Designing Gate Driver Circuit and Switching Mechanism for Modified Sine Wave Inverter

In the previous tutorial, basic operation of a modified sine wave inverter was discussed. It was mentioned in the previous tutorial that the H-bridge MOSFET circuit of the quasi sine wave inverter cannot be directly interfaced with the microcontroller circuit. There is a need of a Gate driver circuit as an intermediate circuit between the microcontroller and the H-bridge MOSFET circuit. The H-bridge MOSFET circuit generates a modified sine wave by switching the MOSFETs in a sequential manner with predetermined delay.

This sequential switching of the MOSFETs in the previous tutorial was explained with the help of a truth table and circuit diagrams. In this tutorial, designing and working of the intermediate gate driver circuitry will be discussed. After adding the gate driver circuitry, the resultant circuit will be able to generate a modified sine wave having a peak to peak voltage of 12 V. In the next tutorial, then, the inverter circuit will be completed by adding a switching mechanism and step up transformer. The output of that circuit will be a quasi sine wave having peak voltage of 220 V and frequency of 50 Hz.

Why need gate driver circuit –

In half bridge configuration, there are two MOSFETs where one of them is in low-side configuration and other is in high-side configuration. For driving the high side MOSFET the microcontroller cannot be directly interfaced to its gate terminal. There must be a gate driver circuit for switching the high side MOSFETs contrary to the low-side MOSFETs which can be directly operated without need of any external circuitry.

In order to learn more about low side configuration and high side configuration of MOSFET and the requirement of gate driver circuit for driving high side MOSFET, refer to the following tutorial –

High and low side switching of MOSFET

Method for driving full bridge or half bridge –

The full bridge configuration is the combination of two half bridge circuits. So, it is first important to understand the half bridge circuit and its driving method. As high side MOSFET cannot be normally operated like low side MOSFETs, there are two methods of driving the high side MOSFET – one is through “dual power supply” and another is by using “bootstrap circuit”. In circuit designed here, bootstrap method is used to drive to the high side MOSFET. But the MOSFET cannot be driven with only bootstrap circuit. Their needs some extra circuit as well. That extra circuit can be incorporated by using some IC having the circuit inbuilt or by designing the circuit explicitly. In the circuit designed here, IR-2110 IC is used. This IC is cheap and easily available. Plus it can be easily used in any circuit. This IC can drive a half bridge circuit one at a time. So for full bridge configuration, there needs two IR2110 ICs.

This IC drives the high side MOSFET using the external bootstrap circuit as well as the low side MOSFET (which do not require any external circuit). So, this IC suits best as per the requirement of the circuit.

Components Required –

Fig. 1: List of components required for Gate Driver Circuit and Switching Mechanism for Modified Sine Wave Inverter

Block Diagram –

Block Diagram of Modified Sine Wave Inverter

Fig. 2: Block Diagram of Modified Sine Wave Inverter

Circuit Connections –

In the previous tutorial, the control circuitry using the Arduino UNO was already designed. Now it’s time to design the switching mechanism and gate driver circuitry. The gate driver circuitry can be designed using IR-2110 IC. IR2110 is a high and low side driver IC. It is a high speed (operational at high frequency) power MOSFET and IGBT driver with independent high and low side referenced output channels. The floating channels can operate up to 500 V or 600 V. The IC is 3.3V logic compatible that is why it can be used with any microcontroller.

The IC comes in a 14 Lead PDIP package. IR2110 has the following pin configuration –

Fig. 3: Table listing pin configuration of IR-2110 IC

The IR-2110 has the following pin diagram –

Fig. 4: Pin Diagram of IR-2110 IC

Representational Image of IR-2110 IC

Fig. 5: Representational Image of IR-2110 IC

Lead/Pin assignments with pictorial representation of IR2110

The VCC is the low side voltage and should be in range from 10 V to 20 V. The VDD is the logic voltage for working of the internal circuitry of the IC. The VDD should range in between 3 V to 20 V (with reference to Vss). The actual voltage of VDD is decided by the input supply of Lin and Hin pin. The Lin and Hin pins are the input logic supply pins for driving the low or high side driver. When the input at Lin is high then this drives the low side driver and output at LO is equal to VCC. If the Lin pin is low this turns OFF the low side driver and output at LO is equal to COM (ground). Similarly, when the input at Hin is high then this drives the high side driver and output at HO is equal to VB. If the Hin pin is low this turns OFF the high side driver and output at HO is equal to Vs. This IC works on TTL or CMOS logic so for input greater than 3V it will give logic 1. As the microcontroller is used for giving input signals to Lin and Hin pins and the controller works on TTL logic, VDD equal to 5V is used to avoid any undermine state at the output due to mismatching in VDD and logic input power supply. The Vss is the logic ground and COM is the low side return. Both are not internally connected to each other so they have been connected separately with the ground. The SD pin is for shut down or to turn OFF the IC. By connecting it to ground the chip has been enabled. In this circuit, SD, Vss, and COM pins are all connected to the ground (as shown in the circuit diagram).

