SiC Modules vs. Discrete Components: Selection Criteria

Module or Discrete – The First Decision When Selecting SiC

Early in the inverter design phase, once the decision to use SiC is made, the immediate question that arises is whether to opt for modules or discrete components. While it may seem like a choice of implementation form, it fundamentally impacts system performance, board area, protection circuit design, and procurement flexibility.

Choosing incorrectly can lead to significant redesign costs. Switching from a discrete design to a module requires rethinking the gate driver circuit, thermal design, and PCB layout from scratch. The reverse is also true.

So, which is the "correct" answer? Ultimately, it's not about one being superior to the other. The optimal choice depends on the system's power level, switching speed requirements, leeway in protection circuit design, and the demand for procurement stability.

The Broad Framework is Determined by Power Level, With Nuances Emerging Later

As a general guideline, discrete components are often favored for systems in the several kW to tens of kW range, while modules are more common for systems exceeding tens of kW. However, this is not an absolute rule.

The advantage of discrete components lies in their design flexibility. Individual devices allow for optimized gate driver, snubber, and protection circuits, making it easier to push for high-speed switching. This is particularly beneficial for applications demanding switching frequencies above 100kHz, such as DC-DC converters or compact onboard chargers (OBCs), where discrete configurations can minimize parasitic inductance.

Modules, on the other hand, feature optimized internal wiring and are packaged with suppressed parasitic inductance. In high-current applications, they can incorporate multiple parallel chips, simplifying current capacity expansion without extensive system redesign. Thermal management is often integrated with a baseplate, streamlining connection to the cooling system.

As this breakdown illustrates, the difference between modules and discretes is not about performance superiority but about which design requirements they best address. The next point to consider is how SiC's unique reliability challenge – short-circuit withstand time (SCWT) – factors into this decision.

Short-Circuit Withstand Time: The "Hidden Selection Criterion"

When selecting SiC MOSFETs, on-resistance (Ron) and voltage rating are always checked. However, the short-circuit withstand time (SCWT) is often overlooked.

SCWT refers to the time a device can withstand a load short-circuit before failure. In essence, it represents the "grace period" before the protection circuit activates. If the protection circuit cannot gate the device off within this time, it will be destroyed.

SiC devices have small chip sizes and high current densities. Consequently, temperature rise during a short circuit is faster compared to Si devices. Protection circuit response time designs that were adequate for Si may not be sufficient.

Specifically, Microchip's SiC MOSFETs (700V/1200V rated) list a typical SCWT of 3μs under certain conditions in their datasheets. This value directly impacts the design margin for protection circuits.

SCWT is equally important for module selection. However, it is necessary to consider that the behavior of current distribution and thermal concentration during a short circuit differs between modules with multiple parallel chips and single discrete components.

Furthermore, SCWT is dependent on conditions such as drain voltage, gate voltage, and junction temperature. As these conditions are relaxed, the withstand time tends to increase. Comparing only the numerical values without correlating them to the actual operating points can lead to misjudgments.

Protection Circuit Design Costs Escalate with Discretes

Desaturation (DESAT) detection is a widely used method for implementing short-circuit protection. This mechanism monitors the on-state drain-source voltage (VDS) and detects when the device exits the saturation region due to overcurrent, subsequently turning off the gate. It has a proven track record as a suitable method for SiC MOSFET protection.

Designing a DESAT circuit requires appropriately setting the trigger threshold (V_DESAT), DESAT current (I_DESAT), and short-circuit blanking time. A shorter blanking time reduces the risk of false triggers but increases the effective response time from short-circuit occurrence to shutdown. For devices with SCWTs as short as approximately 3μs, balancing these factors demands a very tight design.

In discrete configurations, this protection circuit must be implemented individually for each device. For a 3-phase inverter, this means at least six devices, and more for a full bridge. This increases design effort, board area, and component count. With modules, this functionality is often optimized within the package or provided through validated reference designs in conjunction with external gate driver ICs.

However, relying on modules can introduce a different challenge: a lack of visibility into the circuit's internal workings. For applications requiring custom tuning of the protection circuit's operation or pushing response speeds to the absolute limit, discrete configurations offer greater design transparency.

The Trade-off Between On-Resistance and Short-Circuit Withstand Time – How Manufacturers Solve It

In SiC MOSFET design, low on-resistance (Ron) and high short-circuit withstand time are conflicting requirements. Reducing on-resistance increases channel current density, leading to intense thermal concentration during a short circuit. This is the fundamental trade-off.

How manufacturers resolve this trade-off represents their technological differentiation. Mitsubishi Electric has significantly improved SCWT by introducing a p-type protective layer in their trench SiC-MOSFETs, while ROHM's fourth-generation SiC MOSFETs claim to achieve both low RonA and high SCWT through their unique device structures.

While both approaches are described as achieving "both," their specific numerical values and conditions require careful comparison of datasheets and technical documentation. It is important to note that simply comparing catalog values can lead to inaccurate comparisons due to differing measurement conditions.

This graph indicates the same typical value of 3μs for both. Whether this value is "sufficient" depends on the system's protection circuit response time. Verifying the SCWT of the component and then cross-referencing it with the gate driver specifications serves as a crucial decision-making point.

Final Checks – Organizing the Checkpoints

In practical selection, catalog specifications alone are often insufficient. Focusing on the following aspects can facilitate parallel progress in selecting between modules and discretes and narrowing down component choices.

First, narrow down the implementation form based on the system's rated power and switching frequency. For systems exceeding tens of kW with relatively low switching frequencies (e.g., around 20kHz), modules tend to offer higher implementation efficiency. Conversely, for systems below a few kW requiring high-speed switching, discretes can extract better performance by minimizing wiring inductance.

Next, ensure alignment between SCWT and gate driver response time. Check how closely the datasheet conditions for SCWT (drain voltage, gate voltage, junction temperature) match your operating points to assess the available margin. As junction temperature increases, RDSon rises, limiting the saturation current, which tends to improve SCWT. However, this characteristic should also be verified against design conditions.

From a supplier perspective, discretes offer greater procurement flexibility from multiple manufacturers, providing alternative options. Modules, on the other hand, tend to have more limited supply sources. Establishing early relationships, including long-term supply guarantees and price negotiation, is crucial for stability. onsemi offers a wide portfolio from 650V to 1700V SiC MOSFETs, diodes, and modules, making them a candidate for selection based on the breadth of their offerings.

Finally, account for the design effort required for protection circuits. Implementing DESAT in discrete configurations requires practical evaluation to fine-tune the blanking time, V_DESAT, and soft-off settings. Considering the project schedule and design resources, a module combined with an already evaluated gate driver might be a more feasible option.

The choice between modules and discretes is not merely about selecting a component but about defining the system design strategy. By simultaneously considering reliability metrics including SCWT, protection circuit design effort, and procurement stability, a well-founded decision can be made.