MOS transistor is a key component widely used in electronic circuits, especially playing a crucial role in fields such as switching power supplies, motor drives, and power amplifiers. The quality of MOS transistors directly affects the performance and reliability of circuits. Although the most common method for detecting the status of MOS transistors is through electrical performance testing, appearance inspection is also an effective preliminary screening method. Through visual inspection, some obvious signs of damage can be detected in a timely manner, helping us determine whether it needs to be replaced in a shorter period of time.
1. Basic steps of appearance inspection
The appearance inspection of MOS transistors mainly determines whether there is physical damage by visually observing their packaging, pins, surface condition, etc. Here are some key checkpoints.
1.1 Check if the packaging is complete
The packaging of MOS diodes usually adopts various forms such as TO-220, TO-247, SOT-23, etc. The integrity of the packaging is crucial for the normal operation of MOS transistors. Damaged or cracked packaging may cause damage to the internal structure of MOS transistors, affecting their functionality.
Cracks or fragments: If cracks or fragments appear on the packaging surface of MOS transistors, it may be due to excessive mechanical impact or high temperature overload, resulting in package rupture. Package damage not only affects its heat dissipation performance, but may also lead to electrical short circuits or open circuits, causing MOS transistors to malfunction.
Packaging expansion or deformation: High temperature or overcurrent may cause overheating inside the MOS transistor, resulting in packaging deformation or expansion. The deformed packaging may no longer be reliable and may cause internal circuit failures.
1.2 Check the surface condition
The surface of MOS transistor should be kept clean and free of foreign matter adhesion. Surface abnormalities may be external manifestations of thermal damage, current overload, or other faults occurring inside the MOS transistor.
Surface discoloration: Under normal circumstances, there should be no obvious discoloration or coking on the surface of MOS transistors. If there is a significant change in surface color, especially turning black or yellow, it usually indicates that it has experienced overheating or short circuit events and may have been damaged.
Burn or smoke marks: If there are burn or black burn marks on the surface of the MOS transistor, it may be due to overheating caused by overload, short circuit, or long-term high current operation, resulting in local damage.
1.3 Check the condition of the pins
The pins of a MOS transistor are a critical part of its connection to the circuit, and any damage or corrosion to the pins may cause the circuit to malfunction.
Pin corrosion or oxidation: Prolonged exposure to humid environments or high temperatures can cause corrosion or oxidation of MOS transistor pins, resulting in discoloration, blackening, or the appearance of green spots on the surface of the pins. Corrosion and oxidation can cause poor connections between pins and circuits, thereby affecting the operation of MOS transistors.
Pin bending or breakage: If the pin is bent, broken, or loose, it can cause poor electrical contact and even prevent normal connection to the circuit, and the damaged MOS transistor must be replaced.
2. Use auxiliary tools to further detect
In addition to visual inspection, using some simple tools can help us more accurately determine whether the MOS transistor is damaged.
2.1 Digital multimeter inspection
A multimeter is a commonly used tool for detecting the status of MOS transistors. When conducting digital multimeter testing, the main focus is on checking the switching characteristics of MOS transistors, including their conduction characteristics, breakdown voltage, etc.
Leakage current test: Use the diode gear of a multimeter to measure the leakage current of the MOS transistor. Under normal circumstances, the leakage current of MOS transistors should be very small. In the “diode mode” of the multimeter, check the resistance between the source and drain of the MOS transistor. If there is obvious current leakage, it indicates that the MOS transistor may have been damaged.
Gate voltage inspection: Determine the switching characteristics of MOS transistors by measuring the effect of gate voltage on the source. Under normal circumstances, the gate voltage of a MOS transistor needs to reach a certain threshold in order to conduct. If the gate voltage is inappropriate or the gate current is abnormal, there may be a problem with the MOS transistor.
2.2 Infrared thermal imaging detection
In high-power circuits, MOS transistors generate heat during operation. Through infrared thermography detection, we can identify whether MOS transistors are overheated, which is very helpful in determining whether they are damaged.
Uneven temperature or overheating: If abnormal temperature distribution of the MOS tube is detected through thermal imaging, especially if a certain part of the temperature is too high, it indicates that there may be a short circuit or overload inside the MOS tube, leading to overheating damage. At this point, it is necessary to replace the damaged MOS transistor.
2.3 Oscillator Test
For power MOS transistors, switching characteristics testing can be conducted using an external oscillator to check their switching frequency and response speed. If the switching frequency of the MOS transistor is too low or the response time is delayed, it may indicate that its internal structure has degraded or been damaged.
By checking the physical characteristics such as packaging integrity, surface condition, and pin connections, we can preliminarily identify the damage of MOS transistors. Although appearance inspection cannot completely replace electrical performance testing, it can help us quickly detect external damage and take timely measures to avoid the spread of faults.
