Why is the power MOS tube burned? Come in and see!

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2022-03-17 17:14:03 80

Working States in the mos controller circuit: turn-on process (transition from turn-on to turn-on), turn-on state, turn-off process (transition from turn-on to turn-off) and turn-off state.

The main losses of Mos also correspond to these states, such as switching loss (turn-on process and turn-off process), conduction loss, cut-off loss (caused by leakage current, which is ignored), and avalanche energy loss. As long as these losses are controlled within the mos tolerance specification, the mos will work normally, and if it exceeds the tolerance range, it will be damaged. However, the switching loss is often greater than the on-state loss, and the difference between different mos may be very large.

Main causes of Mos damage:

Overcurrent---------the junction temperature caused by continuous heavy current or instantaneous heavy current is too high and burned down;

Overvoltage-overvoltage breakdown of source and drain, overvoltage breakdown of source and gate;

Static electricity-electrostatic breakdown, CMOS circuits are afraid of static electricity;

Principle of Mos Switch (Brief)：Mos is a voltage-driven device. As long as an appropriate voltage is given between the gate and the source, the channel between the source and the drain will be formed. The resistance of this current path is called mos internal resistance, that is, on resistance < Rds(on) >. This internal resistance basically determines the maximum conduction current that the mos chip can bear (of course, it is related to other factors, the most relevant is the thermal resistance). The smaller the internal resistance, the greater the withstand current (because the heat is small).

The problem of Mos is far from simple. The trouble is that it has equivalent capacitance between its gate and source, between source and drain, and between gate and drain. Therefore, the process of charging the gate voltage is the process of charging the capacitor (the capacitor voltage can't suddenly change), so the opening process of mos source and drain from off to on is restricted by the charging process of the gate capacitor.

However, these three equivalent capacitors are a combination of series and parallel, and they influence each other, so they are not independent. If they are independent, it will be very simple.

One of the key capacitances is the capacitance Cgd between the gate and drain, which is called Miller capacitance in the industry.This capacitance is not constant, but changes rapidly with the voltage between the gate and drain. This Miller capacitor is a stumbling block to the charging of the gate and source capacitors, because after the gate charges the gate-source capacitor Cgs to a platform, the charging current of the gate must charge the Miller capacitor Cgd. At this time, the voltage between the gate and the source stops rising, reaching a platform, which is the Miller platform (Miller platform is the process of charging Cgd). Miller oscillation is the first trouble that people think of in Miller platform. (That is, the gate charges Cgs first, and then charges Cgd after reaching a certain platform).

At this time, the voltage between the source and drain levels changes rapidly, and the internal capacitor charges and discharges rapidly accordingly. These current pulses will lead to a large inductance of mos parasitic inductance. There are capacitors, inductors and resistors in the oscillating circuit (which can form two loops), and the stronger the current pulse, the higher the frequency and the greater the amplitude of the oscillation, so the most critical question is how to transition this Miller platform.

Gs adds capacitor to slow down the conduction time of mos transistor, which helps to reduce Miller oscillation. Prevent mos tube from burning.

Too fast charging can lead to fierce Miller oscillation, but too slow charging can reduce the oscillation, but it will prolong the switch and increase the switching loss. The equivalent resistance between the source and drain of Mos during the turn-on process is equivalent to a transformation process from infinite resistance to very small on-resistance (on-resistance is generally only a few milliohms for low-voltage mos).

For example, the maximum current of a mos is 100a, and the battery voltage is 96v. In the process of opening, there is a moment (when we first enter the Miller platform) when the heating power of the mos is P=V*I (at this time, the current has reached the maximum, the load has not run yet, and all the power falls on the MOS tube), P=96*100=9600w! At this time, its heating power is the highest, and then the heating power decreases rapidly until the power becomes 100*100*0.003=30w when it is fully turned on (here, it is assumed that the internal resistance of this mos is 3 milliohms), and this heating power change is amazing during the switching process.

