After reading through power supply circuit diagram, I have found that TL494 IC is used in the circuit and wanted to write a note about it. So what is TL494 IC used in ATX power supply unit. The TL494 is a classic PWM (Pulse Width Modulation) controller IC that was widely used in older ATX power supplies and other switched-mode power supplies (SMPS).
We can think it as an engine driver for the power conversion process — it doesn’t directly make power, but it tells the switching MOSFETs exactly when and how long to turn on, to regulate the output voltage.
- Type: PWM Controller
- Package: DIP-16 or SOIC-16
- Manufacturer: Originally Texas Instruments, also made by others.
- Applications: ATX PSUs, battery chargers, DC-DC converters, motor drivers.
Internal Blocks
A block diagram of the TL494 in an ATX power supply is shown below so that it can bee seen exactly how it connects to MOSFETs, the transformer, and the feedback loop. That’ll make its role crystal clear.
Inside the TL494 you’ll find:
1. Two Error Amplifiers
* Compare the output voltage (via feedback) to a reference voltage.
* One can be used for voltage regulation, the other for current limiting.
2. 5 V Precision Reference
* Used for stable regulation and as a reference for the error amplifiers.
3. PWM Comparator
* Determines the duty cycle (how long MOSFETs are on) based on feedback.
4. Oscillator
*Sets the switching frequency via an external resistor & capacitor.
5. Dead-Time Control
* Prevents both MOSFETs from being on at the same time in push-pull or half-bridge setups.
6. Output Control Logic
* Can be configured for single-ended or push-pull drive.
7. Two Open-Collector Outputs
* These drive the gates/bases of the external power transistors (usually through a driver stage).
How It Works in an ATX PSU
1. Feedback Signal (from TL431 + optocoupler) tells TL494 if output voltage is too high or low.
2. Error Amplifier compares it to the reference voltage.
3. PWM Logic adjusts the pulse width to the MOSFETs.
4. MOSFETs chop the high-voltage DC from the rectifier into pulses for the transformer.
5. TL494 repeats this process thousands of times per second (\~30–100 kHz), keeping outputs stable.
Why It’s Popular
* Very flexible: works in many topologies (push-pull, half-bridge, forward).
* Low cost, easy to find.
* Has built-in reference and dual error amplifiers — minimal external components needed.
* Extremely well-documented (tons of example schematics).
