How to Drive High-Side and Low-Side MOSFETs: A Deep Dive into IR2101S

In power electronics, driving N-channel MOSFETs or IGBTs in a bridge configuration (like H-bridge for motor control or buck converters) is a fundamental challenge. Specifically, switching the high-side MOSFET requires a gate voltage higher than the main power supply rail. This is exactly where the IR2101S Gate Driver IC comes into play.

In this post, we will look into how the IR2101S works, its bootstrap circuit design, and provide a quick microcontroller snippet to get it switching cleanly.

What is the IR2101S?

The IR2101S is a high-voltage, high-speed power MOSFET and IGBT driver with independent high- and low-side referenced output channels. Built on proprietary HVIC technology, it allows the high-side driver to operate up to 600V floating channels.

Key Features:

• Floating Channel Detection: Designed for bootstrap operation up to +600V.
• Gate Drive Range: 10V to 20V standard operational input voltage.
• Independent Channels: Logical inputs are compatible with standard CMOS or LSTTL outputs down to 3.3V, making it friendly for MCUs like Arduino, STM32, or ESP32.
• Output Current: Typ. +130mA source and -270mA sink current.

Understanding the Bootstrap Circuit

To turn on the high-side N-channel MOSFET, the gate voltage must be higher than the source voltage ($V_G > V_S + V_{th}$). When the high-side MOSFET turns on, its source voltage rises to the high-voltage rail ($V_{cc}$). Therefore, the gate needs to be pumped higher than $V_{cc}$.

The IR2101S achieves this using a bootstrap capacitor ($C_B$) and a bootstrap diode ($D_B$):

1. When the low-side MOSFET is ON, the VS pin goes to ground. This allows the capacitor $C_B$ to charge from the $V_{cc}$ rail through $D_B$.
2. When the low-side turns OFF and the high-side turns ON, the VS voltage rises. The diode $D_B$ prevents the charge from flowing back into the supply rail, isolating the floating power supply.

💡 Pro Tip: Always use a fast-recovery or Schottky diode for $D_B$ (like UF4007 or 1N4148) to handle high switching frequencies efficiently without significant reverse recovery loss.

A Simple Microcontroller Control Code

To prevent cross-conduction (shoot-through, where both high-side and low-side MOSFETs are active at the same time and short-circuit the power supply), we must introduce a small dead-time in our code. The IR2101S does not have internal dead-time generation, so it relies entirely on your MCU code or PWM peripheral.

Here is an elegant micro-snippet in C++ (compatible with Arduino or general MCUs) demonstrating proper phase control for the HIN (High-Side Input) and LIN (Low-Side Input) pins:

// Pin definitions connected to IR2101S inputs
const int pinHIN = 5;
const int pinLIN = 6;

void setup() {
  pinMode(pinHIN, OUTPUT);
  pinMode(pinLIN, OUTPUT);

  // Start with both outputs LOW (Safe state)
  digitalWrite(pinHIN, LOW);
  digitalWrite(pinLIN, LOW);
}

void safeSwitchHighSide() {
  // 1. Turn OFF low side first
  digitalWrite(pinLIN, LOW);

  // 2. Dead-time delay (e.g., 2 microseconds) to prevent shoot-through
  delayMicroseconds(2);

  // 3. Turn ON high side
  digitalWrite(pinHIN, HIGH);
}

void safeSwitchLowSide() {
  digitalWrite(pinHIN, LOW);
  delayMicroseconds(2); // Dead-time
  digitalWrite(pinLIN, HIGH);
}

Conclusion
The IR2101S is a versatile and cost-effective gate driver for low-to-medium power conversion layouts. By paying close attention to your bootstrap component choices and ensuring strict dead-time implementations in your firmware, you can build reliable, highly efficient motor drivers or switching regulators.

Have you used the IR2101S or its siblings (like the IR2104 or IR2110) in your projects? Let me know your circuit layout tips in the comments below!