I have written many notes on inverter design. Inverter requires a PWM controller that can turn on and off MOSFET switches which could be in Push Pull configuration or H-bridge configuration. I wrote about 555 timer based inverter, Arduino based inverter, CD4047 based inverter. I previously wrote about why TL494 PWM controller is widely used. They can be used in power supply circuits, PWM motor controller, battery charger, LED lighting controller. But they are designed primarily for switched mode power supply circuits(SMPS). Here I am taking note on how an inverter can be build with TL494 PWM(Pulse Width Modulation) IC.
Below is the circuit diagram of inverter designed with TL494 IC.
This above circuit is basically a push-pull DC–AC inverter circuit using a TL494 (KA7500 equivalent) as the PWM controller and a pair of complementary transistor stages to drive the transformer.
The circuit is explained below.
1. Power Supply
* Vcc (+12V) powers the TL494 (pin 12) and the transistor stages.
* C2 (47 µF) filters the supply to the TL494, stabilizing it.
2. TL494 Configuration
The TL494 is running in push-pull mode to drive two complementary transistor pairs.
Pin Functions in this schematic:
* Pin 12 (Vcc): Main supply to TL494.
* Pin 7 (GND): Ground reference.
* Pin 14 (REF): Internal 5 V reference output, used to bias other parts.
* Pins 8 & 11 (C1, C2): Emitter outputs of the two internal output transistors. These drive the bases of external transistors Q1/Q3 and Q2/Q4.
* Pins 9 & 10 (E1, E2 in datasheet terms — actually the collectors in the datasheet):** Connected together to Vcc, enabling the outputs to pull low actively.
* Pin 5 (CT)** and Pin 6 (RT): Set the oscillator frequency via C1 (1 nF) and R1 (10 kΩ).
* Pin 4 (DTC): Dead-time control. R8 (10 kΩ) pulls it low, allowing near-maximum duty cycle.
* Pins 1, 2, 15, 16: Error amplifier inputs — here set up for feedback/voltage regulation via R2, R3, R4, R5, R6, R7, R11.
The TL494 oscillator generates a sawtooth wave, which is compared with the control voltage from the error amplifier. It alternates the drive between the two outputs (pin 8 and pin 11) to create push-pull action.
3. Drive Stage (BJT Push-Pull Output)
Q1/Q3 NPN (TIP41) and Q2/Q4 PNP (TIP42) are complementary pairs:
* These act as buffer/driver transistors to handle the higher current needed for the transformer.
* When pin 8 of TL494 goes low, Q1 & Q3 conduct, driving current through one half of TR1's primary winding.
* When pin 11 goes low, Q2 & Q4 conduct, driving current through the other half of the primary winding.
*R9 and R10 (10 kΩ each) provide base bias control for Q3 and Q4.
4. Transformer TR1
* Type: Center-tapped primary, secondary provides AC output.
* Driven alternately by the two transistor pairs for push-pull operation.
* The switching creates a high-frequency AC in the primary that’s stepped up in the secondary.
* Output at the secondary is then smoothed (partially) by C4 (10 µF).
5. Feedback & Regulation
* R2, R3, R4, R5, R6, R7, R11 along with C3 form a voltage sensing and feedback network.
* The sensed voltage from the output (or an intermediate winding) is fed into the TL494’s error amplifiers.
* This adjusts the PWM duty cycle to keep the output voltage stable under varying load.
6. Operation Summary
1. TL494 generates two alternating PWM signals.
2. Each PWM output drives one complementary transistor pair.
3. The transistor pairs alternately energize each half of the transformer primary.
4. Transformer steps up voltage to AC output.
5. Feedback network regulates output by adjusting duty cycle.
