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MOS tube conduction characteristics

1. Conduction means as a switch, which is equivalent to switch closure.

2. NMOS features: when the Vgs exceed a certain value, it will be on. It is suitable for the condition of the source electrode grounding device (middle and low drive), if the gate voltage is 4V or 10V.

3, PMOS characteristics, Vgs lower than a certain value will be conductive, suitable for the source pole Vcc condition (high-grade drive). However, although PMOS can be easily used as a high-end driver, NMOS is generally used in high-end drivers because of the large conduction resistor, high price, and less replacement type.

MOS switch tube damage

1. Conduction loss refers to the output power loss caused by the rectifier tube from cut-off to conduction. Whether it is NMOS or PMOS, there are conduction resistors after conduction, in which case the point current will waste kinetic energy on the resistor, which is called conduction loss. Choosing MOS transistors with small conduction resistors will reduce conduction loss. Nowadays, MOS transistors with small output power are generally around tens of millivolts.

2. In the case of conduction and cut-off, MOS must not be carried out in a flash. The voltage on both sides of MOS has a whole process of decreasing, and the current passing through has a whole process of increasing. During this period, the voltage and current of MOS tube damage are multiplied, which is called switch damage. Generally, switch damage is much larger than conduction damage, and the faster the switching frequency is, the greater the damage will be.

3. The multiplication of voltage and current flow at the moment of conduction is very large, resulting in very large damage. Reduce the switching time, can reduce the damage in each conduction, reduce the switching frequency, can reduce the switching frequency per unit time. Both methods can reduce switch damage.

4, soft switch technology, the power supply circuit is in the full bridge rectifier circuit added capacitors and diodes. Diode in the switch tube conduction clamping effect, and constitute the discharge control circuit, discharge discharge flow. The capacitor is charged by the battery under the effect of flyback voltage, the voltage cannot be suddenly increased, when the voltage is relatively high, the current is already 0. That will keep the switch from wearing out too much.

MOS tube driver

1, compared with the dual optical crystal triode, generally think that MOS tube conduction without electricity flow, if the GS voltage is higher than a certain value, it will be ok

2. In the construction of MOS tube, it can be seen that there are parasitic capacitors between GS and GD, while the drive of MOS tube is actually to charge the capacitor battery. The capacitor's battery must be charged at a current rate, which is high because the capacitor's battery can be treated as a short-circuit fault. When selecting/designing a MOS tube drive, be aware of the size of the short-circuit capacity that can be presented in a flash.

3, widely used for high-grade driving NMOS, conducting must be the gate voltage more than the source voltage. The source voltage and drain voltage (Vcc) of high-grade driven MOS tube are the same, so the gate voltage is 4V or 10V larger than the Vcc. If in the same system software, to obtain a higher voltage than the Vcc, you need a professional boost circuit. Many motor drivers are integrated charge pump, it should be noted that the appropriate external capacitor should be selected to obtain sufficient short-circuit capacity to drive the MOS tube.

4. The 4V or 10V mentioned above are the common on-off voltage of MOS tube, so the design scheme must naturally have a certain capacity. And the higher the voltage, the faster the conduction rate and the smaller the conduction resistor. Nowadays, MOS with lower conduction voltage is useful in different industries, but in the electronic control system of 12V car, 4V conduction is generally enough.

MOS tubes use power circuits

The most obvious characteristic of MOS tube is the good switching characteristic, so it is widely used in the power circuit of the switch of necessary electronic devices, such as switch power supply and motor drive power supply circuit, as well as lighting lamps. With today's MOS drivers, there are a few specific requirements:

1. Bottom pressure application

When the 9V switching power supply is applied, at the moment, if the traditional totem pole structure is applied, because the be of the audion only loses 0.7V up and down, the voltage finally loaded into the gate is only 4.3V. At the moment, the MOS tube with the error gate voltage of 4.9V has certain risks. The same problem arises where 3V or other bottom voltage switching power supplies are used.

2. Wide voltage application

The input voltage is not a number; it varies with time or other factors. This change causes the DRIVE voltage shown to the MOS tube by the PWM circuit to be not stable. In order to make MOS tubes safer under high gate voltage, many MOS tubes are embedded with voltage stabilizer tubes to compulsively limit the amplitude of GATE voltage. In such cases, when the presented drive voltage exceeds the voltage of the regulator tube, a large static data function loss occurs. In addition, if the basic principle of resistance voltage division is simply used to reduce gate voltage, the MOS tube works well when the input voltage is relatively high, but the gate voltage is not enough when the input voltage is reduced, resulting in insufficient conduction and thereby improving the function loss.

3, in the power module, the common switch power supply IC immediately drives the MOS tube.

In applications, attention should be paid to the two main parameters, the maximum current of the larger drive and the parasitic capacitance of the MOS tube. Here, the driving capability of the SWITCH power IC, the size of the MOS parasitic capacitance, and the resistance value of the drive resistor will all harm the MOS transistor switching rate. If the parasitic capacitance of MOS tube is relatively large and the driving working ability of switch power supply IC is insufficient, the driving working ability must be improved on the drive power supply circuit. Totem pole power supply circuit is often used to improve the driving working ability of switch power supply IC. At present, none of the various MOS tube drive power circuit is the best. It must be based on the actual application by the customer, combined with the user manual presented by the MOS tube manufacturer, and continuously improve the power circuit and main parameters, so as to polish and polish the best drive plan for its own use.