Siemens SIMOVERT 6SE70 Fault F26: IGBT Firing Board Repair

1. Problem Definition: Fault F26 on Siemens SIMOVERT MASTER DRIVES

The Siemens SIMOVERT MASTER DRIVES family (order code prefix 6SE70) reports Fault F26 as part of a tightly related fault family covering IGBT desaturation events on each output phase. F26 specifically trips when the firing (gating) board monitoring circuitry detects an unacceptably high collector-emitter voltage (V_CE) across the lower or upper IGBT of phase V while the device is commanded ON.

On the chassis-style compact and cabinet units built around the 6SE7021 / 6SE7031 / 6SE7038 / 6SE7090 platforms, F26 typically manifests as a non-resettable fault that latches the drive into a safe state, dropping the gating board command outputs and disabling the inverter. Repeated nuisance trips — or single trips followed by recovery on power cycle — are the hallmark signature of a degrading firing board rather than a true short circuit in the load or DC link.

2. Fault Code Reference: F025, F026, F027 (UCE Trip Family)

The UCE-monitoring fault family is decoded in the SIMOVERT MASTER DRIVES parameter documentation as follows:

The UCE (German: UCE-Abschaltung, collector-emitter shutdown) circuit is the standard desaturation protection on every SIMOVERT MASTER DRIVES IGBT module. Each gate driver board contains a comparator that monitors V_CE(sat); if the voltage exceeds roughly 7 V for more than a few microseconds while the IGBT is gated ON, the comparator latches, the gate is pulled low, and a fault is transmitted back to the control board via fiber optic.

When the firing board components age or the gate-driver supply rails drift, the comparator reference or the V_CE sense path can produce a false trip — sometimes accompanied by no actual device stress, sometimes masking a real short that should have tripped cleanly. This is the core ambiguity that makes F025/F026/F027 difficult to diagnose in the field.

3. Affected Drive Models and Configurations

The following order codes share the firing-board architecture documented in this article. All are SIMOVERT MASTER DRIVES (later renamed SINAMICS MASTER DRIVES) Vector Control and Servo Control variants.

The "-Z" suffix on the example order code (e.g., 6SE7021-3EB71-Z) indicates a customized Z-option configuration. Refer to the Siemens order documentation for the specific Z-code list because some Z options add auxiliary inputs, special encoder feedback, or modified firmware that can interact with the UCE monitoring path.

4. Root Cause Analysis: Firing Board Degradation

The dominant root cause of repeated F25/F26/F27 faults on aging SIMOVERT MASTER DRIVES is degradation of the firing board (also called gating board, gate driver board, or Ansteuerplatine in German documentation). Each firing board performs four functions:

1. Receives gate-fire commands from the control board via fiber optic (transmit fiber, white-coded).
2. Generates an isolated +15 V / -5 V gate-drive rail for the upper IGBT and lower IGBT of the phase.
3. Monitors V_CE on each IGBT through the integrated desaturation comparator.
4. Returns the IGBT status (OK / Fault) back to the control board via a separate fiber optic (receive fiber, black-coded).

Failure modes observed in the field:

• Optocoupler / opto-receiver LED aging: The transmit and receive fiber-optic transceivers on the firing board use LEDs that lose radiant intensity over thousands of hours. Reduced optical budget causes intermittent command reception, leading to spurious "missing pulse" detection that can be misread as a desaturation event by the control board.
• Gate-driver IC degradation: The hybrid gate-driver ICs that source/sink gate charge develop timing drift. Symmetric turn-off of the upper and lower IGBTs is essential; any timing skew of even 1 – 2 µs permits momentary shoot-through (both devices ON), producing a real UCE event.
• V_CE sense resistor drift: The desaturation sense network uses precision components. Drift or thermal cycling causes false trips at high DC-link voltage or high di/dt.
• Power interface module (PIM) faults: The PIM supplies the firing boards and processes current-transformer (CT) feedback. PIM failures often present as combinations of F025/F026/F027 plus possible F011 (overcurrent) or F029/030 measurement-system faults.
• Fiber-optic connector contamination: Dust, oil, or scratched end-faces attenuate the optical signal. Because firing boards interpret a weak signal as "device fault", contamination manifests as an F26 trip.

