Evaluating Superjunction Power MOSFETs for Performance and Efficiency

Superjunction power MOSFETs have dominated high voltage switching applications for so long that it’s tempting to think there must be better alternatives. However, their ability to continue delivering a balance of performance, efficiency, and cost-effectiveness makes them indispensable in optimizing electronic power designs for many new applications.

Commercially available since the turn of the century, silicon-based superjunction MOSFETs were created by stacking alternating p-type and n-type layers of semiconductor material to create PN junctions that resulted in reduced on-state resistance (R_DS(ON)) and gate charge (Q_g), in comparison to traditional planar MOSFETs. These benefits have been quantified in a Figure of Merit (FOM) calculation, where FOM = R_DS(ON) x Q_g.

FOM quantifies how much resistance the MOSFET has when it’s on and how much charge is required to switch on and off.

Q_g provides a handy comparison of switching performance, but sometimes that can be overemphasized. Modern gate drivers are available to meet most gate charge requirements, so designers chasing after even greater optimization risk driving up their costs at the expense of improving other critical parameters.

The charge balance design in superjunction MOSFETs allows for thinner and more heavily doped regions. Their efficiency in power conversion stems from the ability to turn the MOSFET on and off more quickly, reducing switching losses. Thermal management issues are also simplified as the improved efficiency generates less heat during operation.

When or whether to use them depends, of course, on the specific requirements of the application. They are popular in applications where high-voltage switching efficiency and compact design are desired, such as AC/DC power supplies and converters, variable frequency motor drives, solar inverters, and others.

Don’t overlook Q_rr values

Another factor to consider when selecting superjunction MOSFETs for an application is reverse recovery charge (Q_rr)—the charge that builds up in the PN junction as current flows through the MOSFET’s body diode during a switching cycle. When high, that can lead to voltage spikes and additional losses, so a lower recovery charge is important for improving efficiency and minimizing switching losses.

Transient events due to high Q_rr can also generate electromagnetic interference (EMI), impacting negatively on sensitive components and signal integrity.

Reducing Q_rr is beneficial for enhancing performance, especially in high-frequency applications where these effects are magnified, and to ensure optimal operation and compliance with EMI parameters. From a product design perspective, a lower charge can provide the following benefits:

• Reduced switching losses as energy dissipation is minimized
• Enhanced efficiency due to better energy utilization
• Improved thermal performance, with reduced heat generation during switching
• Mitigated EMI through reduced voltage spikes and ringing
• Longer-term reliability due to less stress during switching cycles

Generally, the higher the application’s frequency, the greater the priority of utilizing a lower Q_rr. It’s also important to determine how this factor contributes to heat generation in the application and the consequent cooling requirements.

After settling on one or more potential MOSFETs, designers can use simulation tools to model the MOSFET and how the Q_rr will behave in the application and impact its performance. Experimental testing with an oscilloscope and a current probe can produce measurements of switching events with a particular MOSFET.

Matching these values to the needs of an application depends on finding its appropriate balance with efficiency, and other parameters such as thermal performance, transconductance, threshold voltage, and diode forward voltage.

Selecting the right power MOSFET

Nexperia offers two superjunction power MOSFET product families aimed at providing product designers with a range of options to match the right combination of switching performance to various application requirements.

The company’s NextPower 80 V and 100 V MOSFETs are suited for designers focused on high-efficiency switching and high-reliability applications such as power supplies, industrial design, and telecommunications. The devices deliver Q_rr down to 50 nanocoulombs (nC), with lower reverse recovery current (I_rr), lower voltage spikes (V_peak), and reduced ringing features.

Available in LFPAK56, LFPAK56E, and LFPAK88 copper clip packaging, the devices provide space-saving flexibility without compromising thermal performance or reliability. The LFPAK56/LFPAK56E packaging has a footprint of 5 mm by 6 mm, or 30 mm², an 81% space saving compared to D²PAK at 163 mm², and 57% compared to DPAK at 70 mm² (Figure 1).

Figure 1: Comparison of LFPAK56 package (right) with D²PAK (left) and DPAK footprints. (Image source: Nexperia)

The LFPAK56E (Figure 2) is an enhanced version of the LFPAK56 that achieves lower resistance while maintaining the same compact footprint, leading to improved efficiency. An example in this enhanced package is the PSMN3R9-100YSFX, a 100 V, 4.3 mOhm, N-channel MOSFET with a continuous current rating of 120 A. Qualified to +175°C, it is recommended for industrial and consumer applications, including a synchronous rectifier in AC/DC and DC/DC, a primary side switch for 48 V DC/DC, BLDC motor control, USB-PD adapters, full-bridge and half-bridge applications, as well as flyback and resonant topologies.

Figure 2: The LFPAQK56E package of the PSMN3R9-100YSFX and other NextPower 80/100 V superjunction power MOSFETs. (Image source: Nexperia)

The NextPower PSMN2R0-100SSFJ, a 100 V, 2.07 mOhm, 267 Amp, N-channel MOSFET, comes in an LFPAK88 package which has an 8 mm by 8 mm footprint. It is also qualified to +175°C and is recommended for industrial and consumer applications such as a synchronous rectifier in AC/DC and DC/DC, a primary side switch, BLDC motor control, full-bridge and half-bridge applications, and battery protection.

For designers looking to prioritize high performance and reliability, the NextPowerS3 MOSFETs are available in 25 V, 30 V, and 40 V versions with a Schottky-Plus body diode that delivers low R_DS(ON) and demonstrated continuous current capability up to 380 A. The PSMN5R4-25YLDX, for example, is a NextPowerS3 N-channel 25 V, 5.69 mΩ logic level MOSFET in standard LFPAK56 packaging.

Nexperia’s “Schottky-Plus” technology delivers the high efficiency, low spiking performance usually associated with MOSFETs with an integrated Schottky or Schottky-like diode, but without problematic high leakage current, delivering <1 μA leakage at +25°C.

The NextPowerS3 devices are recommended for a range of applications, including on-board DC-to-DC solutions for server and telecommunications, voltage regulator modules (VRM), point-of-load (POL) modules, power delivery for V-core, ASIC, DDR, GPU, VGA and system components, and brushed/brushless motor control.

NextPowerS3 devices are also available in a 3.3 mm x 3.3 mm LFPAK33 footprint (Figure 3), including the 30 V PSMN1R8-30MLHX, suitable for applications such as a synchronous buck regulator, a synchronous rectifier in AC/DC and DC/DC applications, BLDC (brushless) motor control, along with eFuse and battery protection.

Figure 3: An illustration comparing NextPowerS3 LKPAK33 packaging (right) with DPAK packaging. (Image source: Nexperia)

Conclusion

Silicon-based superjunction power MOSFETs are indispensable in achieving the balance of performance, efficiency, and cost-effectiveness needed for many new power electronic applications. Nexperia’s portfolio of NextPowerS3 and NextPower 80/100 V MOSFETs offer product designers a range of characteristics to meet these demands, and are available in compact and thermally enhanced LFPAK packages for improved power density and reliability.