“I believe that many Electronic design workers know MOS tubes, so do you really know how to use MOS tubes? MOS tubes are conducted by majority carriers, also known as unipolar transistors. It belongs to a voltage-controlled semiconductor device. It has the advantages of high input resistance (10^7~10^12Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, wide safe working area, etc., and has now become a bipolar Transistor and A strong contender for power transistors.
I believe that many electronic design workers know MOS tubes, so do you really know how to use MOS tubes? MOS tubes are conducted by majority carriers, also known as unipolar transistors. It belongs to a voltage-controlled semiconductor device. It has the advantages of high input resistance (10^7~10^12Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, wide safe working area, etc., and has now become a bipolar transistor and A strong contender for power transistors.
All MOS integrated circuits (including P-channel MOS, N-channel MOS, complementary MOS – CMOS integrated circuits) have an insulating gate to prevent voltage breakdown. The thickness of the insulating gate oxide layer of the general device is about 25nm, 50nm and 80nm. There is also a resistor-diode network in front of the high-impedance gate of the integrated circuit for protection. However, the protection network in the device is not enough to avoid electrostatic damage (ESD) to the device. The device may also fail due to the accumulation of multiple lower voltage discharges.
There are various forms of electrostatic damage according to the severity of the damage. The most serious and most likely to occur is the complete destruction of the input terminal or the output terminal, so that it is short-circuited or opened with the power terminal VDD GND, and the device completely loses its original function. The less serious damage is intermittent failures or performance degradation, which is harder to detect. There is also some electrostatic damage that can increase leakage current and degrade device performance.
Some precautions for MOS tube application
As a voltage-driven high-current device, MOS tubes are widely used in circuits, especially power systems. MOS tubes have some characteristics that we should pay special attention to in practical applications.
1. MOS tube body diode, also known as parasitic diode, exists in a single MOS tube device, but not in integrated circuit lithography. This diode can play the role of reverse protection and freewheeling in high current driving and inductive loads. , Generally, the forward voltage drop is about 0.7-1V. Because of the existence of this diode, the MOS device cannot simply see the function of a switch in the circuit. For example, in the charging circuit, after the charging is completed, the battery will reverse after the power is removed. Supplying power to the outside, this is usually the result that we do not want to see. The general solution is to add a diode at the back to prevent reverse power supply. Although this can be done, the characteristics of the diode must have a positive voltage of 0.6-1V. The voltage drop will cause serious heat generation in the case of high current, and at the same time, it will cause waste of energy and low energy efficiency of the whole machine. Another method is to add a back-to-back MOS tube, and use the low on-resistance of the MOS tube to achieve the purpose of energy saving. Another common application of this feature is low-voltage synchronous rectification.
2. The MOS tube has no directionality after it is turned on. After the MOS is turned on under pressure, it is similar to a wire. It only has resistance characteristics and has no on-state voltage drop. Usually, the saturated on-resistance is several to tens of milliohms. , and non-directional, allowing DC and AC to pass.
Precautions for using MOS tube
(1) In order to use the MOS tube safely, the power dissipation of the tube, the limit values of the parameters such as the maximum drain-source voltage, the maximum gate-source voltage and the maximum current cannot be exceeded in the design of the circuit.
(2) When each type of MOS tube is used, it must be connected to the circuit strictly according to the required bias, and the polarity of the MOS tube bias must be observed. For example, there is a PN junction between the gate, source and drain of the junction MOS tube, and the gate of the N-channel tube cannot be positively biased; the gate of the P-channel tube cannot be negatively biased, and so on.
(3) Due to the extremely high input impedance of the MOSMOS tube, the lead pins must be short-circuited during transportation and storage, and metal shielding should be used to prevent the gate from being broken down by external induced potential. In particular, it should be noted that the MOSMOS tube cannot be placed in a plastic box. It is best to store it in a metal box. At the same time, pay attention to the moisture-proof of the tube.
(4) In order to prevent the inductive breakdown of the gate of the MOS tube, it is required that all test instruments, workbenches, soldering irons, and the circuit itself must be well grounded; when the pins are soldered, solder the source first; before connecting to the circuit, All lead ends of the tube are kept short-circuited to each other, and the short-circuit material is removed after welding; when the tube is removed from the component rack, the human body should be grounded in an appropriate way, such as using a grounding ring; of course, if it can be used Advanced gas-heated electric soldering irons are more convenient and safe to weld MOS tubes; when the power is not turned off, the tubes must not be inserted into the circuit or pulled out of the circuit. The above safety measures must be paid attention to when using MOS tubes.
(5) When installing the MOS tube, pay attention to avoid the installation position as close to the heating element as possible; in order to prevent the vibration of the tube, it is necessary to fasten the tube shell; when the pin lead is bent, it should be 5 mm larger than the root size. , to prevent the pins from being bent and causing air leakage, etc.
(6) When using a VMOS tube, a suitable radiator must be added. Taking VNF306 as an example, the maximum power can only reach 30W after the tube is installed with a radiator of 140×140×4(mm).
(7) After multiple tubes are connected in parallel, due to the corresponding increase of inter-electrode capacitance and distributed capacitance, the high-frequency characteristics of the amplifier are deteriorated, and the high-frequency parasitic oscillation of the amplifier is easily caused by feedback. For this reason, there are generally no more than 4 parallel composite tubes, and an anti-parasitic oscillation resistor is connected in series on the base or grid of each tube.
(8) The gate-source voltage of the junction MOS tube cannot be reversed, and can be stored in an open state. When the insulated gate MOS tube is not in use, due to its very high input resistance, each electrode must be short-circuited to avoid external electric fields. damage to the tube.
(9) When welding, the shell of the electric soldering iron must be equipped with an external ground wire to prevent the tube from being damaged due to the electrification of the electric soldering iron. For a small amount of soldering, you can also heat the soldering iron and unplug it or cut off the power supply before soldering. Especially when welding the insulated gate MOS tube, it should be welded in the order of source-drain-gate, and the welding should be powered off.
(10) Use 25W electric soldering iron to solder quickly. If soldering with 45~75W electric soldering iron, use tweezers to clamp the root of the pin to help dissipate heat. The quality of the junction MOS tube can be qualitatively checked by the meter resistance file (check the forward and reverse resistance of each PN junction and the resistance value between the drain and source), while the insulated gate field effect tube cannot be checked with a multimeter, but a tester must be used. Moreover, the short circuit of each electrode can be removed after the tester is connected. When removing, it should be short-circuited first and then removed. The key is to avoid the gate being suspended.
When using in occasions requiring high input impedance, moisture-proof measures must be taken to prevent the input resistance of the MOS tube from being reduced due to the influence of temperature. If a four-lead MOS tube is used, its substrate lead should be grounded. The sesame tube encapsulated in ceramic has photosensitive properties and should be used in the dark.
For power MOS tubes, good heat dissipation conditions are required. Because the power MOS tube is used under high load conditions, a sufficient heat sink must be designed to ensure that the case temperature does not exceed the rated value, so that the device can work stably and reliably for a long time. In short, to ensure the safe use of MOS tubes, there are various matters to pay attention to, and various safety measures to be taken. The majority of professional and technical personnel, especially the majority of electronic enthusiasts, should proceed according to their actual situation. Take practical measures to make good use of MOS tubes safely and effectively. The above are the places that MOS tubes should pay attention to during use, and designers need to strictly follow the specifications.
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