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Posted on 29 June 2019

Shielded Gate Power MOSFET Technology Enables Lower Conduction and Switching Losses

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Power supply designer boost efficiency and power density to new levels

Regulatory bodies as well as end customers find themselves striving for higher efficiency in DC-DC power supplies. New designs require lower Specific On-resistance while not sacrificing Unclamped Inductive Switching (UIS) capability or increasing switching losses.

By Mike Speed, Joe Yedinak and HL Lin, Fairchild Semiconductor Corporation

 

The shielded gate MOSFET has provided the answer for designers of DC-DC power supplies in the 30V to 200V range. RDS-on reductions of 50% or greater have been realized with improved switching performance leading to higher efficiencies and opening the door for higher frequency operation. This article discusses the advantages of the shielded gate MOSFET in the mid-voltage MOSFET area (40- 300V).

Power Design Challenges

Designers are continually facing the challenge of designing higher power density DC-DC designs with increased efficiency. Advancements in power MOSFET technology has helped to keep this initiative possible. The power MOSFET designer must consider the tradeoffs between RDS-on and Qg as reducing one typically increases the other. A new trench MOSFET process allows for a reduction in RDSon WITHOUT incurring a Qg penalty. This technology, known as shielded gate, enables reduction of the epi resistance associated with achieving the BVdss, the key component of RDS-on, in Mid-voltage MOSFETs. As shown in Figure 1, this technology has particular benefits in the >100V area.

Components of RDS-on for Conventional Trench

Figure 1 shows the RDS-on components comparing a 30V with a 100V rated conventional trench MOSFETs. The RDS-on contribution from the epitaxial is much larger percentage for the 100V. Using a charge balance technique like the shielded gate, this epitaxial resistance can be reduced by more than half without increasing the total Qg or the Qgd component.

The Charge Balance Technique

Figure 2 compares the cross-sections of a conventional and a shielded gate trench device. By incorporating a shield electrode for charge balancing, the resistance and length of the region supporting the voltage is reduced and a significant reduction in RDS-on can be realized.

(a) Conventional vs (b) Shield Gate Charge Balance Trench Structure

Further, the shield electrode resides below the gate electrode converting most of the gate to drain capacitance (Cgd or Crss) at the bottom of the conventional trench MOSFET to gate to source capacitance (Cgs). Hence, the shield electrode shields the gate electrode from the drain potential.

Figure 3 compares the capacitive components of the conventional and the shielded gate trench MOSFET having equivalent RDS-on. By reducing the Crss the switching losses are minimized by shortening the time it takes to transition from the off to the on state or the on to the off state. In particular, reducing the Qgd, as show in Figure 4, reduces the switching energy losses by minimizing the time the device has the simultaneous application of high voltage and current.

Comparison of Capacitive components for Conventional and Shield Gate Trench with equivalent 20A RDS-on 5.7mΩ.

Comparison of Qg curves at 20A and 50V for Conventional and Shield Gate Trench

Further, the shield and its resistance act as a built in snubbing resistance (Rshield) and capacitance (Cdshield) network depicted as component of the Coss in Figure 3. This snubbing network slows down the transition of the switching from low to high voltage. This feature of the shielded gate helps to reduce EMI, dv/dt induced turn-on, and avalanching during switching transitions.

Performance Improvements in DC-DC 1/16 Brick Module

The Fairchild FDMS86252 150V shielded gate MOSFET was compared against competitive MOSFETs in the primary of a 48V input and 3.3V output isolated DC-DC converter operating at 400kHz over the current range of 10 to 20 amps. In Figure 5 the FDMS86252 shows a minimum of 0.4% improvement in efficiency with shielded gate. That translates to at least a 0.32W power improvement that seems small but is crucial in DC-DC design where every percentage point is crucial when trying to meet regulatory requirements.

Comparison of Fairchild FDMS86252 150V shielded gate MOSFET against a competitor in a 48V VIN

Summary

Fairchild’s new PowerTrench® MOSFET Technology described above exhibits better RDS-on and Qg improvement over previous generations. This technology has enabled the power supply designer to boost efficiency and power density to new levels. Shielded gate MOSFETs are available from Fairchild Semiconductor Corporation.

 

 

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