Understanding Current Imbalance in Power MOSFETs

The article discusses the impact of current imbalance on paralleled power semiconductor devices. It also describes the types of current imbalance that can occur, such as static and dynamic imbalance, and provides strategies for managing and mitigating current imbalance over time.

To boost the current rating of power conversion systems, power semiconductor devices are frequently coupled in parallel. However, the current of paralleled power semiconductor devices may be out of balance because of mismatched circuit characteristics or differences in semiconductor production.

How Much Can a Current Imbalance Impact Parallel Semiconductor Devices?

It could result in accelerated aging and long-term reliability problems. The current imbalance between paralleled power devices may result in uneven mechanical performance, inconsistent thermal conductivity, and ultimately reliability problems. As a result, there is no quick fix to optimize the current imbalance between the paralleled dies. A thorough investigation of the factors causing the current imbalance must be done before it can be mitigated.

Types of Current Imbalance

The current imbalance can be broken down into two distinct types: static and dynamic. Static imbalance can cause mismatched conduction losses, while dynamic imbalance can cause unequal switching losses and  peaks, both of which are relevant to the current stress. This article presents Zhao's concept of imbalance degree.

Static Degree of Imbalance

Usually, the mismatched ON-state resistance    causes a static imbalance. The current in power MOSFETs is proportional to the conductance. The MOSFET's ON-state resistance controls the current through it.    on each paralleled MOSFET is the same. The MOSFET with a lower    has a higher current     , according to    =     . The MOSFET with lower    can withstand higher conduction losses and junction temperatures because conduction loss can be calculated using    =  . Due to the positive temperature coefficient of   , the static imbalance is typically not fatal.

Dynamic Degree of Imbalance

The mismatched switching trajectory, which is a result of various factors such as mismatched stray inductance, gate resistance   , gate threshold voltage   , etc., causes dynamic imbalance. Theoretically, a synchronous gate signal with a different slew rate and an asynchronous gate signal with the same slew rate are two special cases that can be combined to represent the dynamic imbalance.

Thermal Imbalance and Long-Term Reliability

A thermal imbalance and long-term reliability issues can result from the current mismatch. A MOSFET's overall power loss can be separated into conduction loss and switching loss, as given in equation (1). 

The Current Imbalance Mechanism

Usually, a number of parameters that cause the current distribution are coupled. On the current distribution, pertinent parameter effects are discussed in this article.

Pertinent Parameters

Fig. 1 shows the equivalent circuit for the paralleled power devices. Device parameters, circuit parameters, and status indicators are three categories for the parameters in Fig. 1 that affect the current distribution. Typically, during the die fabrication process, the intrinsic electric parameters, which are the die's parameters, are determined.

It includes elements such as transconductance, internal gate resistance, and junction capacitance. The package stray, bonding wires, PCB, and cables all introduce parasitic inductance, which is one of the external parameters.

Fig. 1. Equivalent circuit of the parallel-connected power devices. Source: IEEE Transactions on Power Electronics

The status indicators stand for the parameters that are important to statuses, such as junction temperature  , humidity, load current, and dc bus voltage. 

Table 1 provides a summary of some typical parameters. 

Table. 1. Parameters That Affect the Current Distribution. Source: IEEE Transactions on Power Electronics

Some equivalent parameters, like gate threshold voltage    versus ON-state resistance    and junction temperature    versus ON-state resistance   , are tightly coupled. Additionally, certain parameters only affect a specific kind of current imbalance. For instance, gate-source capacitance    only affects dynamic distribution, while ON-state resistance    only affects static current distribution. Some variables, like    and junction temperature   , can have an impact on both the static and dynamic distribution of current.

Device Parameter Variations

ON-state Resistance    Variation

Since the current through the two paralleled MOSFETs is directly proportional to their conductance, their    can be determined using equation (2):

The MOSFET with the lower    naturally distributes more current than the MOSFET with the higher   . Increase in    have been shown to decrease    . An in-depth analysis of   is required for understanding the mechanism of static imbalance.   of a SiC mosfet is made up of several parts, as shown in Fig. 2.

