Correct understanding of the RDS (on) Temperature Coefficient Characteristics of power MOS

In general, many materials and materials believe that the on-resistance of the mos hasPositive temperature coefficientAnd can work in parallel. When the temperature of a parallel transistor rises, the on-resistance with a positive temperature coefficient also increases, so the flow current decreases and the temperature decreases, so that the automatic average flow reaches the balance. For a power transistor device, many small cells are connected in parallel. The on-resistance of the unit cells has a positive temperature coefficient, so parallel operation is no problem. However, when you have a thorough understanding of the transmission characteristics and temperature of the power MOS, and the equivalent circuit model of each unit cell, you will find that, the above theory can be established only when the MOSFET enters the state of steady-state conduction. However, the above theory is not true in the transient process of switch conversion. Therefore, some problems may occur in practical application, this article will discuss these problems in detail to correct the limitations and one-sidedness of traditional cognition.

Power MOS transmission features

The transistor has three work zones: the cut-off zone, the zoom-in zone, and the saturation zone. The MOs correspond to the shutdown zone, the saturation zone, and the linear zone. The saturation zone of the mos corresponds to the amplification zone of the transistor, while the linear zone of the mos corresponds to the saturation zone of the transistor. MosLinear ZoneIt is also called a three-pole zone or a variable resistance zone. In this zone, MOSFETs is basically completely turned on.

When the mos work in the saturated area, the mos have the signal amplification function, and the gate voltage and drain current maintain a certain constraint relationship based on their cross-guide. The relationship between the gate voltage and the drain current is the transmission characteristics of the MOs.

Among them, μ n is the mobility of electrons in the inverse layer, Cox is the ratio of oxide dielectric constant to oxide thickness, W and l are the channel width and length, respectively.

Influence of Temperature on power MOS Transmission Characteristics

Figure 1 MOS Transfer Characteristics

Typical transmission characteristics can be found in the data sheet of the MOs. Note that there is an intersection between the 25 ℃ and 175 ℃ curves, which corresponds to the corresponding vgs voltage and ID current value. If the vgs at the intersection is called the turning voltage, you can see that the higher the temperature, the larger the flow of current when the vgs voltage is fixed, the temperature and current form a positive feedback, that is, the RDS (on) of the MOS is a negative temperature coefficient, which can be called the negative temperature coefficient area of the RDS (on.

In the upper-right curve of the vgs turning voltage, when the vgs voltage is fixed, the higher the temperature, the smaller the flow current, and the negative feedback is formed for the temperature and current, that is, the RDS (on) of the MOs) positive temperature coefficient. This region can be called the RDS (on) positive temperature coefficient region.

Equivalent Model of the internal unit cell of the power MOS

Inside a power transistor, the Unit is composed of many units, namely a small unit cell, which is connected in parallel. In the area of the unit, the larger the parallel unit cell is, the more on-resistance RDS (on) of the MOs) smaller. Similarly, the larger the area of the crystal element, the larger the unit cell of the produced MOs, and the smaller the on resistance rds (on) of the MOs. The G pole and S pole of all units are connected by the internal metal conductor to gather at a certain position of the crystal element, and then lead to the pin, so that the G pole serves as a reference point at the collection of the crystal element, the resistance to each unit cell is not completely consistent. The larger the equivalent series resistance of the G pole, the farther away from the collection point.

It is precisely because of the partial pressure of the equivalent gate and source resistance in series that the vgs voltage of the unit cell is inconsistent, resulting in the current inconsistency of each unit cell. In the process of activating the MOs, due to the influence of the gate capacitor, the current of each unit cell is inconsistent.

Thermal imbalance of crystalline cells during power switch Transient Process

As shown in figure 2, the ID of the drain current gradually increases during the activation process, the voltage of the unit cell close to the gate pin is greater than that of the unit cell far from the gate pin, that is, vg1> vg2> vg3> ..., The Unit with high vgs voltage, that is, the unit cell close to the gate pin, flows through a large amount of current, while the unit cell far from the gate pin flows through a small amount of current, the unit cell in the farthest distance may not even be turned on, so there is no current flowing through. Large cell units with high current increase in temperature.

Figure 2 internal equivalent model of power MOSFET

As the vgs voltage gradually increases to the driving voltage during the activation process, the vgs voltage crosses the negative temperature coefficient area of the RDS (on). At this time, the Unit Cells with higher temperature are, due to positive feedback, the flow of current is further increased, and the unit temperature is further increased. If the vgs operates or stays longer in the negative temperature coefficient area of the RDS (on), the more likely these unit cells are to have thermal breakdown, resulting in local damage.

If the vgs does not cause local damage when it reaches the positive temperature coefficient area of the RDS (on) from the negative temperature coefficient area of the RDS (on) in the positive temperature coefficient area, the higher the temperature of the unit cell, the lower the flow of current, the negative feedback is formed between the unit cell temperature and current, and the unit cell automatically fl the flow to achieve a balance.

Correspondingly, the voltage of the unit cell far away from the gate pin decreases slowly during the case of power-off. It is easy to cause local overheating and damage in the negative temperature coefficient area of the RDS (on.

Therefore, accelerating the activation and shutdown of the mos enables the MOs to quickly reduce local energy accumulation through the negative temperature coefficient area of the RDS (on) to prevent local overheating and damage of the unit cell.

Based on the above analysis, we can see that when the MOs are partially damaged, if the damaged hotspot is located in the area close to the gate pin, the local damage may be caused by the slow activation speed; if the damaged hotspot is located in a region far from the gate pin, it may be local damage caused by slow shutdown.

When a large capacitor is added to the gate and source pole, the MOs are often damaged during startup, it is precisely because of an extra large input capacitor that causes a larger imbalance in the vgs voltage of the unit cell, which may lead to local damage.

Conclusion

1. during the activation process, the RDS (on) is converted from the negative temperature coefficient area to the positive temperature coefficient area. During the shutdown process, the RDS (on) transition from the positive temperature coefficient area to the negative temperature coefficient area.

2. the voltage of the vgs of the unit cell is inconsistent with that of the unit cell. As a result, the current of each unit cell is inconsistent, local Overheating and damage are formed during opening and shutdown.

3. Fast Opening and shutdown of the mos can reduce the aggregation of local energy and prevent local overheating and damage of the unit cell. The activation speed is too slow. Local Overheating and damage may occur in areas close to the gate pin, and the shutdown speed is too slow. Local Overheating and damage may occur in areas far from the gate pin.

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