Understanding the RDS(ON) temperature coefficient characteristics of power MOSFETs

Generally, many materials and textbooks believe that the on-resistance of MOSFET has a positive temperature coefficient, so it can work in parallel. When the temperature of one of the parallel MOSFETs rises, the on-resistance with a positive temperature coefficient also increases, so the current flowing through decreases and the temperature decreases, thereby achieving automatic current sharing and achieving balance. Similarly, for a power MOSFET device, there are many small cells in parallel inside it, and the on-resistance of the cell has a positive temperature coefficient, so there is no problem with parallel operation. However, when you deeply understand the transmission characteristics of power MOSFET and the influence of temperature on its transmission characteristics, as well as the equivalent circuit model of each cell unit, you will find that the above theory is only valid when the MOSFET enters the steady-state conduction state, and the above theory is not valid in the transient process of switch conversion. Therefore, some problems will arise in practical applications. This article will discuss these problems in detail to correct the limitations and one-sidedness of traditional understanding.

Transmission characteristics of power MOSFET

The transistor has three working regions: cut-off region, amplification region and saturation region, while the corresponding MOSFET is the turn-off region, saturation region and linear region. The saturation region of MOSFET corresponds to the amplification region of triode, while the linear region of MOSFET corresponds to the saturation region of triode. The linear region of MOSFET is also called triode region or variable resistance region. In this region, MOSFET is basically fully turned on.

When MOSFET works in the saturation region, MOSFET has a signal amplification function, and the gate voltage and drain current maintain a certain constraint relationship based on its transconductance. The relationship between the gate voltage and the drain current is the transfer characteristic of MOSFET.


Among them, μn is the mobility of electrons in the inversion layer, COX is the ratio of the oxide dielectric constant to the oxide thickness, and W and L are the channel width and length, respectively.

The influence of temperature on the transmission characteristics of power MOSFET

In the data sheet of MOSFET, its typical transmission characteristics can usually be found. Note that the two curves of 25℃ and 175℃ have an intersection, and this intersection corresponds to the corresponding VGS voltage and ID current value. If the VGS at this intersection is called the turning voltage, it can be seen that: in the lower left part of the curve of the VGS turning voltage, when the VGS voltage is constant, the higher the temperature, the greater the current flowing, and the temperature and current form a positive feedback, that is, the RDS(ON) of the MOSFET is a negative temperature coefficient, and this area can be called the negative temperature coefficient area of ​​RDS(ON).

Figure 1 MOSFET transfer characteristics

In the upper right part of the curve of the VGS transition voltage, when the VGS voltage is constant, the higher the temperature, the smaller the current flowing, and the temperature and current form a negative feedback, that is, the RDS(ON) of the MOSFET is a positive temperature coefficient, and this area can be called the RDS(ON) positive temperature coefficient area.

The equivalent model of the internal unit cell

of the power MOSFET is composed of many units, that is, small MOSFET units connected in parallel. The more MOSFET units connected in parallel per unit area, the smaller the on-resistance RDS(ON) of the MOSFET. Similarly, the larger the area of ​​the wafer, the more MOSFET units are produced, and the smaller the on-resistance RDS(ON) of the MOSFET. The G poles and S poles of all units are connected by internal metal conductors and gathered at a certain position of the wafer, and then led out to the pins by wires. In this way, the G pole is the reference point at the wafer convergence point, and its resistance to each unit cell is not completely consistent. The farther the unit is from the convergence point, the larger the equivalent series resistance of the G pole.

It is precisely because of the voltage-dividing effect of the series-equivalent gate and source resistances that the VGS voltage of the unit cell is inconsistent, resulting in inconsistent currents in each unit cell. During the MOSFET turn-on process, the influence of the gate capacitance will aggravate the inconsistency of the currents of each unit cell. The

thermal imbalance of the unit cell during the transient state of the power MOSFET switch

can be seen from Figure 2: During the turn-on process, the drain current ID is gradually increasing, and the voltage of the unit cell close to the gate pin is greater than the voltage of the unit cell far from the gate pin, that is, VG1>VG2>VG3>…, the unit with a high VGS voltage, that is, the unit cell close to the gate pin, has a large current flowing through it, while the unit cell far from the gate pin has a small current flowing through it, and the unit cell far from the gate pin may not even be turned on, so no current flows through it. The unit cell with large current has its temperature increased.

Figure 2 Internal equivalent model of power MOSFET

Since the voltage of VGS gradually increases to the driving voltage during the opening process, the voltage of VGS crosses the negative temperature coefficient region of RDS(ON). At this time, due to the effect of positive feedback, the current flowing through the higher temperature unit cells is further increased, and the temperature of the unit cells rises further. If the longer the VGS works or stays in the negative temperature coefficient region of RDS(ON), the more likely these unit cells are to overheat and break down, causing local damage.

If VGS does not cause local damage when it reaches the positive temperature coefficient region of RDS(ON) from the negative temperature coefficient region of RDS(ON), at this time, in the positive temperature coefficient region of RDS(ON), the higher the temperature of the unit cells, the smaller the current flowing through, the unit cell temperature and current form negative feedback, and the unit cells automatically share the current to achieve balance.

Correspondingly, during the MOSFET shutdown process, the voltage of the unit cells far from the gate pin decreases slowly, and it is easy to form local overheating and damage in the negative temperature coefficient region of RDS(ON).

Therefore, speeding up the turn-on and turn-off speed of MOSFET and making MOSFET pass through the negative temperature coefficient region of RDS(ON) quickly can reduce the accumulation of local energy and prevent the local overheating and damage of the unit cell.

Based on the above analysis, it can be concluded that: when the MOSFET is locally damaged, if the damaged hot spot is located in the area close to the gate pin, it may be a local damage caused by too slow turn-on speed; if the damaged hot spot is located in the area far from the gate pin, it may be a local damage caused by too slow turn-off speed.

If a large capacitor is added to the gate and source, MOSFET damage will often occur during the startup process. It is precisely because the additional large input capacitor causes a greater imbalance in the VGS voltage of the unit cell, which makes it easier to cause local damage.

Conclusion

1. During the turn-on process of MOSFET, RDS(ON) transforms from the negative temperature coefficient region to the positive temperature coefficient region; during the turn-off process, RDS(ON) transitions from the positive temperature coefficient region to the negative temperature coefficient region.

2. The voltage divider effect of the equivalent gate and source resistances in series with the MOSFET and the influence of the gate capacitance cause the VGS voltage of the cell unit to be inconsistent, which leads to inconsistent currents in each cell unit, resulting in local overheating damage during the opening and closing process.

3. Rapidly opening and closing the MOSFET can reduce the accumulation of local energy and prevent local overheating and damage to the cell unit. If the opening speed is too slow, the area close to the gate pin is prone to local overheating damage. If the closing speed is too slow, the area far from the gate pin is prone to local overheating damage.