Breadboard a Bootstrapped MOSFET Motor Driver with Arduino PWM Control

[APDahlen]

What is a motor drive?

A motor drive is a semiconductor-based circuit used to power and control a motor. Many of these devices use an intelligent microcontroller to produce digital control signals for pairs of semiconductors known as bridge arms. Some controllers contain 2 bridge arms (4 semiconductor switches) while others may control 3 bridge arms (6 semiconductor switches). The microcontroller’s coordinated Pulse Width Modulation (PWM) signals are used to control Metal Oxide Field-Effect Transistors (MOSFET) in fractional horsepower drives while Insulated Gate Bipolar Transistors (IGBT) are typically used for motors drives up to about 1000 horsepower. A common motor driver is the Electronic Speed Controller (ESC) used in Remote Control (RC) cars, airplanes, and quadcopters.

This engineering brief is the second in a series of articles exploring the technology used in motor drives. This installment presents a minimalist, but functional bridge arm that may be assembled on a breadboard. It may be used to control the speed, but not direction, of a brush type DC motor as shown in Figure 1. The next installment will show how to construct a full H-bridge using two bridge arms that will allow both directional and speed control of the DC motor. The schematic for the Figure 1 assembly is shown in Figure 2.

You are encouraged to construct a copy of the circuit to gain experience prototyping with electronics. You can then modify the circuit to better understand the operation.

Figure 1: The MOSFET based bridge arm may be used to control a DC motor. The Arduino Nano Every is used to produce the PWM signals. The motor speed is determined by the potentiometer setting.

If you have not done so already, you are encouraged to review the previous installment of this series. This earlier article explores the concept of a bootstrap capacitor and how it provides energy for the high-side MOSFET.

Figure 2: Schematic of the bridge arm featuring the IR2110 driver and the Arduino Nano Every. Capacitor C2 performs the critical bootstrap operation. The Arduino is powered via its USB port (not shown).

What is a bridge arm?

A bridge arm is a subsection of an H-bridge or a full three phase bridge. Each bridge arm consists of two semiconductors: typically, MOSFETs or IGBTs. The high-side semiconductor is designed to pull the load high while the low-side semiconductor will pull the load low.

The Figure 2 schematic presents a typical bridge arm and driver. It contains a pair of N-channel MOSFETS. Note that the load is connected to the middle of the MOSFETs. Relative to this central point, MOSFET Q1 is designated the high-side device and Q2 is the low-side.

Characteristics of the MOSFET driver

The circuit shown in Figure 2 features the Infineon Technologies IR2110PBF MOSFET driver. This component was originally designed by International Rectifier in the 1990s. It continues to be relevant, and the 14-pin DIP version is especially useful from a learning perspective, as it may be used in breadboard circuit such as shown in Figure 1.

The IR2110 driver is intended for Pulse Width Modulation (PWM) operation with frequencies up to approximately 100 kHz. The functional block diagram from the datasheet is included as Figure 3. Observe that there are two semi-independent circuits within the IC package including the low side and high side driver with outputs Low Out (LO) and High Out (HO). The inputs Low In (LIN), High In (HIN), and Shut Down (SD) are all low-level logic with 3.3 VDC compatibility.

Figure 3: Functional block diagram of the IR2110 showing the low and high side drivers. Note that the high-side driver includes level shifting circuitry to support bootstrap operation.

There are several features that make the IR2110 desirable for driving a bridge arm including the low impedance output drivers and the bootstrap capabilities of the high-side driver.

Low impedance

A MOSFET is a voltage-controlled device. In theory, all we need to do is apply 15 VDC to a device such as the IRF520PBF and it will be in the on state with a low channel resistance. In practice this is not an easy task especially when the switching frequency is in the 1- KHz to 100 kHz range.

Recall that the MO in MOSFET refers to the Metal-Oxide gate material used to insulate the gate from the channel. Further recall that a capacitor is constructed as two conductors separated by a dielectric. The MOSFET gate structure therefore is a capacitor with a relatively large capacitance that must be charged and discharged to turn the MOSFET on and off. This can be problematic when the device is operating at high frequencies.

A strong driver with a low impedance output is required to both charge and discharge the MOSFET gate capacitance. This is why the IR2110’s drivers are rated to provide a pulse current of up to 2.5 A.

Tech Tip: A condition called shoot through (cross conduction) occurs when both bridge arm switches are simultaneously activated. This effectively shorts out the power supply such as when Q1 and Q2 from Figure 2 are both turned on. Something will break in this situation – by Murphy’s Law it will be the most expensive and hardest to replace parts. This fault may occur if the driver’s logic malfunctions. It can also occur if the MOSFETs are not turned on and off in a timely manner especially when operating at high frequencies. This plus low on resistance requirement is one of the primary reasons for using a low impedance driver.

