Foreword

In the field of power electronics design, the design of MOSFET driver circuits directly determines the switching efficiency and reliability of the devices. Engineers’ focus on fast turn-off far outweighs that on turn-on speed. This design tendency is no coincidence; it stems from the deep coupling between the inherent characteristics of MOSFETs and practical application requirements. This article will analyze the topic from three dimensions—circuit topology, loss mechanism, and physical principles—and reveal the necessity and implementation path of fast turn-off by combining Hekotai’s technical optimization practices for MOSFETs.

Typical Turn-Off Acceleration Circuits: From Topology to Principle

The switching speed of a MOSFET is essentially determined by the charging and discharging rate of the gate capacitance: the higher the series resistance in the gate loop, the longer the charging and discharging time, and the slower the switching action. To address the issue of slow turn-off speed, a diode D and an auxiliary resistor Rs_off (sometimes directly short-circuited to 0Ω) are incorporated into the classic driver topology to form a turn-off acceleration loop:

• Turn-On Phase: The drive signal Vg_drive outputs a high level. The diode D is reverse-biased and cut off, so the drive current charges the gate only through Rs_on, and the turn-on speed is determined by Rs_on.
• Turn-Off Phase: The drive signal is pulled down to GND. Since the gate voltage is higher than the drive terminal voltage, the diode D is forward-biased and conducts. The gate charge is discharged through the parallel loop consisting of Rs_on and Rs_off. According to the parallel resistance principle, the equivalent resistance Req = Rs_off // Rs_on is much smaller than Rs_on alone, thereby significantly accelerating the discharge speed.

Why Is Fast Turn-Off the Core of the Design?

There is an inherent asymmetry between the turn-on and turn-off processes of a MOSFET: even if the gate charging and discharging resistances are the same, the turn-off time is still much longer than the turn-on time. This difference directly affects the power loss and reliability of the device.

From the Perspective of Loss Intervals

Turn-on losses are concentrated in the t2 phase, where the gate voltage rises from VGS(th)

​

 to the Miller plateau voltage VGP

​

; and the t3 phase, the Miller plateau period, during which the Miller charge QGD

​

 is processed.

Turn-off losses are concentrated in the t6 phase, the Miller plateau period, when QGD

​

 is discharged; and the t7 phase, where the gate voltage drops from VGP

​

 to VGS(th)

​

.

The asymmetry of the physical mechanism causes the time consumed in the t6/t7 phases to be much longer than that in the t2/t3 phases. Without turn-off acceleration, turn-off losses will surge sharply, leading to device heating, efficiency degradation, and even device failure. This is particularly critical in high-frequency scenarios such as power conversion and motor drive.

In-Depth Analysis at the Physical Level: The Root Cause of Asymmetry

The asymmetry in the switching speed of MOSFETs essentially stems from the characteristics of RC charge-discharge curves and differences in drive current:

1. Difference Between the Fast and Slow Regions of the RC CurveThe gate threshold voltage VGS(th)
2. ​
3.  of a MOSFET is typically 1–3 V, the Miller plateau voltage VGP
4. ​
5.  is approximately 2–4 V, while the drive voltage VG_DRIVE
6. ​
7.  is mostly above 10 V.

• Turn-on (Phase t2): The gate is charged from 0 V to VGP
• ​
• , which falls in the steep rising region of the RC curve (large voltage difference, fast charging speed).
• Turn-off (Phase t7): The gate is discharged from 10 V to VGP
• ​
• , which falls in the gentle falling region of the RC curve (small voltage difference, slow discharging speed).

2. The "Magnitude Difference" in Drive Current

The amount of charge processed during the Miller plateau phase (phases t3/t6) is the Miller charge QGD

​

 in both cases, but:

• Turn-on current: IG(on)
• ​=(VG_DRIVE
• ​−VGP
• ​)/R
•  (large voltage difference, high current)
• Turn-off current: IG(off)
• ​=VGP
• ​/R
•  (small voltage difference, low current)

With the same amount of charge, the lower the current, the longer the time required — which is why the duration of phase t6 is much longer than that of phase t3.

Hekotai’s Collaborative Optimization from Device to Driver

To address the inherent characteristic of MOSFETs — slow turn-off — Hekotai delivers more efficient solutions for customers through device parameter optimization and collaborative driver circuit design.

1. Device-Level: Optimization of Key Parameters

Hekotai’s MOSFET series (e.g., industrial-grade MOSFETs) reduces Miller capacitance QGD

​

 and gate input capacitance CISS

​

 via process improvements, fundamentally minimizing the charge discharge demand during turn-off. Meanwhile, we optimize the range of VGS(th)

​

 (e.g., controlled within 2–3 V) to narrow the voltage difference during turn-off and enhance discharge efficiency.

2. Driver-Level: Integrated Acceleration Design

Hekotai provides technical support for diverse application scenarios such as power adapters and motor controllers. For industrial-grade MOSFET drivers, we adopt the standard acceleration topology featuring a diode plus low-resistance RS_OFF

​

, ensuring a 30%+ reduction in turn-off time.

Conclusion

Fast turn-off design in MOSFET driver circuits is a critical strategy for balancing device characteristics and application requirements. As a professional manufacturer of discrete devices and passive components, Hekotai always prioritizes technology-driven quality. Through in-depth research on physical mechanisms and scenario-specific adaptation, we provide customers with efficient and stable MOSFET solutions, empowering power electronics systems to achieve low-loss and high-reliability operation.

Company Profile

Founded in 1992, Hekotai is a high-tech and specialized, sophisticated, and innovative enterprise integrating R&D, design, production, and sales of electronic components. We specialize in providing cost-effective component supply and customization services to meet the R&D needs of enterprises.

Product Range: We offer a comprehensive portfolio covering discrete devices and passive components such as chip resistors, including MOSFETs, TVS diodes, Schottky diodes, zener diodes, fast recovery diodes, bridge rectifiers, diodes, triodes, resistors, and capacitors.

Two Smart Manufacturing Centers: Our manufacturing hubs in South China and Southwest China (Huizhou: 75,000 ㎡; Nanchong: 35,000 ㎡) are equipped with over 3,000 sets of advanced production and testing equipment. In 2024, we added three semiconductor material subsidiaries to control production capacity and delivery efficiency from the source.

OEM Packaging and Testing Services: We support sample customization and small-batch trial production. Backed by more than 100 patented technologies and compliance with ISO9001 and IATF16949 certification systems, we ensure that "quality first" is implemented throughout every stage from R&D to delivery.

Adhering to the core philosophy of "customer-centric, innovation-driven", Hekotai is committed to providing stable and reliable components for enterprises.

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