How to understand the MOSFET specification / datasheet

As an electronic engineer and technician, I believe that everyone is familiar with MOSFETs. When engineers are selecting a specific type of MOSFET, the first thing they look at is the datasheet. However, once you receive the datasheet, which can be dozens of pages long, how do you interpret its contents effectively?

The purpose of this article is to share my understanding of MOSFET datasheets and some personal insights. If there are any inaccuracies or areas for improvement, please feel free to point them out. I welcome your thoughts and experiences as well—let’s learn together!

PS: 1. The content/datasheet mentioned in the following text will be referred to as "datasheet".
2. The data about MOSFET datasheets used in this article comes from the Infineon IPP60R190C6 datasheet.

1. VDS (Breakdown Voltage)

The first electrical parameter listed on the datasheet is V(BR)DSS, also known as the drain-source breakdown voltage. This is the voltage rating we care most about when selecting a MOSFET.

Here, the minimum value of V(BR)DSS is 600V. Does this mean the MOSFET can safely operate as long as the voltage across it doesn't exceed 600V? Many people might say "Yes," but the correct answer is actually "No."

This parameter is conditional. The 600V minimum value is specified at Tj = 25°C. That means the MOSFET can only withstand up to 600V when the junction temperature is 25°C. However, V(BR)DSS has a positive temperature coefficient. As the junction temperature increases, the breakdown voltage decreases.

For example, if the MOSFET is used in a cold environment where the ambient temperature drops below -40°C, the V(BR)DSS value may fall below 560V. At that point, the 600V rating would no longer be safe. Therefore, during design, it's important to leave some margin for VDS, not only to account for lower temperatures but also to handle voltage spikes during switching.

2. ID (Drain Current)

MOSFETs were initially labeled with current ratings like "xA" or "xV" (e.g., 20N60). Over time, the naming convention shifted to include Rds(on) and voltage ratings (e.g., IPx60R190C6, where 190 represents Rds(on)).

The relationship between ID and Rds(on) is crucial. Before diving into that, let’s briefly discuss the package and junction temperature:

1) Package: Key factors in choosing a MOSFET include power dissipation, thermal performance, creepage distance, parasitic parameters, and size. For high-voltage MOSFETs, creepage distance is critical. For low-voltage ones, parasitic inductance and resistance matter.

2) Junction Temperature: The maximum junction temperature (Tj_max) for most MOSFETs is 150°C. It's recommended not to exceed 70–90% of this in actual applications.

The relationship between ID and Rds(on) is influenced by the package's thermal resistance and the losses generated by the current. Understanding this helps in optimizing both performance and reliability.

3. Rds(on) (On-State Resistance)

Rds(on) increases with temperature, making it a positive temperature coefficient. This characteristic allows MOSFETs to be easily paralleled, as higher temperatures cause more resistance, balancing the current distribution among multiple devices.

4. Vgs(th) (Gate Threshold Voltage)

Vgs(th) is the gate-to-source voltage required to turn the MOSFET on. Unlike Rds(on), Vgs(th) has a negative temperature coefficient. This means that at lower temperatures, Vgs(th) can drop significantly, potentially causing unintended conduction in low-voltage MOSFETs.

For example, the BSC010NE2LS has a Vgs(th) range of 1.2–2V at room temperature, but it can drop to nearly 0.8V at lower temperatures. A small gate voltage spike could accidentally trigger the MOSFET, leading to system failure. Always be cautious when using low-voltage MOSFETs with this characteristic.

5. Ciss, Coss, Crss (Input, Output, and Reverse Transfer Capacitances)

MOSFETs have parasitic capacitances: Ciss = Cgd + Cgs, Coss = Cgd + Cds, and Crss = Cgd. These values vary with VDS and affect switching performance.

In LLC topologies, minimizing dead time improves efficiency. Choosing a MOSFET with a smaller Coss at low VDS can help achieve zero-voltage switching (ZVS), reducing switching losses.

6. Qg, Qgs, Qgd (Gate Charge)

Qg is the total gate charge, and it's not simply the sum of Qgs and Qgd. Higher Qg leads to greater drive loss. Therefore, selecting a MOSFET with a lower Qg can improve efficiency in high-frequency applications.

7. SOA (Safe Operating Area)

The SOA curve shows the limits of current and voltage the MOSFET can handle without damage. It includes four main regions: Rds(on) limit, pulse current limit, breakdown voltage limit, and thermal limit.

SOA curves are typically provided for Tc = 25°C and 80°C, but real-world conditions often differ. Converting these curves to actual operating temperatures requires knowledge of transient thermal resistance (ZthJC) and power dissipation.

8. Avalanche Capability

Avalanche energy (EAS, EAR) and current (IAR) are key parameters. During avalanche, the MOSFET must dissipate energy without failing. The body diode's reverse recovery time (trr) and charge (Qrr) also play a role in determining switching losses.

9. Body Diode Parameters

The body diode's forward voltage (VSD), reverse recovery time (trr), and reverse recovery charge (Qrr) are important. Smaller trr and Qrr values reduce switching losses, especially in soft-switching topologies.

10. Choosing MOSFETs for Different Topologies

The requirements for MOSFETs vary depending on the circuit topology:

• Flyback:** Focus on VDS rating and minimize voltage spikes due to transformer leakage inductance.
• PFC/Forward:** Consider switching losses and choose faster MOSFETs with lower Qg.
• LLC/Phase-Shifted Full Bridge:** Prioritize fast-recovery body diodes with low trr and Qrr.
• Anti-Reverse/Oring:** Minimize conduction loss by focusing on Rds(on).

Choosing the right MOSFET involves understanding the application, environmental conditions, and thermal behavior. Each parameter plays a role in ensuring reliable and efficient operation.