UC3865NG4

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UC3865NG4 Application Field and Working Principle

The UC3865NG4 is a high performance voltage mode Pulse Width Modulation (PWM) controller from Texas Instruments which is specifically designed for Off-Line and DC-to-DC converter applications offering the designer outstanding features and improved performance. It is suitable for both single-ended and double-ended topologies, such as forward converters, flyback converters, push-pull converters and forward-flyback converters. This device is designed to operated in Continuous Conduction Mode (CCM) which offers a sinusoidal input current and further reduces the size of the external filters and power loss. This makes it well-suited for use in Off-Line applications.

The UC3865NG4 is available in 4 pin DIP package and consists of a variably deadtime generator, Slope compensation and an adjustable Precision Current Limiting circuit (1.75A). It has a high-current totem pole output stage for driving a MOSFET or IGBT. The UC3865NG4 has an oscillator which is based on a saw-toothed waveform and provides a pulse width modulated output. An internal voltage reference ranges from 4.096V ±10%. Line frequency operation is available with guarantee by means of an internal zener diode. Output current sensing is achieved with the help of external sense amplifier.

The UC3865NG4 voltage mode PWM controller is used in applications such as SMPS, AC/DC and DC/DC converter, battery charger, and audio power amplifiers. It is also suitable for telecommunication, computer, monitors and cable TV.

UC3865NG4 Working Principle

The UC3865NG4 working principle relies on a pulse-width regulated-drive type of power converter. This power converter taps the AC input voltage through a high-frequency switching device, like a MOSFET or IGBT. During this process, the output voltage and current drawn from the ripple-regulated DC path is regulated correctly.

The controller features a linear element, Figure 1, in series with the MOSFET to keep a fixed deadtime (“DT”) between the two arms of the switch, which is necessary to reduce circulating current and losses. In case of failure of either the primary or secondary MOSFET, the circuit will latch off in the On-Time (“ON”) of the pulse, thereby preventing mixed-mode operation and total failure of the supply.

The heart of the UC3865NG4 is an oscillator whose frequency can be adjusted with an external resistor. It also features accurate current limit, analog current setting (ACS) and a dead-time generator which all helps to adjust the output voltage. At the same time, the output is maintained close to its set point voltage, thereby eliminating the need to provide external feedback.

The UC3865NG4 can operate in all modes of the switch, i.e. On-time (“ON”), off-time (“OFF”), and deadtime (“DT”). The deadtime resistor sets the pulse width of the output waveforms while the PWM frequency is set by the internal oscillator.

Conclusion

The UC3865NG4 is an advanced, high performance pulse width modulation (PWM) controller that is specifically designed for off-line and DC-to-DC converter applications offering the designer outstanding features and improved performance. It is suitable for both single-ended and double-ended topologies and provides a sinusoidal input current. Output current sensing is achieved with the help of external sense amplifier and output voltage is maintained close to its set point voltage eliminating the need for external feedback.

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