5. Diagnostic Procedure: Step-by-Step Isolation

1. Capture fault status. Before resetting, scroll the OP1S / PMU parameter view to r947 (fault number) and r949 (fault value / time stamp). Record whether the fault is reproducible on cold start, hot start, or only under load.
2. Inspect the fiber-optic bundle. With drive powered off and DC-link discharged (verify < 50 V on the DC-link test points), remove each firing board's fiber connector. Inspect end-faces under a magnifier for contamination, cracks, or oil film. Clean with a lint-free swab and isopropyl alcohol if needed.
3. Measure the gate-driver supply. With the drive powered but not enabled, measure the firing-board supply rails at the test points specified in the SIMOVERT MASTER DRIVES operating instructions. Expected: +15 V ±0.5 V and -5 V ±0.5 V referenced to each firing board's COM terminal. Out-of-tolerance rails indicate a failing board or a failing PIM.
4. Verify fiber-optic transmission. Use a fiber-optic power meter (or temporarily swap a known-good transmit fiber from the control board into the suspect channel) to confirm the optical transmit power meets the Siemens specification of ≥ -18 dBm at the firing-board receiver. Low power points to control-board transmitter aging rather than firing-board failure.
5. Rule out real short circuits. With the drive isolated and the motor disconnected, megger the motor cable and motor windings phase-to-phase and phase-to-ground. Values below 1 MΩ per kV of rating indicate a real insulation fault. Also measure each IGBT module's C-E resistance with the drive fully discharged — a shorted module will show low resistance in both polarities.
6. Inspect the power interface module. Check the PIM for discolored components, swollen capacitors, or burnt solder joints around the CT input stage and DC-link voltage divider. Replace the PIM before condemning the IGBT stack.
7. Swap-test firing boards. Swap the phase-V firing board with the phase-U firing board. If F26 re-appears immediately as F025 (or remains F26 because the V-channel is hard-faulted in the control board's mapping), the firing board is confirmed defective.
8. Reset and re-test. Acknowledge the fault with the PMU "RESET" key, run the drive under no-load for 15 minutes, then apply partial load. Monitor r029 (current actual value) and the UCE fault buffer for any repeat trip.

6. Component Identification and Replacement

Firing-board part numbers vary by frame size and firmware generation. Refer to the Siemens SIMOVERT MASTER DRIVES spare-parts list for the exact replacement, but commonly observed order codes include:

Replacement procedure (per phase):

1. Lock out and tag out the drive's input disconnect. Wait the full DC-link discharge time printed on the drive (typically 5 minutes for compact units, up to 15 minutes for large chassis/cabinet units). Verify with a CAT IV 600 V meter on the DC-link test points.
2. Disconnect all fiber-optic connectors from the firing board. Mark each connector with phase ID and direction (TX vs RX).
3. Disconnect the gate-drive leads and auxiliary power leads from the firing board terminals. Photograph or label each connection.
4. Remove the mechanical fasteners holding the firing board to the IGBT module. On long boards, the board may have 4 – 6 M4 / M5 screws along its length.
5. Lift the board straight up to clear the IGBT gate pins; do not pry or twist — the gate pins are fragile and can bend.
6. Place the new board on a clean, anti-static surface and visually inspect for transit damage.
7. Install the new board, hand-tightening fasteners first to ensure gate-pin alignment, then torque to specification (typically 2.5 – 3.5 Nm for M4, 4 – 6 Nm for M5 — verify against the Siemens mechanical drawing for the frame).
8. Reconnect gate-drive and auxiliary leads. Reinstall fiber-optic connectors following the procedure in Section 7.
9. Torque-check all electrical terminals to the values on the drive's nameplate or terminal diagram.

7. Fiber Optic Cable Handling Procedure

The firing boards communicate with the control board through polymer optical fiber (POF) cables. Each phase uses two fibers: a transmit (white-coded, carries control pulses FWD/REV from control board to firing board) and a receive (black-coded, returns IGBT status from firing board to control board). On a three-phase drive, the bundle therefore contains 6 transmit fibers (3 phases × 2 commands FWD/REV) plus 3 receive fibers.

