FAN7382 Half-Bridge Gate Driver, up to 600V, 350mA / 650mA, DIP-8

In this tutorial we put the FAN7382 circuit into operation, a gate driver for a half bridge that controls two power transistors, one on the high side and one on the low side. It is used in switching power supplies, inverters, DC-DC converters and motor drivers, where a small logic signal must control transistors operating at high voltages.

The circuit does not generate the control signals on its own, so we need a microcontroller. We use an Arduino Uno board, which has digital outputs with 5 V logic, sufficient for the TTL inputs of the driver. The Arduino Uno is the simplest choice for this circuit, because it requires neither WiFi, nor additional memory, nor a special interface.

How it works:

The driver has two logic inputs, HIN and LIN, and two outputs, HO and LO, in phase with the inputs. Logic level 1 is considered above 2.9 V, and logic level 0 below 0.8 V. The LO output drives the gate of the low-side transistor, and the HO output drives the gate of the high-side transistor. Each channel typically supplies 350 mA when turning on and 650 mA when turning off.

The source of the high-side transistor sits at the middle of the half bridge, which rises and falls between ground and the bus voltage. That is why the supply of the high-side channel must follow this point. It is obtained with a bootstrap capacitor connected between VB and VS. When the low-side transistor is on, VS is pulled close to ground, and the capacitor charges from VCC through a fast diode. The high-side channel voltage is approximately VBS = VCC - VF, where VF is the voltage drop across the diode. For VCC = 12 V and VF = 0.7 V, VBS is about 11.3 V, above the typical 9.2 V UVLO turn-on threshold.

The switching speed depends on the gate charge Qg of the transistor and the driver current, according to the relation t = Qg / I. For a transistor with Qg = 30 nC, at 350 mA this gives about 86 ns on turn-on, and at 650 mA about 46 ns on turn-off. In reality the times are longer, because as the gate charges, the current decreases.

The signal generated by the program has the period T = tHigh + tDead + tLow + tDead. With 40 us, 5 us, 40 us and 5 us this gives 90 us, that is about 11.1 kHz, to which the time of the digitalWrite instructions is added. The dead time prevents both transistors from conducting at the same time, which would short-circuit the bus.

Connection:

For the first test use a 12 V power bus, not the maximum voltage of 600 V. High voltages are dangerous, so make the connections only with the setup disconnected. Power VCC with 12 V, within the range 10 - 20 V. The HIN and LIN inputs have TTL levels, so they connect directly to the Arduino Uno pins, without a level converter. The Arduino ground, the power supply ground and COM must be connected together. The transistors are two N-channel MOSFETs, and the load is connected between the middle of the half bridge and ground or the bus.

Board and libraries:

In the Arduino IDE select the Arduino Uno board at Tools > Board from the Arduino AVR Boards package, which is installed from the Boards Manager and usually comes with the Arduino IDE. The program uses only the basic functions of the platform, so no library needs to be installed.

The code:

The program briefly charges the bootstrap capacitor, then generates on pins 9 and 10 two complementary signals with dead time between them.

/*
  FAN7382 half-bridge gate driver test
  Board: Arduino Uno (Tools > Board > Arduino AVR Boards > Arduino Uno)
  Libraries: none, only the core Arduino functions are used
  Wiring:
    FAN7382 pin 1 VCC -> +12 V (with 100 nF and 10 uF to COM, bootstrap diode anode)
    FAN7382 pin 2 HIN -> Arduino D9
    FAN7382 pin 3 LIN -> Arduino D10
    FAN7382 pin 4 COM -> Arduino GND, supply GND, low-side MOSFET source
    FAN7382 pin 5 LO  -> low-side MOSFET gate (series resistor)
    FAN7382 pin 6 VS  -> half-bridge midpoint, bootstrap capacitor minus
    FAN7382 pin 7 HO  -> high-side MOSFET gate (series resistor)
    FAN7382 pin 8 VB  -> bootstrap diode cathode, bootstrap capacitor plus
  Test first at a low bus voltage (12 V), never at 600 V.
*/
const uint8_t HIN_PIN = 9;                 // Input of the high-side driver
const uint8_t LIN_PIN = 10;                // Input of the low-side driver
const unsigned int HIGH_ON_US = 40;        // High-side on time in microseconds
const unsigned int LOW_ON_US = 40;         // Low-side on time in microseconds
const unsigned int DEAD_US = 5;            // Dead time between the two switches
const unsigned int BOOT_CHARGE_MS = 10;    // Low-side on time that charges the bootstrap capacitor
void setup() {
  Serial.begin(9600);                      // Start the serial monitor at 9600 baud
  pinMode(HIN_PIN, OUTPUT);                // Configure the control pins as outputs
  pinMode(LIN_PIN, OUTPUT);
  digitalWrite(HIN_PIN, LOW);              // Start with both switches off
  digitalWrite(LIN_PIN, LOW);
  Serial.println(F("FAN7382 half-bridge test"));
  // Turn the low side on to charge the bootstrap capacitor
  digitalWrite(LIN_PIN, HIGH);
  delay(BOOT_CHARGE_MS);
  digitalWrite(LIN_PIN, LOW);
  Serial.print(F("Bootstrap capacitor charged for "));
  Serial.print(BOOT_CHARGE_MS);
  Serial.println(F(" ms"));
  // Compute the nominal period and frequency without the digitalWrite overhead
  unsigned long periodUs = (unsigned long)HIGH_ON_US + LOW_ON_US + 2UL * DEAD_US;
  Serial.print(F("Nominal period: "));
  Serial.print(periodUs);
  Serial.println(F(" us"));
  Serial.print(F("Nominal frequency: "));
  Serial.print(1000000UL / periodUs);
  Serial.println(F(" Hz"));
  Serial.print(F("Dead time: "));
  Serial.print(DEAD_US);
  Serial.println(F(" us"));
  Serial.println(F("Switching started"));
}
void loop() {
  // Low side off, wait for the dead time, then high side on
  digitalWrite(LIN_PIN, LOW);
  delayMicroseconds(DEAD_US);
  digitalWrite(HIN_PIN, HIGH);
  delayMicroseconds(HIGH_ON_US);
  // High side off, wait for the dead time, then low side on
  digitalWrite(HIN_PIN, LOW);
  delayMicroseconds(DEAD_US);
  digitalWrite(LIN_PIN, HIGH);
  delayMicroseconds(LOW_ON_US);
}

