn off the output. This is one of the most common faults on site.

Chapter 2: Detailed Explanation and Selection Points of BOP 1KW Series Models

2.1 MG Standard Type (Universal Switch Type)

The BOP 1KW MG series covers 8 voltage/current combinations ranging from ± 6V/± 125A to ± 100V/± 10A, with specific parameters as follows (according to the datasheet):

Model Voltage Range Current Range Voltage Channel Gain Current Channel Gain Series Resistance (m Ω) Series Inductance (μ H)

BOP 6-125MG ±6V ±125A 0.6 12.5 0.05 1.5

BOP 10-75MG ±10V ±75A 1.0 7.5 0.13 2.0

BOP 20-50MG ±20V ±50A 2.0 5.0 0.40 8.3

BOP 36-28MG ±36V ±28A 3.6 2.8 1.30 25.6

BOP 50-20MG ±50V ±20A 5.0 2.0 2.50 50.0

BOP 72-14MG ±72V ±14A 7.2 1.4 5.14 104.0

BOP 100-10MG ±100V ±10A 10.0 1.0 10.0 163.0

When selecting, it should be noted that the series resistance of low voltage and high current models (such as 6-125MG) is extremely low (0.05m Ω), but the switching frequency is 50kHz, resulting in slightly lower efficiency (56%); High voltage models (100-10MG) have a switching frequency of 70kHz and an efficiency of 65%. For high impact loads such as solenoid valves and motor starting, it is recommended to choose a current margin of at least 1.5 times the peak current.

2.2 GL ultra-low ripple type (optimized inductive load)

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:

Resistive load bandwidth: 13.5kHz

10 μ F load bandwidth: 16.3kHz (non-uniformity leads to even larger bandwidth)

Rise time: 32 μ s (resistive)/95 μ s (capacitive)

Load recovery time (from infinity to rated resistance): approximately 100 μ s

The MC series is opposite to the ML series in voltage mode - the bandwidth may increase with capacitive loads, but the stability decreases, so it needs to be corrected through internal compensation capacitors.

4.2 Internal user installation of compensating capacitors

The MC model provides optional compensation capacitor positions internally (non user adjustable external pins). The user needs to open the chassis and find the solder pad labeled Ccomp on the motherboard. They can select capacitors with different capacitance values according to the table below to reduce the voltage mode bandwidth and improve stability. For example, BOP 20-10MC:

Compensation capacitor bandwidth (kHz)

1nF 11.5

2.2nF 9.0

4.7nF 7.5

15nF 3.5

33nF 2.2

47nF 1.3

100nF 0.5

Safety warning: Before opening the chassis for internal modifications, the AC power must be disconnected and wait for at least 5 minutes for the internal capacitors to discharge. It is recommended to have a qualified electronic engineer operate it, otherwise it may damage the equipment or cause personal injury.

4.3 Troubleshooting of MC series: Output oscillation

Fault phenomenon: When unloaded or with capacitive load, the output voltage exhibits a continuous sine wave or peak oscillation of 10kHz~50kHz.

Reason: The load capacitance value exceeds the maximum rated value of MC (10mF), or the compensation capacitor is not installed/the selected value is too small.

Exclusion steps:

Verify whether the total load capacitance (including cable parasitic capacitance) is ≤ 10mF.

Check if internal compensation capacitors are installed. If not, select 47nF or 100nF as the starting point according to the table above.

Connecting a 10 Ω~100 Ω resistor in series at the voltage feedback end (remote detection of the rear terminal) can increase the phase margin.

If the load is a pure capacitor without parallel resistance, it is recommended to parallel a minimum power resistor (R ≥ rated voltage ²/10W) at both ends of the load to provide a DC discharge path.

Chapter 5: Application and Driver Integration of Solar Cell Testing (I-V Trace)

5.1 Traditional difficulties in completing I-V curve testing on a single machine

The characterization of solar panels requires scanning the output voltage from short circuit to open circuit, while measuring the current. The traditional solution requires a programmable electronic load (or power supply) to be paired with two digital multimeters (DVMs) and solves the problem of synchronous triggering. This not only incurs high costs, but also results in uneven curves due to cable noise and trigger jitter.

The Kepco BOP 1KW series is equipped with a built-in waveform generator and high-speed measurement capability, coupled with the free LabVIEW sub VI, allowing for complete I-V Trace and Dark I-V testing with just one BOP without the need for an external DVM.

5.2 Testing Principles and Advantages

BOP operates in voltage source mode, scanning from 0 to Voc (open circuit voltage) according to the set step size. At each step, BOP simultaneously measures the output voltage and output current (internal high-precision ADC), and returns the data points to the upper computer. Key Performance:

Scanning speed: 20ms per point (far faster than the second level response of DVM+electronic loads)

No trigger line required: Measurement and stepping are completely synchronized within the BOP, with no external jitter

Energy feedback: When testing the power generation status of the solar panel, the power absorbed by the BOP is fed back to the grid rather than dissipated as heat

In addition, dark current testing requires negative bias, and the four quadrant capability of BOP can seamlessly output negative voltage, scanning the reverse characteristics of the diode.

