0 Preface:
Switching power supply has the outstanding advantages of high efficiency, light weight, small size and wide voltage regulation range. Since its inception in the middle of the 20th century, it has developed extremely rapidly, and has been widely used in computer, communication, aerospace, office and household appliances. There is a great potential to replace the linear regulated power supply. Improving the integration of circuits is one of the pursuits of switching power supplies. For small and medium power switching power supplies, monolithic integration is achieved. The switch integrated voltage regulator refers to the integration of the control circuit, the power switch tube and the protection circuit in one chip, and the switching power supply composed of the switch integrated voltage regulator is called a single-chip switching power supply. American PI company's single-chip switching power supply series is a remarkable representative.
1 250W switching power supply designed with TOPswitch-GX
The TOPSwitch-GX designed 250W switching power supply circuit is shown in Figure 1. The DC voltage is applied to the drain D of the TOPSwitch-GX via the primary side of the transformer; the frequency selection terminal F and the limit current setting terminal X are connected to the source S, and the functions at both ends are useless, that is, the limit is not set from the outside. Current, the internal self-protection current ILIMIT is automatically set, the switching operating frequency is 132K; the control pole is connected to the optocoupler LTV817, and the feedback signal is received to realize the control of the internal integrated high-voltage power MOS tube duty cycle; the line detection terminal L passes a 2MΩ The resistor is connected to the positive terminal of the DC high voltage input to achieve 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 voltage regulator, and the feedback circuit mainly consists of an optocoupler LTV817 and three Zener diodes connected in series. Capacitor C1 is a high-frequency filter capacitor; transient voltage suppressor P6KE200 and ultra-fast recovery diode BYV26C form a clamp circuit, and a RC snubber circuit (composed of R2, R3 and C6) is connected in series, so that it can absorb part of the leakage inductance. In addition to the energy in the middle, the voltage can be clamped at 200V, which can make the drain voltage of the TOPSwitch-GX internal integrated MOS tube not exceed 700V when the switching power supply is started or overloaded; the optocoupler LTV817 and the Zener diode (VR2~) VR4) constitutes the feedback circuit, R6 is the current limiting resistor of the LED in the optocoupler, 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 light is sent to the chip control pole C. The current also changes accordingly. The duty cycle of the PWM signal generated by the chip is changed to finally stabilize the output voltage. The secondary output of the high-frequency transformer T1 is rectified by MURl640CT and filtered by C9, C10 and C11, and then passed through the magnetic beads L1. After C12 filters out the switching noise, the output voltage is obtained; VD4 and C14 form 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 program uses the PI Expert 7.0 expert system. Figure 2 shows the structure of a transformer designed with this software.
3 test results
The power output power is 250W, the efficiency is at least 85%, the load regulation is ±5%, the ripple voltage peak-to-peak is less than 100mV, and the no-load power consumption is no more than 1.4W.
This program uses TOP249Y design, the output power is 250W, it works at its upper limit power, so it is necessary to maintain good heat dissipation conditions (the chip temperature should be kept below 110 °C), and the component can also be replaced with TOP250Y.
4 Conclusion
The single-chip switching power supply has the advantages of monolithic integration, simplest peripheral circuits, best performance indicators, and complete isolation with no power frequency transformers. It is an ideal choice for designing switching power supplies below 290W.
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