Monolithic Switching Power Supply Efficiency Technology Solution-EEWORLD

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Integrated switching power supplies have been developing in two directions: the first is to integrate the core unit of the switching power supply - the control circuit. The second direction is to achieve monolithic integration of medium and low power switching power supplies.
Monolithic switching power supply integrated circuits have the advantages of high integration, high cost performance, the simplest peripheral circuit, the best performance index, and the ability to form a high-efficiency isolated switching power supply without an industrial frequency transformer. At present, it has become the preferred integrated circuit for the development of medium and low power switching power supplies, precision switching power supplies, special switching power supplies and power modules in the world. At present, monolithic switching power supplies have formed dozens of series and hundreds of products. However, switching efficiency has always been a concern of everyone. This article puts forward some of its own views on this issue.
1 Application of Topswitch chips in switching power supplies
Since the 1970s, power supply products have set off a wave of high frequency, miniaturization and modularization. This has effectively promoted the development of monolithic switching power supplies. For switching power supplies below 200W, compared with other circuits, the circuit using Topswitch series devices is relatively simple, small in size, light in weight, complete in self-protection functions, and easy to design. In addition, TOPSwitch devices do not need to have a separate heat sink, which also saves costs. The internal PWM controller and MOSFET power switch tube are connected inside the tube shell, and the connection is extremely short, which eliminates high-frequency radiation, improves the electromagnetic compatibility of the power supply, and reduces the device's transient requirements for circuit board layout and input bus.
TOPSwitch-Ⅱ is an improved model of TOPSwitch. Compared with the first generation product, the device has greatly improved in performance. It increases the maximum power from 100W to 150W when the single voltage input is used, and the electromagnetic compatibility is also enhanced. It has a higher performance-price ratio and greatly reduces the size and weight of the power supply. Because it integrates 700 V power MOSFET, crystal oscillator, high-voltage switching current source, current limiting and thermal shutdown circuits, and provides a high-efficiency switching power supply design with its breakthrough design, it is a converter with bias and self-protection, current linear duty cycle, and the device uses open-drain output. The
third-generation TOPSwitch-FX series is a five-terminal monolithic switching power supply integrated circuit that uses new technologies such as "skipping cycles". If the load of the switching power supply is very light, so that the output power provided by the switching power supply under the minimum duty cycle (Dmin=1.5%) still exceeds the load power consumption, TOPSwitch-FX will use the skip cycle mode to further reduce the output power and improve the voltage stability under light load. This method is equivalent to fixing the duty cycle at 1.5% (or lower value) first, and then adjusting the U0 value under light load by pulse frequency modulation (PFM). In this way, according to the change of load, the switching power supply can automatically switch between normal operation and skip cycle mode without other control. If skip cycle is not required, the minimum load RLmin can be connected to the output of the power supply, and D is greater than Dmin, which is a duty cycle of 1.5%. Using skip cycle mode can not only obtain extremely low output power, but also reduce noise voltage.
TOPSwitch-GX is the fourth generation product. It uses the same topology circuit as TOPSwitch to integrate high-voltage power MOS-FET, pulse width modulation (PWM) controller, fault automatic protection and other control circuits into a monolithic CMOS chip, and increases the operating frequency to 132 kHz, while also expanding the power range of the TOP-Switch series, increasing the maximum power at single voltage input to 250 W. In addition, it also integrates a number of new functions, thus effectively reducing system costs, improving design flexibility, as well as functionality and performance.

2 Main factors affecting the efficiency of monolithic switching power supplies
As part of a monolithic switching power supply, the TOPSwitch series chip has a certain impact on the efficiency of the power supply. Figure 1 shows the internal circuit of the ST204A monolithic switching power supply module. In fact, most of the power loss of the power supply in the figure is caused by TOP204Y, clamping diode (VDZ), output rectifier tube (VD2), common mode choke (L2), rectifier bridge (BR), high frequency transformer (T) and input capacitor (C1), output capacitor (C2), etc. They are also the main factors affecting the efficiency of the power supply.
3 Methods for improving the efficiency of monolithic switching power supplies
3.1 Correctly determine the components of the primary circuit
(1) Selection of input rectifier bridge (BR)
Selecting a rectifier bridge with a larger capacity and making it work at a smaller current can reduce the voltage drop and power loss of the rectifier bridge and improve the efficiency of the power supply. The nominal power supply current IN of the rectifier bridge (BR) composed of diodes should be greater than the primary effective current when the input voltage is the minimum value (Umin), and the power factor should be between 0.6 and 0.8. Its specific value depends on the input voltage u and input impedance.
(2) Selection of clamping diode (VDZ)
The clamping circuit is mainly used to limit the peak voltage generated by the leakage inductance of the high-frequency transformer and reduce the ringing voltage generated by the drain. In the monolithic switching power supply module circuit shown in Figure 1, the input clamping protection circuit is composed of VDZ and VD1. In order to reduce its loss, VDZ can use P6KE200 transient voltage suppression diode; VD1 can use BYV 26C fast recovery diode.
(3) Input filter capacitor (C1)
The input filter capacitor C1 is used to filter out the high-frequency interference introduced at the input end. The selection of C1 is mainly to correctly estimate its capacitance. Generally, when the input voltage U1 increases, the capacitance corresponding to each watt of output power can be reduced.

Reference address：Monolithic Switching Power Supply Efficiency Technology Solution

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