Simple IC SG3525 Inverter Circuit using IRFZ44 MOSFETs

This post shows you how to build a converter that changes car battery power 12V DC into household current 220V AC using a chip called the SG3525 and special transistors called MOSFETs.

It uses a special method called Pulse Width Modulation PWM to make the AC current.

This converter can be used to power regular appliances or even be part of a system that uses renewable energy like solar panels.

WARNING: Building circuits with high voltage can be dangerous.

Only do this with adult supervision.

What is a IC SG3525 Inverter Circuit using IRFZ44 MOSFETs:

The SG3525 is a PWM controller IC commonly used in inverter circuits to control the switching of power transistors such as MOSFETs or IGBTs.

The IRFZ44 is a commonly used power MOSFET in inverter applications.

Below is a basic SG3525 inverter circuit using IRFZ44 MOSFETs.

Understanding the SG3525 IC:

The SG3525 IC serves as a pulse width modulation PWM control integrated circuit commonly applied in switching power supplies and inverters.

Its versatile functionality extends to applications like motor control, lighting and uninterruptible power supplies UPS.

The SG3525 regulates output voltage and frequency by adjusting the duty cycle of the PWM signal offering protection features such as over current and thermal shutdown.

Internal Blocks of SG3525:

The SG3525 integrates several internal blocks to achieve precise control over power delivery:

A stable internal voltage source providing a reference voltage to the error amplifier.

A differential amplifier generating an error signal proportional to the difference between the output and reference voltages.

An internal timer generating the clock signal that sets the PWM waveforms switching frequency.

Logic that implements the PWM control algorithm utilizing the error signal and clock signal to generate the PWM waveform.

SG3525 Features and Functionality:

The SG3525 boasts features like soft start under voltage lockout, and over current protection making it suitable for demanding applications.

Adjusting parameters allows control over output voltage, current, or power, offering a cost effective solution for diverse power control needs.

Pin Configuration and Functionality:

Understanding the pin configuration is crucial for successful implementation:

Pins 1 and 2: Inputs to the onboard error amplifier influencing duty cycle based on voltage differentials.

Frequency of PWM: Dependent on timing capacitance CT timing resistance RT and dead time resistance RD.

Oscillator Frequency: Determined by RT, CT and RD within a range of 100Hz to 400kHz.

Pins 11 and 14: Outputs for drive signals with a continuous current rating of 100mA and peak rating of 500mA.

Pin 10: Shutdown pin enabling PWM when low and triggering immediate PWM latch when high.

Power Transformer and MOSFET Selection:

To achieve the 300W PWM output use the IRFZ44 MOSFET with a maximum power rating of 300W.

Choose a power transformer with a 9-0-9V configuration capable of handling 10 Amperes.

The power source should be 12V, 5A.

Circuit Working:

Parts List:

Power Supply Section:

• The circuit begins with a 12V DC power supply from a battery feeding into the SG3525 IC.
• This power supply should be within the recommended range specified in the datasheet.

SG3525 IC Operation:

• Pins 1 and 2 are connected to the onboard error amplifier.
• The error amplifier compares the reference voltage from the internal voltage source with the output voltage, generating an error signal.
• The timing resistor RT and timing capacitor CT determine the PWM frequency.
• The oscillator generates a clock signal setting the switching frequency of the PWM waveform.
• The control circuitry utilizes the error signal and clock signal to generate the PWM waveform with an adjustable duty cycle.
• The totem pole circuit within the SG3525 drives the low side connected MOSFETs in this case, the MOSFET IRFZ44.
• Pin 10 is the shutdown pin when low PWM is enabled, and when high the PWM latch is immediately set.

PWM Output:

• Pins 11 and 14 are the outputs providing drive signals.
• These are connected to the gate of the MOSFET IRFZ44.
• The MOSFET acts as a power switch turning on and off rapidly according to the PWM signal allowing the conversion of DC to AC.

Filter Section:

• The PWM output is then passed through a filter section which typically includes an inductor and capacitors.
• This section smoothens the PWM waveform producing a more sinusoidal AC output.

