How to find the substitute for MOSFET
For most of MOSFET transistors it is enough to find an analog which is similar in its parameters. Often it is impossible to substitute the defective MOSFET transistor with the same one. In this case, in order to find an analog, it is necessary to do the following:
- Find out the full name of the transistor by its label. For MOSFET transistors in SMD body it is possible to find out the name on the MOSFET transistor by labeling: SMD-codes 🔗.
- Study the MOSFET transistor connection layout for determining its operating mode (key in switching circuits, impulse device, linear stabilizer, etc.).
- Find the datasheet with the description of the defective transistor.
- Fill in the fields of the selection form of the transistor analog.
- Choose the most suitable analogue of MOSFET transistor from the proposed variants, considering its operating mode in the device.
To what is it necessary to pay attention?
Having opened the PDF datasheet, it is first necessary to find out the transistor type (MOSFET or JFET), polarity, type of case, and pin configuration (pinout).
From the numerical parameters, this is, first, limiting characteristics such as Pd (maximum dissipation power), Vds (maximum drain-source voltage), Vgs (maximum gate-source voltage), and Id (maximum drain current). The selected transistor must have no parameters less than those of the original transistor.
The open transistor drain-source resistance (Rds) is an important parameter of a MOSFET-transistor. The power allocated to the transistor depends on the Rds value. The smaller the Rds value, the less the transistor will heat up.
It is necessary to take into account that the higher the Id and the lower the Rds, the greater the gate capactity of the MOSFET transistor. This leads to a requirement of needing more power to control this gate.
If the circuit does not provide the necessary power, then dynamic losses will increase due to the slow transistor switching speed (the MOSFET will run hotter). So it is necessary to check the heat increase of the transistor after turning on the device. If the transistor becomes very hot, then the problem can be hidden both inside the transistor itself or inside the elements of its binding.
Deciphering the main parameters of MOSFET transistors
Transistor type – different transistor types can be used in real devices: JFET or MOSFET.
Electrical polarity - field effect transistors can be forward or reversed conduction (P- or N-channel).
Maximum Power Dissipation (Pd) – it is necessary to be sure that the chosen transistor can dissipate enough power. This parameter depends on the maximum transistor operating temperature – if the temperature increases, the maximum dissipation power decreases. If the maximum dissipation power is not enough – some transistor features become worse. For example, Rds resistance can double as the temperature increases from 25 ° C to 125 ° C.
Drain-Source Breakdown Voltage (Vds) – is the maximum drain-source voltage that does not cause avalanche breakdown at 25 ° C. It depends on the temperature: the voltage decreases if the transistor temperature decreases too. For example, at -50 ° C, a voltage that does not cause avalanche breakdown may be 90% of Vds at 25 ° C. The maximum accessible drain-source voltage (Vgs) – when a more accessible voltage is applied on a gate, damage to the isolating gate oxide layer is possible (it may also be static electricity). You should not use transistors with a large redundancy of Vds and Vgs voltages, because usually they have the worst speed features.
Gate Threshold Voltage Vgs(th) - if the voltage on the gate is higher that Vgs(th), MOSFET-transistor begins to conduct current through the drain-source channel. Vgs (th) has a negative temperature coefficient: if the temperature increases, the MOSFET transistor begins to open at a lower gate-source voltage.
Continuous Drain Current (Id) – you should bear in mind that some outputs from a transistor body limit the maximum accessible direct drain current (switching current can be greater). If the temperature increases, the maximum accessible current decreases.
Maximum Junction Temperature (Tj) - this parameter limits the temperature of the transistor channel in the enabled state. If it is exceeded, the service life of the transistor may be reduced.
Rise Time (tr) - the time for which the drain current increases from 10% to 90% of the specified.
Drain-Source On-state Resistance (Rds) is the resistance of an open drain-source channel for established parameters: Id, Vgs and Tj.
The above are the most important parameters of MOSFET transistors. The manufacturer indicates a lot of additional parameters in the datasheets: gate charge, gate leakage current, impulse drain current, capacity, etc.
Important considerations to take into account when mounting a MOSFET transistor
When working with MOSFET transistors, it is necessary to take into account that they are highly sensitive to static electricity. Before mounting one on a printed circuit board, it is necessary to connect the transistor outputs to each other with a thin wire. When soldering, it is better to use a soldering station, rather than a regular electric soldering iron. This will reduce the breakup of a gate breakdown caused by static electricity. Or use an anti-static wrist band or anti-static mat.
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