Constant current control circuit, control method and flyback circuit

Abstract

The invention discloses a constant current control circuit, a control method and a flyback circuit, when a system is in a continuous mode, the constant current control circuit controls the product of a median current of a primary side inductor and a first conduction duty ratio of a secondary side to be in a set first threshold interval; and when the system is in an interrupted mode, the constant current control circuit controls the product of half of the peak current of the primary side inductor and the second conduction duty ratio of the secondary side to be in the first threshold interval. According to the constant-current control circuit, the constant-current control method and the flyback circuit, the constant-current precision of the system working in a DCM mode of high-voltage input can be greatly improved, sampling errors caused by CS voltage waveform oscillation are avoided, and meanwhile, the influence of median sampling position deviation on the constant current of the system is also eliminated.

Description

technical field [0001] The invention belongs to the technical field of circuit control, and relates to a flyback circuit, in particular to a constant current control circuit, a control method and a flyback circuit. Background technique [0002] figure 1 is a schematic diagram of a typical flyback circuit; see figure 1 , to realize the constant current function of the secondary output through primary sampling, it is necessary to sample the CS median value of the primary current and the conduction duty cycle of the secondary current. [0003] Figure 2a The key waveform diagram for the flyback circuit working in discontinuous mode DCM, Figure 2b Key waveform diagrams for the flyback circuit operating in continuous mode CCM; see Figure 2a , Figure 2b , the chip controls the average output current by calculating the average value of the secondary inductor current or the primary inductor current in each switching cycle: [0004] [0005] Among them: Is(t) is the real-...

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Problems solved by technology

[0014] In practical applications, the DCM working mode usually appears under the condition of high voltage input. At this time, the following points will affect the sampling accuracy of the above-mentioned CS median voltage and the degaussing time T DEM sampling accuracy, resulting in a decrease in constant current accuracy in DCM mode:

[0015] (1) When high voltage is input, the CS waveform is more likely to oscillate, resulting in a large deviation in the sampling of the median voltage;

[0016] (2) Due to the high voltage input, the on-time Ton of the primary side becomes shorter, which increases the deviation of the median position Ton / 2;

[0017] (3) During degaussing detection, due to oscillation factors, the ratio T obtained by sampling DEM The actual value is slightly larger

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