Kepco BOP Bipolar Power Supply Troubleshooting and Maintenance Guide

Kepco BOP Bipolar Power Supply Troubleshooting and Maintenance Guide

The Kepco BOP series bipolar programmable DC power supply (100W/200W/400W) is a commonly used high-precision voltage/current source in laboratories and industrial automation, as well as a bipolar amplifier. Its unique four quadrant output capability (which can serve as a source or trap) makes it indispensable in applications such as battery simulation, motor drive, and magnetic material testing.

However, due to its high bandwidth and complex preamplifier structure, BOP power supplies often experience oscillations, output loss of control, and even damage during on-site use due to improper grounding, poor load inductance matching, remote programming wiring errors, or digital control card configuration issues. This article combines the key technical points in the BOP operation manual to systematically sort out common types of faults, diagnostic methods, and preventive maintenance measures, providing reference for on-site engineers.

Common fault phenomena and root causes

2.1 Output oscillation: instability under inductive load

When BOP drives inductive loads in voltage mode or capacitive loads in current mode, loop oscillation may occur due to additional phase lag. The typical manifestation is high-frequency jitter or significant periodic fluctuations in the output voltage/current.

Root cause: When the load inductance is greater than 0.5 mH (current mode) or the load capacitance is greater than 0.2 μ F (voltage mode), the turning frequency of the load is lower than the closed-loop bandwidth of BOP, and the phase margin decreases, resulting in system instability.

Solution (from manual "IMPORTANT NOTES"):

Method 1: Connect capacitors in parallel at the output end (0.1 μ F to 1.0 μ F, depending on the inductance).

Method 2: Connect an RC series network in parallel at the output end (resistance 100-500 Ω, capacitance 0.1-0.5 μ F).

For BOP 200W/400W models that require driving a wide range of inductive loads and cannot eliminate oscillations using the above methods, they can be sent back to the factory to upgrade to the "ML" version (with component changes and recalibration).

On site experience: If the oscillation frequency is between several hundred Hz and several kHz, a 0.22 μ F capacitor should be tried first; If the load inductance is extremely large (such as the primary of a transformer), RC combination should be used.

2.2 Communication input power failure causing damage to output terminal

When the BOP drives a large inductive load and the output current is high, if the AC input suddenly loses power, the internal circuit of the BOP cannot quickly dissipate the energy stored in the load, which may cause damage to the BOP or the load. The manual suggests taking the following preventive measures:

Use UPS to supply power to BOP.

Before cutting off the BOP power supply, manually reduce the output voltage/current to zero, and then turn it off after the output current actually returns to zero.

Parallel appropriate rated bidirectional transient suppression devices (such as bidirectional thyristors or Zener diodes) at the output end, or use normally closed AC contactors directly in parallel at the output end.

2.3 Damage caused by grounding errors

The programming signal return path of BOP is strictly separated from the output circuit. If the output terminal (+or -) is directly grounded and the programming device is also grounded, the output current will flow through the internal circuit through the programming signal return path, causing irreversible damage.

Key Warning (bolded content in the manual):

It is strictly prohibited to ground the BOP output terminal (or the side of the load connected to this terminal).

It is strictly prohibited to ground both the load return terminal and the programming signal return terminal simultaneously.

It is strictly prohibited to use the programming return terminal as a tap point for load return.

Correct approach: The output common terminal (Common) should be used as the only signal ground and connected to the chassis ground (Chassis) through a "Grounding Network" terminal, which is internally connected to the chassis through an RC series network to reduce common mode noise. If the system must ground the output common terminal, ensure that the programming signal source is completely floating or use an isolated amplifier.

Preventive maintenance and daily inspections

3.1 Cooling and Ventilation

BOP relies on forced air cooling to maintain the internal power semiconductor temperature. The side panel openings and top cover must be kept unobstructed and not blocked. When installing the rack, it is necessary to ensure that the ambient temperature does not exceed 55 ° C. Regularly cleaning the internal dust (recommended every six months) can effectively extend the lifespan.

