SG3525 High Frequency 100W Inverter Circuit

The SG3525 IC-based two-MOSFET inverter circuit board is a square wave inverter that oscillates the DC input voltage at an 18-24 kHz high frequency, using an onboard ATX transformer to generate a 220V AC output voltage.

This compact inverter can be handy for camping and outdoor emergencies where electricity is unavailable to power AC light bulbs.

Here, we will discuss and explain the workings of this Chinese DC-AC power converter along with its circuit diagram.

Circuit Diagram

Schematic of sg3525 high frequency 100w inverter circuit is shown below.

Working Explanation

The working principle of the SG3525 inverter circuit is simple, converting 12V DC input into a 220V square wave output with a high frequency. The main component is the DIP-16 SG3525 integrated circuit, which generates the PWM signal.

• Download/View: SG3525 Datasheet - Onsemi.

The input DC supply is smoothed and filtered by the parallelly connected capacitors (C1 & C2) and powers the SG3525 IC (U1) through its VC pin (13). A series circuit consisting of a current-limiting resistor (R1) and an LED (D1) emits light to indicate the device is turned on.

The IC is typically a multi-function PWM generator, but in this configuration, it is set in an inverting/non-inverting or Push-Pull configuration.

Therefore, the inverting input pin (1) receives a low logic signal from the ground through resistor (R1). An internal 5.1V reference is provided through the Vref pin (16), which supplies a high logic signal from the voltage divider formed by resistors (R6 & R3) to the non-inverting input pin (2).

The external resistors (R4 & R5) and a capacitor (C4) are used to set up the frequency of the inbuilt oscillator stage or circuit in the IC (U1). The CT pin (5) is connected with a calculated capacitor (C4), while the RT pin (6) with a resistor (R4) to optimize the frequency. The Discharge pin (7) is connected to a resistor (R5) to determine the dead time of the IC.

The electrolytic capacitor (C5) is connected to the Soft-start pin (9) to initiate the operations of the IC softly, avoiding a sudden or abrupt start.

The two outputs of the IC, via OutputA pin (11) and OutputB pin (14), are switched by the supply connected to the VC pin (13). These switching push-pull outputs efficiently control the IRF3205 MOSFETs (Q1, Q2) at high frequency, typically around 18kHz to 24kHz.

In Case 1. When the IC's pin (11) generates a high output pulse, the voltage divider formed by resistors (R8 & R9) provides sufficient voltage to trigger the gate of MOSFET (Q1). The 12V input supply flows through the primary winding (L1) and completes the circuit via the Drain-Source of MOSFET (Q1) to the ground. Initially, mutual induction is induced and converted into a step-up +220V in the secondary winding (L3).

In Case 2. When pin (14) generates a high output pulse, the voltage divider formed by resistors (R10, R11) provides sufficient voltage to trigger the gate of MOSFET (Q2). The 12V input supply flows through the primary winding (L2) and completes the circuit via the Drain-Source of MOSFET (Q2) to the ground. Similarly, mutual induction is induced and converted into a step-up -220V in the secondary winding (L3).

Hence, the above oscillation creates a 220V AC voltage in the secondary winding of the ATX transformer (TRF1).

Additionally, the switching pulse of IC (U1) is continuously monitored at the Drain of MOSFET (Q1) through a resistor (R7). If a continuous logic high or low signal is detected at this pin, the PWM pulses instantly decrease to the maximum level, causing the MOSFETs to switch at minimal levels.

This way, the SG3525-based high-frequency inverter, combined with an onboard ATX transformer, converts the 12V DC input into a square wave 220V AC output. The inverter's output power ranges from 5W to 100W, depending on the input current.

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