Learn more about the IR-2110 IC from the following Link –

All about IR2110

The diode D1, capacitors C3 and C4 are part of the bootstrap circuit (as shown in the figure below). The capacitors C1 and C2 are used as filtering capacitor for removing unwanted voltage spikes and ripples from the supply line. The ceramic capacitors C2, C4, and C6 are used in parallel with high-value electrolyte capacitors just to reduce the overall ESR. The resistors R3 and R4 are connected from gate to source for discharging the gate capacitance. The diode D2 and D3 are also used to fast discharging the parasitic capacitance of the MOSFET so the MOSFET turns OFF fast and creates the dead zone. Otherwise, the MOSFET may get damaged due to the continuous charging of the parasitic capacitor and it will exceed the limit of the gate to source breakdown voltage. The resistors R1 and R2 are used to avoid any parasitic oscillation (ringing). The presence of parasitic capacitance at the gate of MOSFET can cause the ringing effect and this will heat up the MOSFET (due to slow turn ON). A low value resistor can solve this problem at the gate of the MOSFET. So a resistor of value 10E is connected at the gate of MOSFET. Any resistor having a value in range from 10 E to 500 E can in fact be used.

While assembling the circuit following precautions must be taken care of –

1. The input power supply to the gate must be greater than or equal to the threshold voltage (Vgs(the)) of the MOSFET otherwise, it will not turn ON the MOSFET. For this refer to the datasheet of the MOSFET used in the circuit.

2. Do not exceed the input voltage (drain voltage and gate voltage) of MOSFET greater than its breakdown voltage as it can damage the MOSFET permanently.

3. Always use a drain to source resistance to avoid any external noise at the gate and to discharge the parasitic capacitance of the MOSFET. Otherwise, the MOSFET can get damaged as this parasitic capacitor keeps on charging and exceed the limit of the gate to source breakdown voltage. A diode (like diodes D2 and D3 in the circuit) can be used to allow fast discharging of MOSFET.

4. Use a small value of resistor at the gate of MOSFET. This will solve the problem of ringing (parasitic oscillations) and voltage spike in the MOSFET.

5. The diode D1 should have a low forward voltage drop and it should sustain a reverse voltage of 24V. The frequency of this circuit is 50Hz so a normal diode can work fine here with 10ms switching speed.

6. The generated code for half bridge must contain a dead time otherwise an abrupt result may be obtained at the output or the circuit may get damaged.

7. The capacitor used in the circuit must be of higher voltage rating than the input voltage. Otherwise, the capacitors will start leaking the current due to the excess voltage at its plates and can burst out.

8. The capacitors should be used at 5V and 12V supply so they can handle the unwanted voltage spikes and noise. Always use a small value of ceramic capacitor in parallel with electrolyte capacitor to reduce the overall ESR.

9. Make sure all the filter capacitors are discharged before working on a DC power supply. For this short the capacitor with a screwdriver wearing insulated gloves.

Prototype of Gate Driver Circuit for Modified Sine Wave Inverter

Fig. 6: Prototype of Gate Driver Circuit for Modified Sine Wave Inverter

How the circuit works –

The operation of this circuit is based on the microcontroller programming which is used to provide signals for the gate driver circuitry. The microcontroller board used in the circuit is Arduino UNO. The Arduino is programmed for generating PWM signal for the input logic pins of the IR-2110 IC. For reference, check out the snapshot of the controller code given below –

Fig. 7: Screenshot of Arduino Code for Modified Sine Wave Inverter

For driving the high side MOSFET, the IR2110 is used along with the bootstrap circuitry. For input logic signal at Lin and Hin pin, two square waves of 180-degree phase difference are applied as both the MOSFETs should not turn on at the same time. In such a situation the MOSFETs can get damaged due to short circuiting at the output. As shown in the figure below the MOSFET Q1 is in high side configuration and MOSFET Q2 is in the low side configuration.