How the code works:

The constants HIN_PIN and LIN_PIN indicate pins 9 and 10, connected to the HIN and LIN inputs. The constants HIGH_ON_US, LOW_ON_US and DEAD_US set the conduction time of each transistor and the dead time, in microseconds. The constant BOOT_CHARGE_MS sets how long the low-side channel stays active to charge the bootstrap capacitor.

In the setup() function the serial monitor is started at 9600 baud, both pins are configured as outputs and set to LOW, so that both transistors start off. Then LIN is set to HIGH for BOOT_CHARGE_MS milliseconds, which charges the bootstrap capacitor, and returns to LOW. At the end the nominal period and frequency are calculated and displayed, in the variable periodUs.

The loop() function repeats the same sequence endlessly. First LIN is set to LOW, and the dead time is waited out. Then HIN is set to HIGH for HIGH_ON_US microseconds. After that HIN returns to LOW, the dead time is waited again, and LIN is set to HIGH for LOW_ON_US microseconds. At no moment are the two pins HIGH at the same time.

Result:

Open the serial monitor at 9600 baud. After resetting the board the startup messages appear, together with the bootstrap charging duration and the nominal values of the period and frequency. With an oscilloscope or logic analyzer on the HO and LO pins you see two complementary signals, with a pause of about 5 us between them.

FAN7382 half-bridge test
Bootstrap capacitor charged for 10 ms
Nominal period: 90 us
Nominal frequency: 11111 Hz
Dead time: 5 us
Switching started

Common problems:

The HO output does not switch. The most common cause is an uncharged bootstrap capacitor or a diode mounted backwards. Check that the diode has its anode at VCC and its cathode at VB, and that LO is activated long enough before the first HO command.

Both channels remain inactive. Check the VCC supply, which must be above the typical 9.2 V UVLO threshold, and check that the COM pin is connected to the Arduino ground. Without the common ground the logic levels are not recognized.

The transistors heat up strongly. They are possibly conducting at the same time. Check the HO and LO signals with the oscilloscope and increase the dead time DEAD_US until they no longer overlap.

Oscillations appear on the gates or the switching is erratic. Mount the 10 - 47 ohm series resistors as close as possible to the gates, shorten the traces and add the 100 nF capacitor directly between VCC and COM.

The high-side channel stops after a long conduction time. The bootstrap capacitor discharges gradually. Use continuous switching, with pauses in which the low-side transistor charges the capacitor again.

Limitations and tips:

The circuit has two independent channels and does not include internal protection against simultaneous conduction, so the dead time remains the responsibility of the control signal. Supplying the high-side channel through bootstrap requires periodic switching, so the high-side transistor cannot be kept on indefinitely.

The test program with digitalWrite reaches about 11 kHz and has small timing variations due to Arduino interrupts. For high frequencies and precise timing use the hardware timers of the microcontroller.

After the circuit works correctly at 12 V, the bus voltage can be increased gradually, with care for the dangerous voltages and with transistors rated for it. On this basis one can build a motor half bridge, a Buck converter or a complete bridge with two such drivers.