Introduction
We have been told that the MOSFET gate draws no current.
Fair enough.
Once the MOSFET is turned on, essentially zero current will flow through the MOSFET’s insulated gate. However, the turn-on dynamics are not so simple. In fact, large MOSFETs in high frequency circuits may demand gate currents of several amps.
Just like the “path of least resistance” myth, the “no MOSFET gate current” myth can get the novice designer into trouble. Actually, there are two tripping points. The first is insufficient static gate drive voltage as addressed here and the second is insufficient dynamic gate current.
MOSFET Gate Model
As a first approximation, the MOSFET gate circuit may be modeled as an internal capacitor positioned between the gate and source terminals. For example, the Vishay IRF510PBF MOSFET has a 180 pF input capacitance. A model with more complexity would include the parasitic capacitance between the gate and drain. This Miller capacitance (beyond the scope of this article) makes the MOSFET even harder to drive.
We can better understand the situation by considering the Multisim Live circuit shown in Figure 1. In this quintessential circuit, an ideal 300 kHz 15 VDC square wave is used to turn on the MOSFET via a 22 Ω resistor. For increased speed, the MOSFET is turned off via the Schottky MSS2P3 diode.
Figure 1: Simulation to measure MOSFET gate current.
Tech Tip: There may be times when you want to slow the transition speed of the MOSFET. As an example, it may be easier to meet EMI requirements if the high energy drain transient waveform is slower. However, this may impact efficiency as the MOSFET spends additional time “sliding” through its linear region.
Post PCB production selection of the components allows you to balance your design. While not shown, you may want to include a pad for a resistor in series with the diode. Use a zero Ohm resistor if it is not needed.
Tech Tip: Slow charge and discharge times represent a danger to the MOSFET. We recognize that the MOSFET is prized for its low on-resistance. Yet, when we are slow to transition, the MOSFET enters a linear region (“Ohmic” mid resistance) where energy dissipation is a function of current, voltage, and time. High power circuits that take too much time to switch tend to destroy the MOSFET due to excessive heating and thermal stress.
The resulting gate current is shown in Figure 2. The green waveform is the 15 V square wave driver. The resulting gate current is shown in the blue waveform. The charge pulse has a peak of 0.7 A while the discharge is approximately 1 A.
Clearly, significant gate currents are associated with a MOSFET.
Figure 2: Gate drive using a 15 VDC square wave (green) with resulting gate current (blue).
Necessity of MOSFET Drivers
Figure 2 demonstrates that the MOSFET gate drive current can be high. Dedicated gate driver ICs are available. As an example, consider the TI LM5114BSD/NOPB. This 3x3 mm device is designed to sink 7.6 A from the MOSFET’s gate (see the qualifications in the next section).
Tech Tip: MOSFET drivers are available as low-side (ground referenced as featured in this article) and high-side (load referenced). To better understand the distinction, consider this BLDC motor application. Be sure to grok the term bootstrap as it is an essential consideration for the high-side driver.
What is the split gate drive associated with a MOSFET driver?
To further our exploration of MOSFET drive circuits, consider the split drive as shown in Figure 3. In this application, the TI LM5114 has two outputs associated with the MOSFET gate. The sourcing and sinking specifications are asymmetrical. The IC is capable of sourcing 1.3 A peak (to lift the gate to 12 VDC) while sinking 7.6 A (discharging the MOSFET gate capacitance). Again, the designer is free to slow down the MOSFET by adding appropriate resistors in series with the P_OUT and N_OUT lines.
Figure 3: Simplified boost converter as described in TI’s LM5114 datasheet featuring a split drive.
Parting Thoughts
We must consider the static as well as the dynamic characteristic of a digital switch. It is true that the gate of a static (always on) MOSFET draws no current. However, the dynamic transitions between the on and off states require significant current pulses. The speed of this transition and the resulting system efficiency is dependent on a stout driver.
Do you agree with the implication of overdesigning the driver and then slowing it down with series resistors to meet EMI requirements?
Please leave your comments and suggestions in the space below.
Best wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.