Let us now understand how this circuit drives both high and low side MOSFET. For turning ON the MOSFET the Vgs should be greater than the threshold voltage (Vth) of the MOSFET. That means,

Vgs = Vg – Vs>Vth

In this circuit, IRF840 MOSFET are used. These have a threshold voltage (Vth) in range from 10V to 12V. So the Vcc is equal to 12V and the threshold voltage is also 12V. When Hin is low and Lin is high so the transistor Q2 turns ON without any external circuit ( as Vg-Vs = 12 – 0 = 12V ) and transistor Q1 is OFF, it can be said that the low side MOSFET is ON in this case. Therefore the diode D1 is forward bias and the capacitor C3 starts charging from point a to b up to 12V through transistor Q2 and a low voltage is obtained at the output (as shown in the figure below).

Circuit Diagram showing Charging of Bootstrap Capacitor

Fig. 8: Circuit Diagram showing Charging of Bootstrap capacitor

In the next cycle when Hin is high and Lin is low then transitor Q2 turns OFF and transistor Q1 turns ON. That means the high side MOSFET is ON in this case.

Now let us see how this bootstrap capacitor helps in switching ON the high side MOSFET. In this state, the capacitor C3 starts discharging through internal MOSFET (as shown in image below) following the path from point c to d. For figuring out the internal MOSFET in IR-2110, check out its internal circuitry given in its datasheet. The capacitor C3 now tries to maintain the 12 V across it and this raises the source voltage of transistor Q1 to 12 V. This makes the diode D1 get reverse biased as its cathode voltage is now 24 V for maintaining the 12 V across the capacitor. So at the HO pin, a voltage of 24 V is obtained which is then applied to the base of transistor Q1. So the gate of transitor Q1 is at 24V and its Source is at 12 V. This makes Vgs of transistor Q1 equal to 12 V which is sufficient enough to drive the transistor Q1.Therefore, at the output, high voltage (12 V) is obtained.

Circuit Diagram showing Discharging of Bootstrap Capacitor

Fig. 9: Circuit Diagramm showing Discharging of Bootstrap capacitor

The capacitor C3 is called bootstrap capacitor as it boosts up the 12 V input signal to 24 V for driving the high side MOSFET. The value of this bootstrap capacitor can be calculated by a standard formula. The standard formula requires many values like leakage current of the capacitor, the gate charge of MOSFET etc which are not generally known. So, the value of the capacitor can be determined by hit and trial method. It was observed that for 50Hz frequency, a 100uF polarized capacitor along with 1uF ceramic works fine. The same working priciple applies to the other half bridge so, a square wave of 12V is obtained at the output.

It should be noted that a dead time of 1 us is provided when the signal changes polarity. This dead time is generated by the Arduino code after turning OFF the Hin and Lin pin. For recalling the importance of generating this dead time during the polarity change, check out the previous tutorial.

Testing the circuit –

On observing the output waveform of the half bridge circuit on a Cathode Ray Oscilloscope, the following waveform was observed.

Fig. 10: Graph showing Waveform at Gate of high side MOSFET

The Gate voltage should be 24 V as per the theoretical derivation, but practically, the waveform observed has a peak to peak voltage of 22 V. This gives Vgs equal to 10 V (22 V -12 V) and that much voltage is sufficient enough to turn ON the MOSFET.

Graph showing Output Waveform of Half Bridge FET Circuit

Fig. 11: Graph showing Output waveform of half bridge FET circuit

The circuit designed in this tutorial can be used in designing of SMPS (switched mode power supply). It can be used in DC to AC converters and in induction heating applications. In the next tutorial, the modified sine wave inverter circuit will be completed by making full bridge MOSFET circuit and adding a step up transformer.

Project Source Code

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//Program to

/* code of bootstrap circuit for driving the high side MOSFET*/


// the setup function runs once when you press reset or power the board

void setup() {

  // initialize digital pin 2 and 3 as an output.

  pinMode(2, OUTPUT);   // at Hin pin of Ir2110 IC

    pinMode(3, OUTPUT);  // at Lin pin of Ir2110 IC

}


// the loop function runs over and over again forever

void loop() {

  

  //initially 2 and 3 pin are low 

  digitalWrite(3, HIGH);    //  Lin pin high

  delay(10);              // wait for 10ms second

  digitalWrite(3, LOW);     // Lin pin low

  delayMicroseconds(1);     // dead time of 1us

  digitalWrite(2, HIGH);     //  Hin pin high

 delay(10);              // wait for 10m second

 digitalWrite(2, LOW);     //  Hin pin LOW

 delayMicroseconds(1);     // dead time of 1us

  } 

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