3842-series chip based PFM application circuit

Abstract

The invention discloses a circuit using an 8-pin 3842-series controller chip as an example, applied in the technical field of switching power supplies and changed from a PWM mode to a PFM mode. The circuit includes a 3842 integrated block IC1, resistors R4 and R14, a capacitor C7 and an optocoupler U1. The 3842-series internal circuit is defined to be a block diagram having 8 different functional pins. The pin 4 of the 3842 integrated block IC1 is connected with one end of each of the resistors R4 and R14 and the capacitor C7, the other end of the capacitor C7 is grounded, the other end of the R14 is connected with the pin E of the U1, the other end of the R4 is connected with the pin 8 of the IC1, the pin 8 of the IC1 is further connected with the pin C of the U1, and the pin 5 of the IC1 is grounded. When the pins C and E of the optocoupler is connected, the charging current of the C7 is increased, the working state of the controller chip 3842 is controlled to be in the PFM mode, the stability and efficiency of the circuit are greatly improved after mode change, and the working amount of design, detection and production is decreased.

Description

technical field [0001] The invention relates to a PFM circuit, in particular to a PFM application circuit based on 3842 series chips. Background technique [0002] Switching power supply controller chip 3842 series is the earliest and most widely used controller chip in the field of switching power supply technology. It is a high-performance, typical PWM mode controller chip. This chip mainly has the following features: (1) fine-tuning the discharge current of the oscillator can accurately control the duty cycle, (2) single pulse measurement latch, cycle-by-cycle current limit, (3) internal reference voltage, under-voltage lockout, (4) ) High current totem pole output (5) Low starting and operating current. The peripheral circuits of this series of controller chips are simple and cheap. At present, it is widely used in the field of power supply technology. There are two types of conventional application circuits for this series of controller chips, see attached figure 1 ...

AI Technical Summary

Problems solved by technology

To give a few examples: (1) The controller chip 3842 is a current-type controller, which naturally has many advantages, (not introduced), but the closed-loop control of each power supply product requires a certain amount of time to fine-tune, and a little improper work may occur. Stable, self-excited in severe cases

(2) When the external load changes from zero to full load and the input voltage of the power supply changes from the lowest to the highest, there are always some points that are not stable, especially when the dummy load is small and the external load is zero.

(3) The controller chip 3842 needs slope compensation when working. If the compensation is not in place, the operation will be unstable if the closed-loop adjustment is slightly improper.

(4) The false load should not be too small, generally 1% to 2%. Small false loads are prone to instability, and large false loads are inefficient. Each design can only adopt a compromise method to solve this problem

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