Power Transformer:

• The power transformer with a 9-0-9V configuration steps up the voltage for the AC output.

Output:

• The final output is a 220V 300W PWM AC waveform suitable for powering AC devices.

Formulas:

Below mentioned formula calculates the oscillator frequency (f) of the SG3525 pulse width modulation PWM controller IC.

Frequency (f) = 1 / (0.693 * (RT * CT + RD * CT))

RT: 2kΩ to 150kΩ, CT: 1nF to 0.2µF, RD determines dead time.

here,

• f: This is an indication of the output signal frequency produced by the SG3525.
• In circuits like inverters and converters, this square wave signal is utilized to regulate the power device switching.
• For these applications, the frequency is usually in the kilohertz (kHz) range.
• 0.693: This constant has to do with the SG3525s internal circuitry.
• It makes figuring out the oscillator frequency easier.
• RT (Timing Resistor): This external resistor is wired between the SG3525s pin 6 and ground.
• The oscillation frequency is determined by the combination of CT and RT values.
• A decreased frequency is the result of higher RT values.
• RT typically has a range of 2kΩ to 150kΩ.
• CT (Timing Capacitor): This is an external capacitor connected between pin 5 of the SG3525 and ground.
• Similar to RT, CT also affects the oscillation frequency.
• Higher values of CT will result in a lower frequency.
• The typical range for CT is 1nF to 0.2µF.
• RD (Dead Time Resistor): This extra resistor is linked to pin 7 (the discharge pin) and pin 5 of the SG3525.

In a push pull design, RD adds a tiny delay between turning off one switching element and turning on the other, even though it is not directly engaged in frequency calculation.

Dead time is the term for this delay, which aids in preventing shoot through current, which can harm components.

How the formula works:

• The product of RT and CT values is calculated as RT * CT.
• This is an illustration of the resistor and capacitor formed RC circuits time constant.
• Another term in the formula is RD * CT.
• Though its effect on frequency is not as great as that of RT * CT, it still makes a small contribution.
• To obtain the frequency (f), the complete expression (0.693 * (RT * CT + RD * CT)) is then inverted (1 / expression).

How to Build:

Building a simple IC SG3525 inverter circuit using IRFZ44 MOSFETs involves a systematic process of connecting components and ensuring proper circuit functionality.

• Refer to the SG3525 datasheet to understand the pin configuration and functionality.
• Design the circuit layout based on the SG3525 PWM inverter application.
• Connect a 12V DC power supply to the SG3525 IC.
• Ensure the power supply is within the specified range.
• Add a voltage regulator to stabilize the supply voltage VCC for the SG3525 IC.
• Connect pins 1 and 2 of the SG3525 IC to the error amplifier circuit.
• Connect the timing resistor RT and timing capacitor CT to determine the PWM frequency.
• Use an external resistor RD to set the dead time.
• Connect the soft start capacitor between pin 8 and ground.
• Utilize the internal Totem Pole Circuit to drive low-side connected MOSFETs.
• Connect MOSFET IRFZ44 to the SG3525 output for power switching.
• Connect pins 11 and 14 of the SG3525 to the gate of the MOSFET.
• Implement a filter circuit using an inductor and capacitors to smooth the PWM output.
• Connect the IRFZ44 MOSFET for the 220V AC output.
• Refer to the datasheet for proper pin connections.
• Use a power transformer with a 9-0-9V configuration and a current rating between 10 Amperes.
• Carefully assemble the components on a PCB following the circuit layout.
• Ensure proper insulation and spacing between components to prevent short circuits.
• Connect the heat sink to the MOSFET to dissipate heat effectively.

Testing

• Power up the inverter and check for proper functioning.
• Measure the output voltage, frequency and waveform using an oscilloscope.
• Ensure that the inverter can handle loads up to 300W without overheating or malfunctioning.

Conclusion:

By following these construction details and incorporating the specified formulas, you can successfully build a simple PWM inverter using the SG3525 IC.

Ensure careful consideration of parameters and component selection for optimal performance.

References

Design of a single-phase power inverter with voltage controller using IC SG3525

Datasheet SG3525A