High-power monolithic switching power supply design

Introduction
Switching power supplies have outstanding advantages such as high efficiency, light weight, small size, and wide voltage regulation range. Since its introduction in the mid-20th century, it has developed extremely rapidly and has been widely used in computers, communications, aerospace, office and household appliances, and has a tendency to replace linear voltage regulators. Improving the integration of circuits is one of the pursuits of switching power supplies. For small and medium-power switching power supplies, it is to achieve monolithic integration. A switching integrated voltage regulator refers to a control circuit, power switch tube, and protection circuit integrated into one chip, and a switching power supply composed of a switching integrated voltage regulator is called a monolithic switching power supply. The monolithic switching power supply series of the American PI company is a prominent representative.

l 250W switching power supply designed with TOPswitch-GX
The 250W switching power supply circuit designed with TOPSwitch-GX is shown in Figure 1. The DC voltage is added to the drain D of TOPSwitch-GX through the primary side of the transformer; the frequency selection terminal F and the limit current setting terminal X are connected to the source S, then the functions of both ends are useless, that is, the limit current is not set from the outside, the self-protection current ILIMIT is automatically set internally, and the switch operating frequency is 132K; the control electrode is connected to the optocoupler LTV817, and receives the feedback signal to realize the control of the duty cycle of the internal integrated high-voltage power MOS tube; the line detection terminal L is connected to the positive end of the DC high-voltage input through a 2MΩ resistor to realize the line voltage detection of overvoltage and undervoltage line voltage feedforward. The whole circuit is a single-ended flyback type, TOPSwitch-GX is a switch integrated regulator, and the feedback circuit is mainly composed of the optocoupler LTV817 and three voltage regulator diodes connected in series with it. Capacitor C1 is a high-frequency filter capacitor; transient voltage suppressor P6KE200 and ultra-fast recovery diode BYV26C form a clamping circuit, and an RC absorption circuit (composed of R2, R3 and C6) is connected in series. In addition to absorbing some of the energy in the leakage inductance, the voltage can also be clamped at 200V, so that the drain voltage of the integrated MOS tube inside TOPSwitch-GX does not exceed 700V when the switching power supply is started or overloaded; optocoupler LTV817 and voltage regulator diodes (VR2~VR4) form a feedback circuit , R6 is the current limiting resistor of the LED in the optocoupler, and it also determines the gain of the control loop. When the output voltage changes, the current flowing through the LED in the optocoupler changes accordingly, so that the current sent to the chip control electrode C also changes accordingly. The duty cycle of the PWM signal generated inside the chip changes accordingly, and finally the output voltage is stabilized; the secondary output of the high-frequency transformer T1 is rectified by MURl640CT and filtered by C9, C10 and C11, and then the switching noise is filtered out by the magnetic beads L1 and C12 to obtain the output voltage; VD4 and C14 constitute a soft start circuit.

2 High-frequency transformer design
For PI's single-chip switching power supply, the high-frequency transformer is designed using PI's related switching power supply computer-aided design software. This solution uses the PI Expert 7.0 expert system. Figure 2 shows the structure of the transformer designed using this software.

3 Test results
The power supply has an output power of 250W, an efficiency of at least 85%, a load regulation rate of ±5%, a ripple voltage peak-to-peak value of less than 100mV, and a no-load power consumption of no more than 1.4W.

This solution uses TOP249Y design, which works at its upper power limit when the output power is 250W, so good heat dissipation conditions must be maintained (the chip temperature must be kept below 110℃). The component can also be replaced by TOP250Y.

4 Conclusion
The monolithic switching power supply has the advantages of monolithic integration, simplest peripheral circuit, best performance index, and complete isolation without power frequency transformer. It is an ideal choice for designing switching power supplies below 290W.