Troubleshooting methods for medium-frequency power supply thyristors

Thyristors (SCRs), as key components in high-power systems such as medium-frequency power supplies and induction heating equipment, can cause equipment shutdown or even damage to other components if they malfunction. The following are systematic and practical methods for troubleshooting thyristor failures:

I. Visual and Physical Inspection

1. Visual inspection: Check the thyristor for obvious damage such as charring, cracks, bulging, blown fuses, or loose solder joints.

2. Heat sink condition: Check if the installation is secure, if the thermal grease is dry, if the heat sink is dusty or blocked, and if the cooling water circuit is unobstructed.

II. Static Testing (with power off)

Use a digital multimeter or transistor tester to perform the following measurements (taking a bolt-type thyristor as an example, A-anode, K-cathode, G-gate):

1. A-K resistance:

Both forward and reverse resistance should be high (usually >1 MΩ). If the resistance is very low (close to 0 Ω), it indicates a short circuit; if it is open, there may be an internal break.

2. G-K resistance:

Forward resistance (red probe to G, black probe to K) should be tens to hundreds of ohms; reverse resistance should be high. If both forward and reverse are open or short-circuited, the gate may be damaged.

3. Trigger conduction test (simple method):

Set the multimeter to the R×1k range, connect the black probe to A and the red probe to K; there should be no conduction at this point;

Briefly short-circuit A-G with your finger or a wire (simulating triggering). If the meter needle deflects and remains conductive, and remains conductive after releasing, the thyristor is normal; if it fails to conduct or cannot maintain conduction, it may be faulty.

⚠️ Note: This method is only suitable for low-power thyristors; a dedicated tester is recommended for high-power devices.

III. Dynamic and Operational Testing (with power on)

1. Observe operating phenomena:

Equipment tripping upon startup, insufficient output power, abnormal current fluctuations, and inverter failure may all be related to the thyristor. 2. Oscilloscope Testing:

Measure the gate trigger pulse: Confirm that the pulse amplitude (usually ≥4 V), width (>20 μs), and timing are correct;

Observe the anode voltage/current waveform: Check for any abnormalities such as missing half-waves, distortion, or oscillations.

3. Temperature Monitoring:

During operation, use an infrared thermometer to measure the case temperature of each thyristor.  Significantly high temperature rises may indicate poor conduction or uneven current distribution.

IV. System-Level Troubleshooting

Check if the RC snubber circuit is open or if the capacitor has failed, leading to overvoltage breakdown of the thyristor;

Check if the fast-acting fuse has blown, determining if the fault was caused by overcurrent;

If multiple thyristors are connected in parallel, check if the current-sharing inductors and connecting copper bars are symmetrical to avoid current concentration.

V. Replacement Verification Method

After ruling out faults in the drive, protection, and peripheral circuits, replace the suspected faulty thyristor with a new one of the same model and batch, and observe whether the equipment returns to normal operation.

Thyristor fault troubleshooting should follow the principles of "external first, then internal; static first, then dynamic; simple first, then complex," combining instrument measurements with experienced judgment. Regular maintenance, standardized operation, and proper selection are fundamental measures to prevent failures.