Guide to Bootstrap Operation in a MOSFET Gate Drive Circuit

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What is a bootstrap circuit?

The term “bootstrap” evokes the image of a person pulling themselves up by their bootstraps. In electronics, the bootstrap circuit is defined as a circuit that uses the output signal to “pull up” on the input signal. In analog circuits, this technique is used to increase the input impedance of an amplifier or provide increased stage linearity. In switching circuits, the bootstrap is the energy source used to turn on the high-side MOSFET as shown in Figure 1.

The bootstrap operation is typically performed with a “flying” capacitor. In this context, the flying bootstrap capacitor is referenced to the output of an active stage as opposed to being ground-referenced. Consequently, the capacitor will follow the output and reflect changes back to the stage’s input. For example, if the stage goes from a ground-referenced 0 to 50 VDC, a charged 15 VDC capacitor’s voltage will go from a ground-referenced 15 to 65 VDC. The voltage as measured across the capacitor is a constant 15 VDC yet, the capacitor is flying relative to ground. This 15 to 65 VDC transition associated with bootstrap capacitor C1 in Figure 1 is the central theme of this engineering brief.

Figure 1: This schematic presents a highly simplified high-side MOSFET gate drive circuit. Bootstrap capacitor C1 provides energy for the MOSFET gate drive.
Figure 1: This schematic presents a highly simplified high-side MOSFET gate drive circuit. Bootstrap capacitor C1 provides energy for the MOSFET gate drive.975×570 81.6 KB

Figure 1: This schematic presents a highly simplified high-side MOSFET gate drive circuit. Bootstrap capacitor C1 provides energy for the MOSFET gate drive.

Tech Tip: The term high-side refers to a device that is referenced to the power supply. In Figure 1 the drain of the MOSFET Q1 is nailed to the 50 VDC supply. In this schematic, D2 is the low-side device. It is nailed to ground. The high-side N-channel MOSFET is challenging to drive as the source is a moving target. This requires a gate drive circuit hat can fly with the MOSFET’s source maintain V_{GS} at 15 VDC when the MOSFET is on and V_{GS} at 0 VDC when the MOSFET is turned off. The gate drive must stay within this window to prevent destruction of the MOSFET.

What is meant by the term ground-referenced?

When applied to a capacitor, the term ground-referenced implies that one side of the capacitor is attached to ground. For example, consider the power supply filtering capacitors used in a complementary output DC power supply. The negative terminal for the positive rail bypass capacitor is connected to ground while the positive terminal for the negative rail bypass capacitor is connected to ground. We could say that these capacitors and the resulting voltage rails are nailed to ground.

Let’s step back and consider how we think about power supplies. We naturally assume all supplies are ground-referenced. We reflexively transform this thought to action when we place the oscilloscope’s ground to the chassis as we probe the circuit.
This is appropriate for most, but not all applications.

What is meant by the term flying capacitor?

The term flying does not imply that the capacitor is free. In no way are we suggesting that it is floating. Instead in this context, the flying bootstrap capacitor is always connected —effectively “nailed” —to the output of the MOSFET. As the MOSFET toggles between on, off, and on, the capacitor’s negative terminal will toggle between 50, 0, and 50 VDC.

The capacitor is flying within a grounded framework.

This is an unnatural way to think about a power supply. It leads to classic measurement errors or worse as described in the tech tip. If we were to use a ground referenced probe, the 15 VDC charged capacitor would display a voltage of 65, 15, and 65 VDC as seen in Figure 2 (left).

Figure 2: Voltage for the bootstrap capacitor when measured with a ground reference (left) and when measured across the capacitor (right).
Figure 2: Voltage for the bootstrap capacitor when measured with a ground reference (left) and when measured across the capacitor (right).979×491 44.7 KB

Figure 2: Voltage for the bootstrap capacitor when measured with a ground reference (left) and when measured across the capacitor (right).

Tech Tip: Never forget that the oscilloscope is ground referenced. When we connect that ground lead alligator clip we are applying a ground to that point of the circuit through the probe to the oscilloscope chassis to the AC ground plug. This is problematic when attempting to measure a flying component such as the voltage across C1. A careless connection of the oscilloscope ground lead to the source of the MOSFET will result in immediate destruction of the MOSFET and could damage the power supply or even the oscilloscope.

How do we measure the voltage across a flying component?

Proper measurement requires a differential probe or an oscilloscope that can perform a subtraction operation between channel 1 (capacitor positive) and channel 2 (capacitor negative).
These real-world ground vs reference ideas may be used in the simulator as shown in Figure 2. Observe that the BootstrapVolts probe measurement has a configuration setting that allows a ground or an arbitrary reference. The left panel in Figure 2 shows the ground referenced signal while the right shows the voltage measured with respect to REF1 as shown in Figure 1.

Analysis of the bootstrap circuit

The circuit as shown in Figure 1 is a highly simplified circuit related to the H-bridge or three-phase H-bridge used to drive brush and brushless motors respectively. Perhaps you have seen one of these circuits in the form of an Electronic Speed Controllers (ESC) for a model Remote Control (RC) car or quadcopter. Recall that the ESC allows variable speed and direction control of a motor.

While the Figure 1 circuit is simple, it does contain the essential components required for a high-side MOSFET driver including:

  • level shifting via optical isolation to allow the low-level ground-referenced logic signal to drive the MOSFET.

  • highly simplified gate driver (see tech tip below)

  • bootstrap capacitor and charging circuit

  • flyback diode to commutate the load when the high-side MOSFET is turned off

Tech Tip: Dedicated integrated circuit drivers provide a reliable simplified solution for MOSFET control. The single-package solution incorporates the level shifting, low impedance MOSFET driver, and varying level of protection. There are MOSFETS driver suitable for the 50 VDC circuit featured in this article all the way to several thousand volts. Drivers are available for single MOSFETS including both the high and low side. Bridge arm drivers are available as are full bridge and three phase bridge drivers.

