Monolithic switching power supply integrated circuits have the advantages of high integration, high cost-effectiveness, the simplest peripheral circuitry, the best performance indicators, can constitute a high-efficiency isolated switching power supply without frequency transformer. It was introduced in the mid-1990s, late successively, it shows a strong vitality, and now it has become the international development of medium and small power switching power supply, precision switching power supply and power supply module of the preferred integrated circuits. The switching power supply composed of it, in terms of cost and the same power linear voltage regulator power supply is comparable to the power supply efficiency is significantly improved, the volume and weight is greatly reduced. This creates good conditions for the promotion and popularization of new switching power supplies.
(1) TOpSWitch-II internal including oscillator, error amplifier, pulse width modulator, gate circuit, high-voltage power switching tubes (MOSFET), bias circuit, over-current protection circuit, overheating protection and power-on reset circuit, shutdown / automatic restart circuit. It completely isolates the output from the power grid by means of a high-frequency transformer, which is safe to use*. It belongs to the current-controlled switching power supply with open drain output. Due to the use of CMOS circuit, the power consumption of the device is significantly reduced.
(2) There are only three leads: control terminal C, source S, drain D, comparable to three-terminal linear regulator, can be the simplest way to constitute the frequency transformer without the flyback switching power supply. In order to complete a variety of control, bias and protection functions, C, D are multi-functional pin out, realizing a pin multi-purpose. Take the control terminal as an example, it has three functions: (1) the end of the voltage VC for the on-chip parallel regulator and gate drive stage to provide bias voltage; (2) the end of the current IC can adjust the duty cycle; (3) the end of the power supply branch circuit and automatic restart/compensating capacitor connection point, through the external bypass capacitor to determine the frequency of the automatic restart and compensation for the control loop.
(3) The range of input AC voltage is extremely wide. For fixed voltage input, 220V±15% AC can be selected, and the maximum output power will be reduced by 40% if it is equipped with 85~265V wide range AC. The input frequency range of the switching power supply is 47~440Hz.
(4)The typical value of switching frequency is 100KHz, and the duty cycle adjustment range is 1.7%~67%. The efficiency of the power supply is about 80%, up to 90%, which is nearly double than the linear integrated regulated power supply. Its operating temperature range is 0 to 70 degree chip maximum junction temperature Tjm=135 degree .
(5) TOpSwitch-II's basic principle of operation is to use the feedback current IC to regulate the duty cycle D, to achieve the purpose of voltage regulation. For example, when the output voltage of the switching power supply VOT due to some reason, after the optocoupler feedback circuit will make Ic↑→error voltage Vrt→D↓→Vo↓, so that Vo remains unchanged. And vice versa.
(6) simple peripheral circuit, low cost. External only need to connect the rectifier filter, high-frequency transformer, primary protection circuit, feedback circuit and output circuit. The use of such chips can also reduce the electromagnetic interference generated by the switching power supply.
DUM (DiscontinuousMode). The switching current waveforms of these two modes are shown in Fig. (a) and Fig. (b), respectively. As can be seen from the figure, in the continuous mode, the primary switching 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 indicates that in continuous mode, the next switching cycle has an initial energy because the energy stored in the high-frequency transformer is not all released in each switching cycle. The use of continuous mode reduces the primary peak current Ip and RMS current IRMS, reducing the power consumption of the chip. However, the continuous mode requires an increase in the primary inductance Lp, which leads to an increase in the size of the high-frequency transformer. In summary, the continuous mode is suitable for lower power TOpSwitches and larger size high frequency transformers.
The switching current in discontinuous mode rises from zero to a peak and then falls to zero. This means that the energy stored in the high-frequency transformer must be completely released during each switching cycle, and its switching current waveform is triangular in shape. The discontinuous mode has larger values of Ip and IRMS, but requires less Lp. Therefore, it is suitable for the use of TOpSwitch with larger output power with smaller sized high frequency transformer.
