Single-chip switching power supply with two working modes
The single-chip 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 efficiency of the power supply 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.
Features of monolithic switching power supply
(1)TOpSWitch-II includes oscillator, error amplifier, pulse width modulator, gate circuit, high voltage power switch tube (MOSFET), bias circuit, overcurrent protection circuit, overheat protection and power-on reset circuit, shutdown/ auto-restart circuit. It uses a high-frequency transformer to completely isolate the output terminal from the grid, which is safe and reliable. It is a current-controlled switching power supply with open-drain output. Due to the use of CMOS circuits, the power consumption of the device is significantly reduced.
(2) There are only three terminals: control terminal C, source S, and drain D, which are comparable to three-terminal linear regulators and can form a flyback switching power supply without a power frequency transformer in the simplest way. In order to complete a variety of control, bias and protection functions, C and D are multi-functional terminals, realizing one pin with multiple functions. Taking the control terminal as an example, it has three functions: ①The voltage VC of this terminal provides bias voltage for the on-chip shunt regulator and the gate driver stage; ②The current IC of this terminal can adjust the duty cycle; ③This terminal is also used as a power supply branch The connection point with the automatic restart/compensation capacitor, the frequency of the automatic restart is determined by an external bypass capacitor, and the control loop is compensated.
(3) The range of input AC voltage is extremely wide. 220V±15% AC power is optional for fixed voltage input, if it is equipped with 85~265V wide-ranging AC power, the maximum output power will be reduced by 40%. The input frequency range of the switching power supply is 47~440Hz.
(4) The typical value of the switching frequency is 100KHz, and the adjustment range of the duty ratio is 1.7% to 67%. The power supply efficiency is about 80%, up to 90%, which is nearly double that of the linear integrated regulated power supply. Its working temperature range is 0～70℃The maximum junction temperature of the chip is Tjm=135℃.
(5) The basic working principle of TOpSwitch-II is to use the feedback current IC to adjust the duty ratio D to achieve the purpose of voltage regulation. For example, when the output voltage VOT of the switching power supply is caused by some reason, the optocoupler feedback circuit will make Ic↑→error voltage Vrt→D↓→Vo↓, so that Vo remains unchanged. vice versa.
(6) The peripheral circuit is simple and the cost is low. Externally only need to connect rectification filter, high frequency transformer, primary protection circuit, feedback circuit and output circuit. The use of such chips can also reduce electromagnetic interference generated by switching power supplies.
Two working modes of monolithic switching power supply
The monolithic switching power supply has two basic working modes: one is the continuous mode CUM (ContinuousMode), and the other is the discontinuous mode
The switching current waveforms of the two modes in Fig.
(a) continuous mode (b) discontinuous mode
DUM (Discontinuous Mode). The switching current waveforms of these two modes are shown in Figure (a) and Figure (b) respectively. It can be seen from the figure that in continuous mode, the primary switch current starts from a certain amplitude, 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.