Bootstrap operation

Note that Figure 3 features three power supply inputs including:

• the VDD / VSS pair for low level logic (5 VDC as shown in Figure 2)

• the VCC / COM pair for the lower driver (15 VDC associated with C3 as shown in Figure 2)

• the VB / VS pair for the upper driver (15 VDC floating bootstrap associated with C2 in Figure 2)

If you have not already done so, you are encouraged to read the DigiKey Guide to Bootstrap Operation. The article describes the bootstrap operation in detail with an emphasis on the distinction between a ground referenced signal and a floating signal. It explores the bootstrap circuit by focusing on the high-side MOSFET gate drive requirements and the necessity of a bootstrap charging circuit.

TL;DR: The bootstrap circuit provides a floating voltage source to drive the high-side MOSFET in a bridge arm. The bootstrap operation requires a charging circuit and a MOSFET driver featuring a level shifting circuit. The bootstrap capacitor (flying capacitor) charges when the low-side switch is activated. This capacitor maintains a steady 15 VDC voltage to the high-side driver. When viewed from a ground referenced oscilloscope, the positive terminal of the bootstrap capacitor flies between 15 VDC and 15 VDC plus the primary input voltage. For example, if the bridge arm has a 50 VDC supply, the bootstrap voltage will jump between 15 and 65 VDC.

Bootstrap charging

With regards to Figure 2, C2 is the bootstrap capacitor. The negative terminal is connected to the output of the bridge arm. The capacitor is then connected to the IR2110 floating VB / VS pair. The bootstrap capacitor is charged via D1 when Q2 is on.

Tech Tip: The phrase “Q2 is on” has two interpretations. The simple state occurs when MOSFET Q2 is physically turned on via the low side driver. The second state occurs when the Q2 body diode is conducting. Assuming the bridge arm is connected to an inductive load, this will occur every time the high-side MOSFET is turn off. In this case, the inductive flyback voltage forces Q2 to conduct. This places the negative terminal of bootstrap capacitor C2 one diode drop relative to ground. In this instance the capacitor charges to approximately 15 VDC via D1.

Tech Tip: Do not operate the high-side driver with a 100% duty cycle. MOSFET Q1 must be allowed to turn off periodically to charge the bootstrap capacitor. Without the relaxation, the capacitor will slowly discharge. This will eventually be detected as an undervoltage condition by the IR2110 driver. However, the resulting output will be erratic and difficult to troubleshoot. Also, while the IR2110 should protect the MOSFET, the undervoltage setting may be too low for your application. The result is a MOSFET operating with high channel resistance leading to overheating and device destruction.

Level shifting

An inspection of Figure 3 reveals the level shifting circuitry for the high side driver. In the previous article we introduced the level shifting using an optocoupler. This allowed the signal to pass while the high-side MOSFET is free to transition between ground and the supply voltage. This coupled with the bootstrap capacitor ensures that the high-side MOSFET receives a clean drive signal that transition from 0 to 15 VDC when measured from gate to source.

To my knowledge, the IR2110 does not use an optocoupler, yet the result is the same. The internal circuitry allows the high-side driver and associated bootstrap capacitor to float from 0 up to 500 VDC within a system with rapidly changing voltage transients (high dv/dt). Once again, it is essential for the high-side driver to fly with the source of the high-side MOSFET. Fail in this for even a moment and the design maximum V_{GS} will be exceeded with destruction of the MOSFET gate insulation.

Tech Tip: A differential probe may be used to measure the high-side gate to source voltage. Recall that a differential probe has two inputs. The probe responds to the difference between the input signals. This measurement may be performed using two conventional probes and a math operation. This technique was used to measure the boost voltage as shown in Figure 4 where a differential calculation is performed as Channel 2 minus Channel 1.

Figure 4: Voltages associated with the bootstrap capacitor Channel 1 (orange) negative terminal of C2 Channel C2 (blue) positive terminal of C2, with red and the mathematical result of Channel 2 – Channel 1. The red line shows the floating voltage across the VB to VS pair used to drive the upper MOSFET.

Test conditions

The circuit as shown schematically in Figure 2 and pictorially in Figure 5 features a single bridge arm. This allows the motor velocity to be changed but not the direction. A full H-bridge (to be explored in a future article) is required for bidirectional speed control of a brush type motor.

With regards to Figure 2, notice that the DC motor is connected from ground to the output of the bridge arm. This implies that the high-side driver Q1 is in command with the low side driver Q2 ready but disabled since the LIN command line is disabled via a direct connection to ground. At the same time, we must recognize that Q2 is essential for circuit operation as the MOSFET body diode is still required to catch the motor’s inductive flyback voltage.

Alternative test conditions:

We could swap the circuit conditions to operate the low-side driver by moving the #2 DC motor connection from ground to the DC source. We would also swap the HIN and LIN inputs.

Tech Tip: A MOSFET’s reverse (body) diode is typically designed to match the characteristics of the associated MOSFET. As an example, suppose Q1 is conducting a 5A through a highly inductive DC motor. As Q1 turns off, this flyback current is naturally commutated through Q2’s diode. Knowing this fact, we can better understand the need to match MOSFET and internal diode.

External diodes may be used for improved performance. However, this is increased parts count is undesirable when we consider the Bill Of Materials (BOM).

