Valley control circuit and valley control method

Abstract

The present invention provides a wave valley control method and a wave valley control circuit which can expand an input voltage range and which will not cause wave valley switching spikes. The wave valley control circuit comprises a current detection circuit, a secondary feedback circuit, and a drive output circuit. The current detection circuit is used to detect a primary peak value current of a source of a transistor, so as to control a turn-on time of the transistor. The second feedback circuit is used to receive a voltage signal reflecting the impedance of a load and fed back by a voltage sampling isolation feedback circuit. The drive output circuit is used to output a drive signal to the transistor. An input voltage detection circuit and a waveform detection circuit are further comprised. The input voltage detection circuit is used to detect an input voltage of a flyback converter, such that the wave valley control circuit can determine, according to the value of the input voltage, at which wave valley the switching cycle of the transistor will start. The waveform detection circuit is used to sample the waveform of a wave at a drain of the transistor, and count the number of wave valleys.

Description

technical field [0001] The invention relates to the field of switching converters, a flyback converter working in a discontinuous mode, in particular to a valley control method and a valley control circuit of the flyback converter in DCDC applications. Background technique [0002] With the development of science and technology, there are more and more electrical equipment, instruments, and electronic products, and the demand for switching power supplies is also increasing. The emergence of some portable electronic products has increased the demand for switching power supplies in the micro-power segment. [0003] At present, the switching power supply scheme used in the micro-power segment is mainly the flyback circuit, because it has a series of characteristics such as low cost, simple and reliable circuit, and mature application. There are two working modes of the flyback circuit, one is continuous mode (CCM) and the other is discontinuous mode (DCM). Generally, the conti...

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Problems solved by technology

Assuming two common prototypes (the first input is 100VDC-400VDC, the reflected voltage is 100V, the second input is 9VDC-36VDC, and the reflected voltage is 10VDC), ignoring the small part of the resonance time in the critical mode, according to the formula Vin*D=Voff*(1-D), L*Ipk=Vin*D*T, Pin=1 / 2*L*Ipk 2 *f, assuming that the efficiency under high and low pressure is the same, it can be concluded that the operating frequency of the first prototype under high pressure is 2.56 times that under low pressure, and the operating frequency of the second prototype under high pressure is 2.73 times under low pressure , so the operating frequency of the switching tube in the traditional scheme varies greatly, and the EMI circuit is difficult to design

Moreover, the operating frequency will be very high under high input voltage, resulting in very low efficiency under high input voltage, so it can only be used for applications with a relatively narrow input voltage range.

[0008] The second defect of the valley control scheme of the traditional control chip is: when the FB voltage corresponding to the load is just near the gear, assuming that the FB voltage reaches the second gear, the resonance valley number of the MOS transistor TR1 will be at the 4th gear. Constantly switch between the first trough and the third trough, because that point is a critical value

The usual idea is to add a hysteresis to make the judgment of the critical point more stable, but under this condition even if the hysteresis is increased, it will cause valley switching, because when the working state changes from a condition with a large number of troughs to a condition with a small number of troughs After the state (FB pin voltage from low to high), the operating frequency of the switch tube is equivalent to a sudden change, and the peak current cannot change immediately, so according to the energy formula Pin=1 / 2*L*Ipk 2 *f can know that the input energy will suddenly increase, which will inevitably cause an increase in the output voltage, and the increase in the output voltage will inevitably cause a decrease in the voltage of the FB pin, so it will jump back to the one with more valleys. original state

The magnitude of the FB pin voltage change will exceed the amplitude of the hysteresis. The fundamental reason is that the switching trough will inevitably cause a sudden change in energy, and the sudden change in energy will further cause a sudden change in the voltage of the FB pin, thus forming an oscillation.

In such an oscillating situation, the output ripple will become very large, especially in DCDC applications. Since the conventional ripple requirement of most products is only about 50mV, when the output capacitance is small, the traditional solution valley switching The time ripple reaches 100-200mV, so the traditional solution is difficult to use in the field of DCDC micropower

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