Single-switch switching power supply efficiency technical solution based on Topswitch - Power Circuit - Circuit Diagram

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The integrated switching power supply has been continuously developed in two directions: the first is to integrate the control circuit of the core unit of the switching power supply. The second direction is to achieve monolithic integration of medium and small power switching power supplies.

The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, the simplest peripheral circuit, the best performance index, and the isolated switching power supply capable of forming a high efficiency and no power frequency transformer. At present, it has become the preferred integrated circuit for developing medium and small power switching power supplies, precision switching power supplies, special switching power supplies and power modules. At present, monolithic switching power supplies have formed dozens of series and hundreds of products. However, switching efficiency has always been a concern for everyone. This article puts forward a little bit of your own opinion on this issue.

1 Topswitch chip in switching power supply

Since the 1970s, power supply products have set off a wave of high-frequency, miniaturization and modularization. This has effectively promoted the development of single-chip 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, self-protecting and complete in design. In addition, the TOPSwitch device eliminates the need for a separate heat sink and saves costs. The internal PWM controller and MOSFET power switch are connected inside the package, and the connection is extremely short, which eliminates high frequency radiation, improves the electromagnetic compatibility of the power supply, and reduces the device layout and input. The transient requirements of the bus.

TOPSwitch-II is an improved version of TOPSwitch that offers significant performance improvements over the first generation. It increases the maximum power from 100W to 150W at a single voltage input, enhances electromagnetic compatibility, and has a higher performance-to-price ratio and greatly reduces the size and weight of the power supply. Because it integrates a 700 V power MOSFET, crystal oscillator, high-voltage switch current source, current limiting, and thermal shutdown circuitry, it provides a high-efficiency switching power supply design with its groundbreaking design. A bias and self-protecting, current linear duty cycle converter with an open-drain output.

The third-generation TOPSwitch-FX series is a five-terminal monolithic switching power supply IC that uses new technologies such as “skip cycles”. If the switching power supply is so lightly loaded that the output power provided by the switching power supply at the minimum duty cycle (Dmin = 1.5%) still exceeds the load power consumption, TOPSwitch-FX uses the skip cycle mode to further Reduce output power while increasing voltage stability at light loads. This method is equivalent to first fixing the duty ratio to 1.5% (or lower), and then adjusting the U0 value at light load by pulse frequency modulation (PFM). Thus, depending on the load, the switching power supply can automatically switch between normal operation and skip cycle mode without additional control. If it is not necessary to skip the cycle, the minimum load RLmin can be connected to the power supply output and D is greater than the duty cycle of Dmin of 1.5%. Using the skip cycle mode not only achieves very low output power, but also reduces noise voltage.

TOPSwitch-GX is the fourth generation. It uses the same topology as TOPSwitch to integrate high voltage power MOS-FETs, pulse width modulation (PWM) controllers, fail-safe protection and other control circuitry into a single CMOS chip and increase the operating frequency to 132 kHz while The power range of the TOP-Switch series has also been extended to increase the maximum power at a single voltage input to 250 W. In addition, it integrates several new features, effectively reducing system cost and increasing design flexibility, as well as functionality and performance.

2 Main factors affecting the efficiency of single-chip switching power supply

As part of a single-chip switching power supply, the TOPSwitch family of chips has a certain impact on power efficiency. Figure 1 shows the internal circuit of the ST204A single-chip switching power supply module. In fact, most of the power loss of the power supply in the figure is from TOP204Y, clamp diode (VDZ), output rectifier (VD2), common mode choke (L2), rectifier bridge (BR), high frequency transformer (T). And input capacitor (C1), output capacitor (C2) and so on. They are also a major factor affecting power efficiency.

3 Ways to improve the efficiency of single-chip switching power supply

3.1 Correct determination of primary circuit components

(1) Selection of input rectifier bridge (BR)

Selecting a rectifier bridge with a larger capacity and operating it at a lower current can reduce the voltage drop and power loss of the rectifier bridge and improve the power efficiency. The nominal supply current IN of the rectifier bridge (BR) consisting of diodes should be greater than the primary effective current when the input voltage is at its minimum (Umin). The power factor should be between 0.6 and 0.8, depending on the input voltage u. And input impedance.

(2) Selection of clamped diode (VDZ)

The clamp 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 FIG. 1, the input clamp protection circuit is composed of VDZ and VD1. In order to reduce its loss, VDZ can use P6KE200 type transient voltage suppression diode; VD1 selects BYV 26C type 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. The choice of C1 is mainly to correctly estimate the capacitance. Generally, when the input voltage U1 is increased, the capacitance corresponding to the output power per watt can be reduced.