Connector seating procedure:

1. Confirm the drive is fully discharged and locked out.
2. Insert the fiber end into the firing-board connector until the white band on the fiber jacket sits flush with the connector opening. This indicates full fiber insertion.
3. Tighten the connector locking screw with finger-tight torque only. Do not use tools. Over-tightening strips the connector body and cracks the fiber end-face.
4. Do not use thread-locking compound, thread sealant, or adhesive on the connector threads. The connector is designed for finger-tight dry assembly.
5. Route the fiber in a gentle curve to the control board, securing with cable ties that do not compress the fiber jacket.
6. Repeat for every fiber connection before applying power.

Fiber-Optic Connector Reference (Inline SVG)

8. Power Interface Module (PIM) Verification

The power interface module sits between the control board and the firing boards. Its functions:

• Distributes isolated DC supply rails to all three firing boards.
• Conditions current-transformer (CT) feedback from the output busbars for the control board's ADC inputs.
• Provides the DC-link voltage divider and scaling for the control board's V_DC measurement.

A failing PIM can present as an F26 because the CT feedback signal becomes noisy, leading the control board's current regulator to command excessive di/dt, which in turn causes a legitimate UCE event in the firing board's view. Before condemning the IGBT stack, replace or swap-test the PIM — it is far less expensive than an IGBT stack and is field-replaceable on most frames.

9. Predictive Maintenance Strategy

Because firing-board failures scale strongly with operating hours and ambient thermal stress, a planned replacement strategy is more cost-effective than reactive replacement. Recommended approach:

1. Log run hours, ambient temperature near the drive, and number of F025/F026/F027 trips per drive in a maintenance database.
2. Compute an estimated MTBF from the historical data. Field evidence from the 6SE7038-class suggests the firing-board population begins to show failures after ~50,000 – 80,000 operating hours in normal industrial environments, but this varies widely with switching frequency, load profile, and cooling effectiveness.
3. When one phase's firing board fails, schedule replacement of all three phase firing boards and the PIM in the next planned outage. Continuing to run with mixed old/new boards typically results in a second trip within 3 – 9 months.
4. Inspect and clean fiber-optic connectors annually. Replace any fiber showing yellowing, kinking, or connector wear.
5. Verify gate-driver supply rails during annual thermographic inspection — a firing board running hot at the gate-driver ICs is the leading indicator of imminent failure.

10. Verification and Commissioning After Repair

1. With the drive isolated, perform a complete insulation test (megger) of the motor cable and motor. Confirm > 1 MΩ per kV of rating.
2. Power the drive with the motor disconnected. Run through the SIMOVERT commissioning wizard or the equivalent parameter set: confirm P060 (ramp-up time), P064 (command source), and P366 (firing enable) are correct.
3. Execute a no-load test run at 10 Hz, then 25 Hz, then 50 Hz (or the motor's rated frequency). Hold each step for at least 5 minutes. Monitor r029 (current), r027 (frequency), and r947 (active fault) throughout.
4. Apply 50% load for 30 minutes, monitoring firing-board temperature with an infrared thermometer or thermal imaging camera. A healthy firing board runs at < 70 °C ambient-rise above inlet air temperature.
5. Apply 100% load for 1 hour. Confirm no F25/F26/F27 events.
6. Record baseline parameters: firing-board temperature at full load, motor current at full load, and DC-link voltage ripple. These become the comparison baseline for future maintenance.
7. Update the maintenance log with the replacement parts used, the operating hours at replacement, and the post-repair running hours.

11. Related Fault Codes and Cross-Reference

For the full fault list, refer to the SIMOVERT MASTER DRIVES parameter list (parameter r947, r949) and the operating instructions manual for the specific frame. The Siemens Industry Online Support portal hosts the complete SIMOVERT MASTER DRIVES documentation archive searchable by order code.

12. Diagnostic Flowchart

13. Frequently Asked Questions