Fig. 2. Component breakdown of SiC MOSFETs    Source: IEEE Transactions on Power Electronics

In Fig. 2,   is broken down into its various parts; the channel resistance  ()  has a negative temperature coefficient, while the resistance in the drift region  () has a positive temperature coefficient. SiC MOSFETs   change over temperature because a thicker drift layer is needed to support a higher breakdown voltage. Most commercially available SiC MOSFETs have positive temperature coefficients of    , allowing the static currents to balance themselves.

   can make up more than 50% of the total    in a 650 V SiC MOSFET, but it can be reduced to less than 30% in a 1.7 kV SiC MOSFET. Due to the higher temperature coefficient of   , paralleling higher-voltage SiC MOSFETs usually results in a stronger self-balancing effect.

There is a trade-off between the conduction loss and the viability of parallel connections with regard to the temperature coefficient of   . For parallel connections, a greater percentage of positive temperature coefficient is advantageous for current balancing, but at high junction temperatures, it causes a higher conduction loss.

Gate Threshold Voltage     Variation

The device's manufacturing process can introduce the    variation. Under prolonged gate stress,    can also be changed. Therefore, it is crucial to consider how variation affects the current imbalance. The following equation (3) could also be used to explain how    variation has a significant impact on the dynamic switching current distribution and    variation primarily affects the static current sharing:

The device with lower    in paralleled MOSFETs has faster turns-ON and slower turns-OFF, which causes higher turn-ON and turn-OFF losses. This occurs as a result of it withstanding more current stress. Due to higher switching losses, the device with lower    among the paralleled devices may have a higher   .

Additionally, the negative temperature coefficient of    can create a vicious cycle with disastrous results at the end.

This self-aggravating feature of   , unlike the self-regulating feature of    variation across paralleled devices, could lead to severe mismatches and even thermal runaway.

Circuit Parameters Mismatch

Drain Inductance   Mismatch

According to researchers,    has little effect on dynamic current sharing. However, it is impossible to ignore  's influence on the current oscillations.

Within a short time of being turned ON and OFF,   affects the current. Smaller oscillation frequencies and smaller damping factors after turn-ON and turn-OFF are characteristics of SiC MOSFETs with larger   . The SiC MOSFET with a larger    has larger current overshoot and a larger current oscillation amplitude after turn-OFF. Additionally, if the drain current is still fluctuating during the ON-state period,    has an effect on the static current sharing.

The loop inductance, , load inductor L, and dc-link voltage are the four variables that can affect the static current difference.

Source Inductance   Mismatch

Unlike   ,    mismatch has a significant effect on dynamic current sharing due to its effect on   . By analyzing the equivalent circuit in Fig. 1, Equation (4) can be derived. It demonstrates that during a switching transient, the channel current is governed by   , while   is influenced by    and source current   .

Gate Inductance    Mismatch

The effect of   mismatch on transient current sharing is minimal. Even though a higher    results in a slower    charging process, gate currents and its    are typically quite small in switching dynamic. Consequently, the   effect on the gate voltage values is less significant than that of   . The influence of   mismatch on the present distribution is not significant. Nevertheless, gate inductance and its mismatch may result in oscillations and instability issues at the gate.

Summarizing the Key Points

• Paralleling power semiconductor devices can boost current ratings, but it can also cause current imbalances due to mismatched circuit characteristics or differences in semiconductor production.
• Mismatched circuit characteristics are the cause of static imbalance, whereas differences in semiconductor production are the cause of dynamic imbalance.
• Strategies for managing and mitigating current imbalances include using matched devices, adjusting gate resistors, and using active current sharing techniques.

Reference

Li, Helong, Shuang Zhao, Xiongfei Wang, Lijian Ding, and Homer Alan Mantooth. “Parallel Connection of Silicon Carbide MOSFETs—Challenges, Mechanism, and Solutions.” IEEE Transactions on Power Electronics 38, no. 8 (August 2023): 9731–49. https://doi.org/10.1109/tpel.2023.3278270.