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These days people are very interested of testing hobby projects using small microcontrollers such as Arduino, AVR, ESP32, ESP8266 etc. Some of these projects needs to drive output loads that requires high current and voltages. As you might know most of these microcontrollers can’t handle more than 500mA of current or more than 5V directly. To overcome this issue, we can use transistors to drive such loads. Although BJTs (Bipolar Junction Transistors) are used in handling small loads when handling high power (high current consuming and high voltage) loads MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) and IGBTs (Insulated Gate Bipolar Junction Transistor).

Why isolation is needed ?

Most of the high powered loads are inductive loads such as motors, coils, magnets etc. These types of inductive loads tend to produce considerable amount of reverse voltage across their connectors due to the back EMF generated while switching these devices. If we are using controls like PWM this back EMF can be generated in very high level as the load is switched in high frequency. This reverse voltage can travel to the controller of the device. This controller can be a microcontroller or any other type. How ever this control side can be damaged due to this reverse voltage as most of the controllers contain sensitive and sophisticated electronics. To avoid these types of damages to the control side of the device we can electrically isolate the control side from the power circuit. This means there is no direct electrical conductivity path between the control side and power side of the device.

There are several methods that can be used to electrically isolate a circuit such as optical isolation, magnetic isolation etc. Optical isolation is the most commonly used type of isolation PC923 also use an optical isolation method.

What is PC923

PC923 is an isolated IGBT or MOSFET driver. It contains two major parts, optical isolation circuit and gate driver circuit. Optical isolation circuit consist of a light emitting diode and a photo detector. When a signal is received to the input of the PC923 that signal will be converted to a light signal by the LED and it will be detected by the photo detector and convert it back to a electrical signal. That is how the optical isolation is achieved.

Other than optical isolation part PC923 consist two transistor gate driver to drive an external MOSFET or an IGBT.

Key Features of PC923

By studying the datasheet of the PC923 we can identify the following key features of the PC923 isolated MOSFET IGBT driver IC

· High speed (0.5µs switching time)

· Wide operating voltage of 15V DC to 30V DC

· High signal noise reduction

· High isolation voltage of 5000V RMS

· 0.4A Gate control current

PC923 Pinout

PC923 isolated MOSFET IGBT driver comes in 8 pin packages like DIP 8 (Dual Inline Pin) package and SOP 8 (Small Outline Pin) package. As of this variety of packages PC923 is easy to use in both THT designs or SMD designs. You can see the physical appearance of the PC923 from below image.

Let’s identify the pin configuration of the PC923 IC. Pinout and pin labels of PC923 is shown in the below image

Let’s see what the functions of these pins of PC923 isolated gate driver IC are. You can get a idea on the functions assigned to each pin from the list below.

· Pin 1 : N.C. — This pin is not connected to anything internally in PC923 IC. It is advisable to connect this pin to GND to avoid any interference

· Pin 2 : Anode — This is the positive pin of the infrared LED inside the PC923 IC. This pin must be supplied a voltage below 1.6V otherwise it can be damaged

· Pin 3 : Cathode — This is the negative pin of the LED this should be connected to the GND of the controller

· Pin 4 : N.C. — This pin is also not connected to anything internally

· Pin 5 : VO — This is the output pin which the gate of the MOSFET or IGBT will be connected

· Pin 6 : VO — This is an output as well

· Pin 7 : GND — This is the ground connection

· Pin 8 : VCC — This is the supply voltage pin to PC923 IC it should be supplied with 15V to 30V DC

Main components

We are going to need following components to build this circuit other than the PC923 IC

1. A microcontroller with digital or PWM output pins

2. Resistors 10kΩ, 220Ω, 47Ω

3. IRF3205 Power MOSFET

4. Any type of DC load (example motor)

Circuit Diagram

You can find the circuit diagram of the isolation circuit. This is shown only for one pin of the microcontroller you can repeat this for any number of pins as long as those pins are outputs and they are configured as a digital output or a PWM output. This will not work with analog output pins properly. You can see the circuit diagram from the below image.