3.2 Input voltage selection and wiring

When leaving the factory, the BOP is set to 115V input. If it is necessary to change to 104V, 208V or 230V, the primary jumper of transformer T201 must be reconnected according to Figure 2-3 and operated by qualified technicians. The circuit breaker is effective at all voltages.

3.3 Requirements for output and sensing wires

The load conductor should be as thick and short as possible, and twisted in pairs to reduce magnetic field disturbance.

When remote error sensing is required, shielded twisted pair cables (minimum 20 AWG) must be used to connect the sensing terminal of BOP to the load end, compensating for a voltage drop of up to 0.5V per wire. Polarity must not be reversed.

All external programming resistors should use low-temperature drift, high stability metal film or wire wound resistors.

Remote programming troubleshooting

BOP can receive external voltage, resistance, or digital signal control through the rear programming connector (PC-12, 50 pin edge socket). The common problems are as follows:

4.1 Output nonlinearity when programming with external potentiometers or resistors

Reason: Wiring error or potentiometer resistance too high/too low. For the voltage channel, a ± 10V reference source (4mA maximum) is connected to the inverting input of PREAMP "A" through a 10k Ω input resistor. If a potentiometer is used, the total resistance should be about 10k Ω, and the voltage division method is shown in Figure 3-4.

Check: Confirm if the jumpers (1-8, 2-4, etc.) on PC-12 are connected correctly. If the local/remote flag (PC-12 pin 35) is grounded, the front and rear panel mode switches will be disabled and need to be set high to restore local control.

4.2 External 1V signal cannot drive BOP at full range

When the input signal is only ± 1V and full range output (e.g. ± 100V) is required, the gain of PREAMP "A" needs to be changed. The manual provides the formula:

Eo(PREAMP “A”) = –Ei × (Rf / Ri)，

The required output is ± 10V (BOP internal reference). If Ri=10k Ω remains constant, then Rf should be 100k Ω. Connect the external 1V signal to the corresponding pin of PC-12, and disconnect the original feedback network according to Figure 3-10, and connect the external 100k Ω resistor. If the signal source has high impedance, the in-phase input terminal of PREAMP "A" should be used (Figure 3-11).

4.3 No response from digital programming interface

BOP supports multiple digital programming methods: the old SN/SNR series, BIT 488/500 card, and the latest BIT 4886 card. Common faults:

BIT 4886 card communication failure: Check RS232 parameters (9600 baud, no checksum, 8-bit data, 1-bit stop bit). Is the GPIB address switch set correctly. Does the driver software support SCPI commands.

BIT 500 parallel card: Confirm that the data bus logic level (positive/negative) selects jumper wires (pins 1, 3, and 5), STROBE pulse width ≥ 2 μ s, and whether the BUSY signal is correctly detected.

SN 488 series: Confirm that the command format is the ASCII character "NCVVV", where N is the channel, C is the control character, and VVV is a hexadecimal or BCD number. Does the address match the controller.

TLD 488-16 system: CILL command syntax needs to be checked, such as FNC DCS: CH00 followed by SET VOLT 10, etc. Note that spaces and carriage returns must be correct.

Diagnostic prompt: All BIT cards provide optical isolation. If the output cannot be programmed, first measure the corresponding analog voltage on the rear programming connector (such as the Main Channel output should be 0-10V corresponding to full range). If there is no voltage, the digital card is not powered correctly or not placed in the REMOTE state (PC-12 pin 35 must be high, corresponding to the BIT card, the corresponding control pin is high).

Calibration and internal adjustment

BOP has been calibrated before leaving the factory and only needs to be readjusted after repair or replacement of components. The position of the potentiometer for internal adjustment is shown in Figure 2-1. Before operation, the load must be disconnected and the upper cover must be removed (note the high voltage hazard).

5.1 ± 10V reference voltage calibration

Connect the precision digital voltmeter to pin 22 (-10V) and pin 28 (+10V) of PC-12, with the common terminal connected to COMMON.

After power on, adjust R31 (+10V) and R32 (-10V) to make the reading accurate to 10.000V ± 1mV.

5.2 Zero point calibration of ammeter

Do not connect the load, connect the digital voltmeter to pin 10 (Io monitoring) and COMMON of PC-12.