Arduino code

The code for the Arduino Nano Every is included below. Note that the microcontroller reads the 10-bit potentiometer value and converts it to an 8-bit value to drive the PWM on pin D3. Note that a saturation block is used to limit the PWM to about 98 %.

#define VREF_PIN A7
#define PWM_PIN 3
void setup() {
  pinMode(PWM_PIN, OUTPUT);
  TCB1_CTRLA = 0b00000011;  // pin D3
}
void loop() {
  uint16_t setpoint = analogRead(VREF_PIN);
  uint16_t drive = setpoint >> 2;
  if (drive > 250) {
    drive = 250;
  }
  analogWrite(PWM_PIN, drive);
  delay(50);
}

Note that direct Special Function Register (SFR) manipulation is used to increase the default PWM frequency from 980 Hz to 32 kHz as described in this article. This higher frequency shifts the motor hum out of the audible range. It also shifts the motor from discontinuous (pulsed) mode to continuous mode. Here, the term “continuous” implies that the motor’s current is nearly continuous when we consider the switching frequency and the inductance of the motor. With a lower frequency, the current is discontinuous (pulsed) as the inductive energy will dissipate to zero for every PWM cycle.

Figure 5: Test setup for the bridge arm using the B&K Precision DC power supply and the Digilent Analog Discovery.

Conclusion

At this point we have a functional albeit limited motor controller. Using a PWM signal from the Arduino and an IR2110 MOSFET driver we were able to control the high-side MOSFET in a bridge arm. This solution features an elegant circuit centered around a level shifter and bootstrap capacitor to provide a stable on off signal while the MOSFET’s source is toggling between ground and the supply voltage.

It may not seem like it, but we have made significant progress toward understanding advanced motor drives. As the next step, we will use two bridge arms to construct an H-bridge. This will allow bidirectional speed control of a brush type DC motor. From there it is a small step to generate a trapezoidal drive for a BLDC motor or the three-phase sinusoidal used by an industrial Variable Frequency Drive (VFD) for a hundred horsepower induction motor.

Please leave your comments and questions in the space below. Be sure to test your knowledge by answering the questions and critical thinking questions at the end of this note.

Please give a thumbs up if you learned something from this note.

Stay tuned for more.

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 160 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.

Helpful links

Please follow these links to related and useful information:

• Digikey’s product selection guides
• Arduino education content

Questions

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

1. Using normally open switches in place of the semiconductors, present the schematic for a bridge arm, an H-bridge, and a three-phase bridge.

2. What is a MOSFET body (reverse) diode? When is it activated in a bridge arm?

3. Describe shoot through as it applies to a bridge arm.

4. Why is shoot through more problematic at high operating frequencies?

5. Describe the steps required to charge the bootstrap capacitor.

6. MOSFETs are voltage-controlled devices, in three words or less, explain why a high current driver is necessary.

7. For simplicity, the Figure 2 bootstrap charging voltage and the primary MOSFET voltage are set to 15 VDC. Without modifying the circuit, what is the maximum voltage that may be applied without damaging components? Hint: The MOSFET is the limiting factor.

8. What is the purpose of the IR2110’s level shifting circuitry?

9. Sketch the output voltage of the bridge arm assuming an inductive load, a 75 VDC source, and a 10 kHz switching frequency applied to the high-side MOSFET. On the same graph, show the boost voltage waveform applied to the IR2110’s VB pin.

10. Explain why the IR2110 could or could not be used as a driver for two independent low-side MOSFETs? For example, could one ground referenced MOSFET be driven at 10 kHz with another ground referenced MOSFET driven at 15 kHz. Hint: Does the IR2110 contain internal circuitry to prevent shoot through?

11. Describe the symptoms and anticipated failures if the system suffered a failure that decoupled the IR2110 VS from the bridge driver. Hint: Did you conduct a failure analysis as part of your systematic troubleshooting?

12. Retrieve the datasheets for the IR2110PBF and the IRF520PBF.
    A) What is the maximum supply voltage for each device.
    B) What modification must be made to Figure 2 to handle the higher voltage.
    C) Select an appropriate bootstrap charging diode (D1) for the system’s maximum operating voltage.

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.

13. Modify the Figure 2 schematic to operate using a 50 VDC supply. For full credit, derive the necessary voltage(s) from the 50 VDC supply. Hint: For simplicity use a linear voltage regulator.

14. Modify the circuit to operate with a 100% duty cycle. For full credit, derive the gate voltage source from the 50 VDC supply.

15. Retrieve the datasheet for an N-channel 50 VDC MOSFET capable of switching 75 A.
    A) What is the DigiKey part number?
    B) What is the input capacitance?
    C) Assuming the driver has a 50 Ω output impedance, calculate the time constant for the resulting RC circuit.
    D) Recommend a maximum operating frequency with the assumption that the on time must be a factor of 10 x for the switching time while the duty cycle is set to 50%.
    E) Recalculate for a driver with a 5 Ω impedance.
    F) Based on these calculations, provide recommendations for PCB design involving high current and high switching speed.