Single-chip switching power supply application and development - Power Circuit - Circuit Diagram

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Abstract: At present, medium and small power switching power supplies are developing in the direction of monolithic integration. This paper elaborates on the performance characteristics and typical applications of various single-chip switching power supplies that have been introduced in recent years.

The single-chip switching power supply integrated circuit has the advantages of high integration, high cost performance, simplest peripheral circuits, optimal performance indicators, and an isolated switching power supply capable of forming a high-efficiency power-free transformer. After its emergence in the middle and late 1990s, it showed strong vitality. At present, it has become the preferred integrated circuit for developing medium and small power switching power supplies, precision switching power supplies and power modules. The switching power supply composed of it is equivalent in cost to the linear power supply of the same power, and the power supply efficiency is remarkably improved, and the volume and weight are greatly reduced. This has created favorable conditions for the promotion and popularization of new switching power supplies.

1 Development of integrated switching power supply

Switching power supply is known as high-efficiency energy-saving power supply. It represents the development direction of regulated power supply and has become the mainstream product of regulated power supply. For more than 20 years, integrated switching power supplies have evolved in two directions. The first direction is the integration of the control circuit of the core unit of the switching power supply. In 1997, foreign countries first developed pulse-width modulation (PWM) controller integrated circuits. Motorola, Silicon General, and Unitrode introduced a number of PWM chips. The typical products are MC3520. SG3524, UC3842. Since the 1990s, high-speed PWM and PFM (Pulse Frequency Modulation) chips with switching frequency up to 1MHz have been developed abroad. Typical products such as UC1825 and UC1864. The second direction is the centering, small power switching power supply to achieve monolithic integration. This is roughly divided into two phases: In the early 1980s, SGS-Thomson was the first to introduce the L4960 series of single-chip switching regulators. The company introduced the L4970A series in the 1990s. The utility model is characterized in that a pulse width modulator, a power output stage and a protection circuit are integrated in one chip, and the power frequency transformer is required to be isolated from the power grid when used, and is suitable for low voltage output (5.1 to 40V) and medium and medium power (400W or less). ), high current (1.5A ~ 10A), high efficiency (can exceed 90%) switching power supply. But in essence, it is still a DC/DC power converter.

In 1994, Power Company first developed a three-terminal isolated pulse width modulation single-chip switching power supply in the world, which was hailed as “top-level switching power supply”. Its first generation is the TOPSwitch series, and the second generation is the TOPSwitch-II series that was introduced in 1997. In 1998, the company introduced the high-performance, low-power, low-cost four-terminal single-chip switching power supply TinySwitch series. After that, Motorola introduced the MC33370 series five-terminal single-chip switching power supply in 1999, also known as the HighVoltage Power Switching Regulator. At present, single-chip switching power supplies have formed four series and nearly 70 models.

2 TOPSwitch-11 series three-terminal single-chip switching power supply

According to the package form, TOPSwitch-II can be divided into three types: TOP221Y~227Y (TO-220 package), TOP221P~224P (DIP-8 package), TOP221G~224G (SMD-8 package), product classification is shown in Table 1. .

Among them, the output power of TOP227Y is the largest.

2.1 TOPSwitch-11 performance characteristics

î—¤(1) TOPSWitch-II includes oscillator, error amplifier, pulse width modulator, gate circuit, high voltage power switch (MOSFET), bias circuit, overcurrent protection circuit, overheat protection and power-on reset circuit, off Shutdown/auto-restart circuit. It completely isolates the output from the grid through a high-frequency transformer and is safe to use*. It is a current-controlled switching power supply with an open-drain output. Due to the use of CMOS circuits, device power consumption is significantly reduced.

î—¤(2) î—¥ There are only three terminals: control terminal C, source S, and drain D, which are comparable to three-terminal linear regulators, and can form a flyback switching power supply without power frequency transformer in the simplest way. . In order to complete a variety of control, offset and protection functions, C and D are multi-function terminals, which realizes multi-purpose. Taking the control terminal as an example, it has three functions: 1 the terminal voltage VC provides a bias voltage for the on-chip shunt regulator and the gate drive stage; 2 the terminal current IC can adjust the duty cycle; 3 the terminal also serves as a power supply branch The connection point with the auto-restart/compensation capacitor determines the frequency of the auto-restart by the external bypass capacitor and compensates the control loop.

