Electronics & Motor Control Advanced 25 min

Discrete N-Channel MOSFET H-Bridge with Bootstrap Gate Drive

Construct an all-N-Channel discrete MOSFET H-Bridge capable of driving 24V 20A motors with high-side bootstrap charge-pump gate drive and hardware dead-time.

2. Prerequisites & Background Theory:

MOSFET gate charge Qg, bootstrap capacitor sizing, shoot-through prevention, and PCB high-current layout.

STEP 1

Goal & Engineering Objectives

Build a high-efficiency discrete motor driver capable of handling 24V at 20A continuous without heatsink thermal runaway.

STEP 2

Governing Mathematical Formulation & Physics

                  All N-Channel Topology Advantages:
N-Channel MOSFETs exhibit 3x lower RDS(on) per unit silicon area than P-Channel devices. However, turning ON a high-side N-Channel MOSFET requires driving its gate 10V above the supply rail (VG >= Vin + 10V).

Bootstrap Capacitor Sizing Formula:
$$C_{boot} \ge \frac{2 \left( 2 Q_g + \frac{I_{leak}}{f_{sw}} + Q_{ls} \right)}{\Delta V_{boot}}$$
Where Qg is MOSFET total gate charge and Delta V_boot is allowable bootstrap voltage drop (typically 0.5V).

Dead-Time Requirement:
$$t_{dead} > t_{off,max} - t_{on,min} + t_{prop\_skew}$$                

STEP 3

System Architecture & Block Flow Diagram

[MCU PWM Signals] --> [IR2104 / HIP4081 Gate Driver IC]
                            |
                     [Bootstrap Diode + Cap]
                            |
              +-------------+-------------+
              |                           |
     [High-Side Q1 (NMOS)]       [High-Side Q2 (NMOS)]
              |                           |
              +------ [DC Motor] ---------+
              |                           |
     [Low-Side Q3 (NMOS)]        [Low-Side Q4 (NMOS)]
              |                           |
              +------------+--------------+
                           |
                     [Current Shunt] --> [PGND]

STEP 4

Required Hardware Bill of Materials (BOM)

Hardware Component	Qty	Engineering Specification
N-Channel Power MOSFETs (IRFB3077PbF)	4	75V, 210A, 2.8 mΩ RDS(on), TO-220
Half-Bridge Gate Driver ICs (IR2104)	2	Integrated 520ns dead-time generator, 600V high-side capability
Ultrafast Bootstrap Diodes (UF4007)	2	1000V, 1A, 75ns reverse recovery time
Bootstrap Ceramic Capacitors (0805)	2	1 µF 50V X7R ceramic
Gate Resistors	4	10 Ω 1/4W metal film for gate ringing damping
TVS Diodes (SMBJ30CA)	2	30V bidirectional transient voltage suppressors across half-bridges

Look up component pinouts in Components Catalog →

STEP 5

Wiring, Pinout & Schematics Mapping

                  • Connect 12V auxiliary gate supply to IR2104 VCC pins.
• Connect UF4007 diode from VCC to VB (Cathode at VB).
• Connect 1µF bootstrap capacitor across VB and VS pins of each driver.
• Connect HO through 10Ω gate resistor to High-Side MOSFET gate.
• Connect LO through 10Ω gate resistor to Low-Side MOSFET gate.
• Connect VS directly to half-bridge center switch node (motor terminal).
• Ground return: Low-side source terminals connected to heavy copper ground plane.                

STEP 6

Firmware Implementation & Algorithms

// Arduino C++: 20 kHz Complementary PWM with Direction Control
#include <Arduino.h>

const int PIN_PWM_LEFT  = 9;  // Connects to Left IR2104 IN
const int PIN_PWM_RIGHT = 10; // Connects to Right IR2104 IN
const int PIN_ENABLE    = 4;  // Connects to IR2104 SD (Shutdown)

void setMotorHBridge(int speed) { // speed from -255 to +255
    if (speed == 0) {
        digitalWrite(PIN_ENABLE, LOW); // Disable both half-bridges
        analogWrite(PIN_PWM_LEFT, 0);
        analogWrite(PIN_PWM_RIGHT, 0);
        return;
    }

    digitalWrite(PIN_ENABLE, HIGH);
    int duty = constrain(abs(speed), 0, 250); // Cap at 98% to preserve bootstrap refresh

    if (speed > 0) {
        analogWrite(PIN_PWM_LEFT, duty);
        analogWrite(PIN_PWM_RIGHT, 0);
    } else {
        analogWrite(PIN_PWM_LEFT, 0);
        analogWrite(PIN_PWM_RIGHT, duty);
    }
}

void setup() {
    pinMode(PIN_PWM_LEFT, OUTPUT);
    pinMode(PIN_PWM_RIGHT, OUTPUT);
    pinMode(PIN_ENABLE, OUTPUT);
    digitalWrite(PIN_ENABLE, LOW);
}

Simulate and Edit in Coding Lab

STEP 7

Bench Testing & Waveform Verification

                  1. Connect dual-channel oscilloscope to High-Side Gate and Low-Side Gate: Verify 520ns dead-time with 0% overlap.
2. Connect 24V power supply and DC motor: Command 50% PWM and verify clean motor rotation in both directions.
3. Run motor at 15A stall torque for 60 seconds: Measure MOSFET tab temperatures with FLIR camera; verify temperature remains under 55°C.                

STEP 8

Troubleshooting & Common Failure Modes

                  • Problem: Both MOSFETs in one half-bridge blow immediately upon power application.
  Solution: Gate driver shoot-through or ground bounce. Verify gate traces are short (< 25 mm) and twisted with source return.
• Problem: High-side MOSFET turns off after 2 seconds at 100% duty cycle.
  Solution: Bootstrap capacitor discharge. High-side gate driver requires periodic low-side switching to recharge C_boot. Limit maximum duty cycle to 98%.
• Problem: Large voltage spikes on motor terminals upon PWM switch-off.
  Solution: Motor inductive energy dump. Add bidirectional TVS diodes and 470µF low-ESR electrolytic capacitor across DC supply rails.                

STEP 9

Extensions & Undergraduate Research Challenges

                  1. Add inline current-sense resistor with INA240 amplifier to implement instantaneous cycle-by-cycle overcurrent protection.
2. Incorporate active freewheeling synchronous rectification in software to minimize body diode conduction heat loss.                