Threshold Frequency Shaking Control System and Method for Primary Side Feedback AC-DC Switching Power Supply

Abstract

The invention discloses a system and a method for controlling threshold jitter frequency of an original-side feedback AC-DC switching power supply. The threshold jitter frequency control system is provided with a current-limiting threshold jitter unit which is connected with a cycle-to-cycle current-limiting unit; the cycle-to-cycle current-limiting unit is provided with a resistor and a comparator; the current-limiting threshold jitter unit is provided with an oscillator, a frequency divider, a plurality of groups of current sources and switches; the current-limiting threshold jitter unit can generate a periodic current signal to regulate a constant-current frequency control unit, so that the working frequency jitters periodically within a certain range when the system works in a constant-current mode, the electromagnetic interference (EMI) energy is dispersed and the EMI energy is reduced greatly; moreover, the constant-current characteristics of the threshold jitter control system are not influenced by the frequency jitter; and the threshold jitter frequency control method corresponds to the threshold jitter frequency control system.

Description

technical field [0001] The invention relates to the technical field of switching power supplies, in particular to a threshold frequency shaking control system for primary-side feedback AC-DC switching power supplies. Background technique [0002] The primary-side feedback AC-DC control technology is a new type of AC-DC control technology developed in the last 10 years. Compared with the traditional secondary-side feedback switching power supply structure, its biggest advantage is that it eliminates the need for isolation chips and isolation A group of components that work together on a chip, which saves space on the circuit board, reduces cost and increases system reliability. The primary-side feedback DC power supply has a large market share in applications with high cost pressure such as mobile phone chargers, and applications with high volume requirements such as LED drive power supplies. [0003] attached figure 1 A structural block diagram of a primary-side feedback A...

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Problems solved by technology

[0005] When the system works in constant current mode, when the input voltage and output voltage are fixed, for the same system parameters, the secondary conduction time Tons=(Ips×Ls) / (Vout+Vz)=Ipp×(NS / NP )×Ls / (Vout+Vz)=(Vcs / R3)×(NS / NP)×Ls / (Vout+Vz), since Vcs is the current limiting threshold inside the chip, it is a fixed value, and other parameters are also fixed value, so the secondary conduction time Tons is a fixed value, so the period T=K×Tons is a fixed value, that is, when the system works in constant current mode, its operating frequency is fixed, so the energy of EMI is concentrated in this The operating frequency and its Nth harmonics make the EMI very large, and it is difficult to meet the EMI requirements of the charger and LED drive system

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