Shielded-gate MOSFETs lower conduction losses

Shielded-gate MOSFETs lower conduction losses

The shielded-gate MOSFET can now provide the answer for designers of dc/dc power supplies in the 40 to 200-V range

BY MIKE SPEED, JOE YEDINAK, and H.L. LIN
Fairchild Semiconductor
www.fairchildsemi.com

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.

The shielded-gate MOSFET can provide the answer for designers of dc/dc power supplies in the 30 to 200-V 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 40 to 300-V range.

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 RDS(on) without incurring a Qg penalty. This shielded-gate technology enables reduction of the epi-resistance associated with achieving the BVDSS , the key component of RDS(on) , in mid-voltage MOSFETs. As shown in Fig 1 , this technology has particular benefits in the >100-V area.

Fig. 1: Components of RDS(on) for conventional trench designs benefit >100-V designs.

Figure 1 shows the RDS(on) components comparing a 30-V with a 100-V rated conventional trench MOSFET. The RDS(on) contribution from the epitaxial is much larger percentage for the 100 V. 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.

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.

Fig. 2: Compares (a) conventional vs (b) shielded-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 Fig. 4 , reduces the switching energy losses by minimizing the time the device has the simultaneous application of voltage and current.

Fig. 3: Compares the capacitive components for conventional and shielded-gate trench with equivalent 20 A, 5.7-mΩ RDS(on) .

Fig. 4: Compares Qg curves at 20 A and 50 V for conventional and shielded-gate trench with equivalent 20 A, 5.7-mΩ RDS(on).

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 Fig. 3. This snubbing network slows down the transition of the switching from low to high voltage. This feature of the shielded gate helps reduce EMI, dv/dt induced turn-on, and avalanching during switching transitions.

Performance improvements in 1/16 brick modules

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

Fig. 5: Compares Fairchild FDMS86252 150-V shielded-gate MOSFET with a competitor’s device in a 48-V VIN ; 3.3-V VOUT , isolated dc/dc converter operated at 400 kHz.

Mike Speed, Marketing Director, Wilkes Barre, PA

Joe Yedinak, Device Concepts Engineer, Wilkes Barre, PA

HL Lin, Application Engineer, Shindian City Taipei, Taipei, Taiwan