The TL494 is a versatile integrated circuit designed for pulse width modulation (PWM) control in switching power supplies. It features a fixed oscillation frequency and includes all the necessary components for efficient power conversion. The TL494 is commonly used in various topologies, such as single-ended forward, half-bridge, full-bridge, and push-pull configurations, making it a popular choice for a wide range of power supply designs. The TL494 operates with an internal linear sawtooth oscillator that generates a fixed-frequency signal. This frequency can be adjusted externally using a resistor and capacitor connected to pins CT and RT. The output pulse width is determined by comparing the sawtooth voltage on the capacitor with two external control signals. These signals are fed into the PWM comparator, which controls the duty cycle of the output pulses. Power transistors Q1 and Q2 are controlled by a NOR gate based on the state of a bistable flip-flop. When the clock signal is low, the flip-flop is triggered, allowing the output pulse to occur only when the sawtooth voltage exceeds the control signal. As the control signal increases, the pulse width decreases, effectively adjusting the output power. The control signals are input from outside the IC and pass through the dead-time comparator before reaching the error amplifier. The dead-time comparator uses a 120mV reference voltage to limit the minimum dead time to about 4% of the oscillator period. This ensures safe operation by preventing both transistors from turning on simultaneously. When the output is grounded, the maximum duty cycle reaches 96%, while connecting the output to a reference level sets the space ratio at 48%. By applying a fixed voltage (between 0 and 3.3V) to the dead-time control pin, additional dead time can be introduced, further enhancing system safety. The pulse-width modulator compares the feedback signal from the error amplifier and adjusts the output pulse width accordingly. When the feedback voltage changes from 0.5V to 3.5V, the maximum on-time percentage drops to zero, ensuring precise control over the output. The two error amplifiers have a common-mode input range from -0.3V to (Vcc - 2.0V), allowing them to respond to variations in output voltage and current. The error amplifier output is always high and is combined with the inverted input of the PWM comparator, enabling the control loop to maintain stable output under varying load conditions. When the CT pin discharges, the dead-band comparator produces a positive pulse, which triggers the bistable flip-flop to stop the outputs of Q1 and Q2. If the output control terminal is connected to a reference voltage, the modulated pulse alternates between the two output transistors at half the oscillator frequency. In single-ended mode, if the maximum duty cycle is less than 50%, the drive signals are taken from either Q1 or Q2. The output transformer has a feedback winding and a diode that provides a feedback voltage to the error amplifier, completing the control loop. In applications requiring higher drive current, Q1 and Q2 can be operated in parallel. To do this, the output mode control pin should be grounded to disable the bistable trigger. In this configuration, the input pulse frequency matches the oscillator frequency, ensuring consistent and reliable operation. Overall, the TL494 offers a robust and flexible solution for designing efficient and stable switching power supplies.
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Introduction to TL494