(3) The range of the input AC voltage is extremely wide. For fixed voltage input, 220V±15% AC can be selected. If the AC power varies from 85 to 265V, the maximum output power should be reduced by 40%. The input frequency range of the switching power supply is 47~440Hz.

(4)  The switching frequency is typically 100KHz, and the duty cycle adjustment range is 1.7% to 67%. The power efficiency is about 80%, up to 90%, nearly double the linear integrated regulated power supply. Its operating temperature range is 0 ~ 70 ° C  chip maximum junction temperature Tjm = 135 ° C.

 (5) The basic working principle of TOPSwitch-II is to use the feedback current IC to adjust the duty cycle D to achieve the purpose of voltage regulation. For example, when the output voltage VOT of the switching power supply is caused for some reason, the optocoupler feedback circuit makes Ic↑→error voltage Vrt→D↓→Vo↓, so that Vo remains unchanged. vice versa.

î—¤(6) î—¥ The peripheral circuit is simple and the cost is low. The external only needs to be connected to a rectification filter, a high frequency transformer, a primary protection circuit, a feedback circuit, and an output circuit. The use of such chips also reduces electromagnetic interference generated by switching power supplies.

2.2 Typical applications

TOPSwitch-II can be widely used in instrumentation, notebook computers, mobile phones, televisions, VCDs and DVDs, camcorders, battery chargers, power amplifiers, etc., and can form a variety of miniaturized, high-density, low-cost switches. Power module. In addition, it is also suitable for forming a backup switching power supply, a non-isolated switching power supply, a constant current constant voltage output switching power supply, a DC/DC power converter for wireless communication, a constant power regulator, a power factor compensator, and the like.

2.3 Circuit design requirements

 (1) The feedback circuit of TOPSwitch-II needs to be equipped with optocoupler and output circuit. When designing a precision switching power supply, a TL431 adjustable precision correlation regulator should be added to form an external error amplifier to replace the voltage regulator in the sampling circuit. The voltage regulation rate Sv and current regulation rate S1 of the precision switching power supply can reach ±0.2%, which is close to the index of the linear integrated regulated power supply.

 (2)  The optocoupler that can change linearly with current transfer ratio (CTR), such as PC817A, NEC2501, 6N137, etc., 4N25, 4N35 and other 4N×× ordinary optocouplers are not recommended. The latter has a poor linearity, which causes distortion when transmitting analog signals and affects the regulation performance of the switching power supply.

î—¤ (3) The primary of the high-frequency transformer must be provided with a protection circuit to absorb the spike voltage caused by the leakage inductance and ensure that the MOSFET is not damaged. This protection circuit should be connected in parallel to the primary. There are four design schemes: 1 consisting of a transient voltage suppression diode (TVS) and an ultra-fast recovery diode (SRD); 2 by TVS and silicon rectifier (VD) The clamp circuit is composed; 3 is an absorption circuit composed of a RC element and an SRD; and 4 is an absorbing circuit composed of a RC member and a VD. In the above scheme, the effect of 1 is the best, and the advantage that the TVS response speed is extremely fast and can withstand high-energy transient pulses can be fully utilized. The program is second.

 (4) Add a suitable heat sink when using the chip. For the TO-220 package, it can be mounted directly on a small dispersion board. For DIP-8 and SMD-8 packages, 4 sources can be soldered to an area of ​​2.3 Instead of the heat sink, the printed board is coated with copper foil.

î—¤ (5) î—¥ In order to suppress the interference introduced from the power grid and prevent the interference generated by the switching power supply from being transmitted to the outside, an EMI filter is added to the power supply line, which is also called a power supply noise filter (PNF). ).

î—¤ (6) When using such chips, the source leads should be as short as possible. In order to stabilize the output voltage at no load or light load, a resistor of several hundred ohms should be connected to the output terminal of the regulated power supply as the minimum load, or a voltage regulator tube can be connected in parallel.