Two working modes of monolithic switching power supply
The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, the simplest peripheral circuit, the best performance index, and can form a high-efficiency isolated switching power supply without power frequency transformer. After it came out in the mid-to-late 1990s, it has shown strong vitality. At present, it has become the preferred integrated circuit for the development of medium and small power switching power supplies, precision switching power supplies and power modules in the world. The switching power supply composed of it is equivalent in cost to the linear regulated power supply of the same power, while the power supply efficiency is significantly improved, and the volume and weight are greatly reduced. This has created good conditions for the promotion and popularization of new switching power supplies.
The monolithic switching power supply has two basic working modes: one is the continuous mode CUM, and the other is the discontinuous mode
The switching current waveforms of the two modes in Fig.
continuous mode/discontinuous mode
In continuous mode, the primary switch current starts at a certain magnitude, then rises to a peak value, and then quickly returns to zero. Its switching current waveform is trapezoidal. This shows that in continuous mode, since the energy stored in the high-frequency transformer is not fully released in each switching cycle, the next switching cycle has an initial energy. Adopting the continuous mode can reduce the primary peak current Ip and the effective value current IRMS, and reduce the power consumption of the chip. However, the continuous mode requires an increase in the primary inductance Lp, which will lead to an increase in the size of the high-frequency transformer. To sum up, continuous mode is suitable for TOPSwitch with small power and high frequency transformer with large size.
The switch current in discontinuous mode rises from zero to peak value and then drops to zero. This means that the energy stored in the high-frequency transformer must be completely released in each switching cycle, and its switching current waveform is triangular. The Ip and IRMS values in the discontinuous mode are larger, but the required Lp is smaller. Therefore, it is suitable for adopting TOPSwitch with larger output power and matching high-frequency transformer with smaller size.