5.3 Troubleshooting: I-V Curve Distortion

Phenomenon: The I-V curve obtained from scanning shows steps or jumps near the maximum power point.

Reason: Resonance between the inductance of the test connection line and the output capacitance of the BOP, or noise coupling caused by strong light exposure on the tested panel.

resolvent:

Use a four wire Kelvin connection to separate the voltage detection line from the current line.

Connect a 0.1 μ F~1 μ F thin film capacitor in parallel at the BOP output (to absorb high-frequency noise).

Reduce the scanning step size or increase the dwell time for each step size (default 20ms can be adjusted to 50ms).

Chapter 6: Programming Control, Calibration, and Common Fault Codes

6.1 Communication Interface and Instruction Set

BOP 1KW has built-in standard GPIB (IEEE 488.2) and supports RS232. All models accept SCPI (Standard Commands for Programmable Instruments) command set and provide VISA driver program. Typical command example:

VOLT 50 sets the voltage to+50V

VOLT -25 sets the voltage to -25V

CURR 10 sets the current limit to 10A

OUTP ON enables output

MEAS:VOLT? Read back the actual voltage

For advanced users, ± 10V analog control can also be performed through the rear analog interface (50 terminal port). The main channel uses -10V to+10V corresponding to full negative to full positive output, and the limiting channel uses 0.05V to 10V corresponding to positive direction limiting.

6.2 Calibration Process and Password Protection

All calibration of BOP 1KW is performed through remote interface or front panel keypad, and the calibration coefficients are stored in non-volatile memory and password protected (default password can be obtained from Kepco). The steps are as follows:

Enter calibration mode (via menu or send CAL: STAT ON).

Connect the high-precision multimeter to the output terminal according to the prompts.

Calibrate the zero point, positive and negative full-scale voltage, and positive and negative full-scale current in sequence.

Save calibration parameters.

Common faults: If the device output deviates from the set value by more than 0.5% and cannot be adjusted, it is usually due to loss of calibration data (battery depletion or motherboard failure). At this point, it is necessary to recalibrate or replace the motherboard.

6.3 Interpretation of Typical Fault Codes

OVP (Overvoltage Protection): If there is a voltage source (such as a battery) on the load side that is higher than the rated voltage of the BOP, or if the internal voltage feedback loop fails. Disconnect the load first and measure whether the output is normal; If normal, check if the load contains back electromotive force.

OCP (Overcurrent Protection): Load short circuit or low current setting value. Check the load impedance and confirm that the current limit setting is higher than the peak demand.

FAN malfunction: Internal fan stalling or loss of speed signal. Clean the fan blades, and if ineffective, replace the fan (standard 92mm 12V).

AC FAIL: Input voltage exceeds the range of 176-264Vac, or PFC circuit failure. Check the voltage of the power grid, if it is normal, it needs to be repaired.

6.4 Parallel and Series Operations

Up to 5 BOPs of the same model can be connected in parallel (increasing current), and up to 3 BOPs can be connected in series (increasing voltage). When connecting in parallel, it is necessary to use a current sharing cable (contact Kepco for customization), and set one as the master and the rest as slaves. When connecting in series, please note that the total voltage should not exceed the insulation withstand voltage of any one unit (usually ± 100V models can reach ± 200V after being connected in series, but the grounding midpoint is required).

Key safety reminder: After series connection, the common mode voltage of each BOP will increase. It is necessary to ensure that the grounding potential of all series connected units is consistent and the output does not exceed the ground withstand voltage (usually ± 150V).

Chapter 7 Maintenance and Lifecycle Management

7.1 Regular inspection items

Every 6 months: Use an internal resistance meter to measure the insulation resistance of the output terminal to ground, which should be greater than 10M Ω; check whether the internal fan filter (if installed) is blocked.

Every year: Perform a complete calibration, especially for current measurement accuracy, as aging of the current divider can affect the accuracy of the absorption mode.

Every 3 years: Check the internal electrolytic capacitors (especially PFC bus capacitors) for bulges or leaks. For devices that frequently operate in absorption mode, capacitors age faster.

7.2 Replacement strategy after shutdown

Although this article mainly focuses on the Kepco BOP 1000M series, the series is still in production. If there is a future shutdown or shortage of maintenance spare parts, the following replacement directions can be considered:

Upgrade directly to BOP 2KW series (higher power)

Use other brands of high-voltage four quadrant power supplies (such as California Instruments, Chroma), but pay attention to whether their energy feedback efficiency and analog bandwidth match.

For scenarios that only require source functionality, it can be downgraded to a single quadrant power supply+independent electronic load, but the system complexity will significantly increase.

It is recommended that users backup the calibration data and configuration parameters of each device in advance (by reading all settings through GPIB), so as to quickly reproduce the working status during future replacements.