Power supply

The circuit contains three power supplies including 5 VDC, 15 VDC, and 50 VDC. The 5 VDC supply represents the logic-level control drive signal originating from a microcontroller or Field Programmable Gate Array (FPGA). The 15 VDC is used for the MOSFET gate charge circuitry. This value was chosen as it is close to the MOSFETS optimal drive signal thereby ensuring low on resistance as explored in this article. Finally, the 50 VDC source is used to provide energy to the load.

Level shifting

The optoisolator is used to level shift the ground referenced logic level signal to the high-side MOSFET. To the left we have the low-level signals as represented by the green signal shown in Figure 2 (left). To the right we have a blue time varying signal as represented in Figure 2 (left). In this example, the optoisolator is operating in a conservative range with a maximum difference of 65 VDC across the optical window.

Gate drive

The gate drive is taken directly from the optoisolator’s output transistor. A common collector configuration is used such that the NPN transistor will pull the MOSFET up to the bootstrap voltage. When turned off, resistor R2 will discharge the MOSFET gate via the R4 current limiting resistor.

At best, this is a poor circuit configuration; however, it is sufficient for demonstration purposes. Perhaps the greatest weakness is the high current drain associated with the emitter resistor. Recommend the use of a dedicated MOSFET driver as suggested in the tech tip.

Bootstrap charging circuit

Diode D1 and resistor R3 are used to charge the bootstrap capacitor. A charging cycle occurs every time the MOSFET is turned off. This cycling is reflected in Figure 2 (right). Recall that the blue line represents the voltage as measured across the capacitor. The voltage increases when the green PWM control signal is turned off. The voltage decreases while the MOSFET is turned on. Careful inspection of the charging cycle reveals the RC charging curve determined by the R3 and C1 time constant. The seemingly linear discharge is the attributed to the longer time constant of R2 and C1.

Flyback

The load consists of a 10 Ω resistive load in series with a 1 H inductor to simulate the inductive nature of a motor. Diode D2 will clamp the inductor flyback voltage preventing the high voltage spike from destroying the MOSFET.

Tech Tip: Do not operate the PWM dive signal at 100% duty cycle with a bootstrap circuit. Recall that the bootstrap capacitor (C1) is charged when the upper MOSFET is turned off. If the PWM never allows the MOSFET to turn off, the charge in C1 will dissipate. With reduced gate voltage, the high-side MOSFET will not have sufficient gate voltage. The MOSFET will enter its linear mode and likely destroy itself.

This duty cycle limitation problem can be mitigated by using a charge pump or other type of power supply to provide an isolated drive signal to the high-side MOSFET. Obviously, this requires additional circuit complexity when compared to the elegance of the bootstrap method.

Conclusion

The high-side MOSFET driver is an elegant circuit. It uses many of the tools in a designer’s toolkit such as level shifting via optical isolation, flyback protection, a steering diode for charging, and the bootstrap capacitor. The circuit is even more impressive when offered as a complete integrated circuit. Next time you get a chance, reverse engineer an RC electronic speed controllers to see how compact these MOSFET circuit can be.

Please leave your comments and questions in the space below. Please give a thumbs up if you learned something from this document.

Stay tuned as a future article will explore how to breadboard an H-bridge and be sure to test your circuit knowledge by answering the questions and critical thinking questions that appear at the end of this note.

Best wishes,

APDahlen

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 (partially interwoven with military experience). 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 educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

Questions

The following questions will help reinforce the content of the article.

  1. What is a high-side MOSFET and how is it differentiated from the low-side MOSFET?

  2. What is meant by the term ground referenced?

  3. Why is an understanding of ground referenced critical to understanding the bootstrap circuit?

  4. What is meant by term flying capacitor. Hint; What is it flying to and from?

  5. What would happen to the Figure 1 circuit is D2 were omitted?

  6. Calculate the maximum reverse bias on D1.

  7. A novice engineer tests the circuit by connecting an oscilloscope probe to the MOSFET gate and the alligator ground clip to the high-side MOSFET source. Describe the result.

  8. Sketch a MOSFET-based bridge arm, H-bridge, and a full three phase bridge.

  9. What happens to the charge on the Figure 1 bootstrap capacitor if the PWN drive signal is set to 100%?

  10. Search the DigiKey offerings to locate a suitable high-side MOSFET driver to replace the optocoupler.

  11. Could the circuit in Figure 1 be used to drive a purely resistive load?

Critical thinking questions

These critical thinking questions expand the article’s content allowing you to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

  1. As a rule, the gate current limiting resistor should be placed as physically close to the MOSFET as possible. Why? Also, why should we limiting our oscilloscope probing to the drive side of the resistor?

  2. Modify the Figure 1 circuit so that the 50 VDC supply may be used as the source of the MOSFET gate drive. Hint: The IRF510 has an absolute V_{GS} of $20 VDC.

  3. Suppose the high-side MOSFET were replaced with a BJT. Describe the necessary changes to the bootstrap and charging circuit.

  4. Describe the advantages of the optocoupler based MOSFET driver when compared to a dedicated driver integrated circuit.

  5. How can a capacitor be “nailed” to ground. Just as important, how can a voltage rail be “nailed” to ground using a large capacitor? What other terms are used instead of nailed?

  6. Contrast and compare the concepts of negative feedback and bootstrap.

  7. Modify the Figure 2 circuit so that the high-side MOSFET would accommodate a 100% duty cycle drive signal.

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