Adjust R50 to read 0.000V. This signal corresponds to 0% of the output current.

5.3 Full range calibration of output current (R316/R314)

Connect the precision shunt and ammeter, place the BOP in current mode, and turn off the current control switch.

Connect an external 10V DC voltage source to the current programming input terminal (both front and rear panels can be used).

After power on, adjust R316 (R314 for BOP 200-1M) until the reading of the ammeter is equal to the rated value (for example, 4.00A for BOP 50-4M).

5.4 Linearization adjustment of optocoupler unique to BOP 200-1M

Use a function generator to output a 20Vp-p, 250Hz triangular wave, and drive the BOP to full range output through voltage programming input terminal.

Connect the oscilloscope probe to the testing point on the A1 control board (see Figure 3-32) and observe the waveform.

Adjust R15A to achieve optimal linearity of the triangular wave ramp (without bending or distortion).

Attention: The above adjustments must be made after sufficient preheating (at least 30 minutes) and ensure stable ambient temperature.

Common problems in master-slave series/parallel configuration

BOP can be connected in series (voltage addition) or in parallel (current addition), and it is recommended to use master-slave control method.

6.1 Uneven or out of control output voltage during series configuration

Possible reason: The grounding network of the slave (SLAVE) is not disconnected. The link (4-5) should be opened on the rear terminal TB201 of SLAVE to isolate the output common terminal of the slave from the chassis. Otherwise, it will cause a grounding loop.

Coupling adaptation resistance RT calculation error: RT (k Ω)=(Eom_max -10), where Eom_max is the rated output voltage of the host. For example, for BOP 100-2M host, RT=100-10=90 k Ω. If the resistance is too small, it will limit the amplitude, and if it is too large, the slave will not be able to follow at full range.

6.2 Uneven current distribution during parallel configuration

Root cause: The slave is not correctly placed in current mode and the current control switch is not turned off. When connecting in parallel, all slave MODE switches must be set to CURRENT, and the CURRENT CON switch must be turned off. The driver signal output from the host is through pin 10 of PC-12 (programming signal for host voltage in voltage mode)? The actual parallel connection should use current mode master-slave connection: from the host PC-12 pin 19 to the slave pin 13? Manual Figure 3-27 clearly states that the host pin 10 (I2) is sent to the slave pin 19. Ensure reliable connection of shielded cables.

Protection measures: All parallel/series BOPs should be interlocked through a "circuit breaker control circuit" (see Figure 3-28). Once a fault occurs in one unit, the SCR triggers the circuit breakers of all units to trip simultaneously, preventing system overvoltage.

Interpretation of indicator lights and status signs

The BOP front panel has five LED status lights:

Eo MODE (green): Voltage mode activated.

Io MODE (green): Current mode activated.

VOLTAGE LIMITED (yellow): The voltage limiting circuit operates (reaching the set upper or lower limit).

Current Limit (yellow): The current limiting circuit is activated.

Remote (blue): Remote control activated (PC-12 pin 35 grounded, or BIT card enabled).

The rear PC-12 provides a corresponding TTL flag (low level valid). If a certain flag logic level is abnormal (such as always low), it is necessary to check the comparator circuit or reference voltage.

Special note: When the GPIB interface card (such as BIT 488) is set to "remote", the front panel MODE switch fails, but the voltage/current limit knob remains valid. To restore local control, disconnect pin 35 of PC-12 from COMMON or set the device back to LOCAL using a command.

Recommendations for spare parts and long-term storage

When not in use for a long time, it should be stored in a dry, dust-free environment at a temperature of -25 ° C to 70 ° C. Every six months, power on and run for 2 hours to maintain the performance of the electrolytic capacitor.

Spare parts suggestion: Keep the original PC-12 connector (jumper for local control), a set of external programming resistors (10k Ω, 100k Ω metal film), and two spare fuses (slow melting type, specifications see manual) provided by the factory.

If you need to change the 200W/400W model to ML version to be compatible with a wide range of inductive loads, please contact Kepco factory for upgrade services.