If the turn-on time is slow, it means that the transition from 9600w to 30w is slow, and the mos junction temperature will rise sharply. So the slower the switch, the higher the junction temperature, and the easier it is to burn mos. In order not to burn the mos, we can only reduce the current limit of the mos or the battery voltage. For example, limit it to 50a or reduce the voltage by half to 48v, so that the heating loss of the switch is also reduced by half, and the tube is not burned.

This is also the reason why high-voltage controller is easy to burn tubes. Only the switching loss of high-voltage controller is different from that of low-voltage controller (the switching loss is basically proportional to the terminal voltage of battery, assuming the same current limit). The conduction loss is completely determined by mos internal resistance and has nothing to do with battery voltage.

Actually, the whole mos opening process is very complicated. There are too many variables in it. In short, it is difficult for Miller to oscillate when the switch is slow, but the switching loss is large, the tube heats up, and the switching speed is fast. Theoretically, the switching loss is low (as long as Miller oscillation can be effectively suppressed). However, the Miller oscillation is very severe (if the Miller oscillation is very severe, the tube may burn on the Miller platform), but the switching loss is also large, and the upper arm mos oscillation is more likely to cause the lower arm mos to misdirect and form a short circuit between the upper and lower arms.

So this is a test of the designer's skills in driving circuit wiring and main circuit wiring. Finally, it is to find a balance point (generally, the opening process does not exceed 1us). Turn-on loss is the simplest one, which is only proportional to the on resistance. If you want to find one with high current and low loss and low internal resistance.

▼ Here's an introduction to the practical point for ordinary users

▼Brief description of important parameters selected by Mos, with datasheet as an example:

Gate charge;Qgs, Qgd：

Qgs：

Refers to the total charge when the grid is charged from 0v to the corresponding current Miller platform (the actual current is different, and the height of this platform is different. The higher the current, the higher the platform, and the greater this value). This stage is to charge Cgs (also equivalent to Ciss, input capacitor).Qgd：Refers to the total charge of the whole Miller platform (called Miller charge here). This process charges Cgd(Crss, this capacitor changes rapidly with gd voltage).

The following is the model stp75nf75:

Our common 75-tube Qgs is 27nc and Qgd is 47nc. Combined with its charging curve.

Charge Cgs before entering the platform, with total charge Qgs 27nc and Miller charge Qgd 47nc on the platform.

In the switching overshoot, the main heating interval of mos is the stage marked by thick red. The main heating interval is from Vgs exceeding the threshold voltage to Miller platform. In which the mos is basically completely turned on after the Miller platform, and the loss is the basic conduction loss (the lower the internal resistance of the mos, the lower the loss).

Before the threshold voltage, the mos is not turned on, and there is almost no loss (only a little loss caused by leakage current). Among them, the loss is the highest at the red corner (Qgs charging is almost over, and the heating power is the highest during the process of approaching the Miller platform and just entering the Miller platform (indicated by thicker lines).

Therefore, under a certain charging current, the tube with small total charge in the red marked interval will pass quickly, so that the heating interval time is short and the total calorific value is low. Therefore, theoretically, selecting mos transistors with small Qgs and Qgd can quickly pass through the switching region.

On resistance: rds (on); Under certain circumstances, the lower the withstand voltage, the better. However, the internal resistance of different manufacturers has different test conditions. The measured value of internal resistance will be different under different test conditions. In the same pipe, the higher the temperature, the greater the internal resistance (this is the characteristic of silicon semiconductor materials in mos manufacturing process, which cannot be changed, but can be slightly improved). Therefore, the internal resistance will increase in high-current test (the junction temperature will increase significantly in high-current test), and decrease in low-current or pulse-current test (because the junction temperature has not increased significantly, and there is no heat accumulation).

The nominal typical internal resistance of some pipes is almost the same as that of your own small current test, while that of some pipes is much lower than the nominal typical internal resistance (because its test standard is high current). Of course, there is also the problem of lax labeling by manufacturers. Don't believe it completely.

Therefore, the selection criterion is-find the mos tubes with small Qgs and Qgd, and at the same time meet the mos tubes with low internal resistance.

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Why is the power MOS tube burned? Come in and see!

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