Development and Application of Single-chip Switching Power Supply

    Abstract: Currently, medium and small power switching power supplies are developing towards monolithic integration. This article elaborates on the performance characteristics and typical applications of various monolithic switching power supplies that have come out in recent years.

    Keywords: Monolithic switching power supply, pulse width modulation, standby power supply, programmable state controller

    The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, simplest peripheral circuits, best performance indicators, and can form a high-efficiency isolated switching power supply without a power frequency transformer. After it came out in the mid-to-late 1990s, it has shown strong vitality. Currently, it has become the preferred integrated circuit for the development of medium and small power switching power supplies, precision switching power supplies and power modules in the world. The switching power supply composed of it is equivalent in cost to a linear regulated power supply of the same power, while the power efficiency is significantly improved, and the volume and weight are greatly reduced. This creates good conditions for the promotion and popularization of new switching power supplies.

    1 Overview of the development of integrated switching power supplies

    Switching power supply is known as high-efficiency and energy-saving power supply. It represents the development direction of voltage-stabilized power supply and has now become the mainstream product of voltage-stabilized power supply. In the past 20 years, integrated switching power supplies have continued to develop in the following two directions. The first direction is to integrate the control circuit, the core unit of the switching power supply. In 1997, foreign countries first developed a pulse width modulation (PWM) controller integrated circuit. Motorola, Silicon General, and Unitrode in the United States successively launched a number of PWM chips. Typical products include MC3520, SG3524, UC3842. Since the 1990s, foreign countries have developed high-speed PWM and PFM (pulse frequency modulation) chips with a switching frequency of 1MHz, typical products such as UC1825 and UC1864. The second direction is centering, and the low-power switching power supply realizes monolithic integration. This can be roughly divided into two stages: In the early 1980s, Italian-French Semiconductor Co., Ltd. (SGS-Thomson) took the lead in launching the L4960 series of monolithic switching regulators. The company launched the L4970A series in the 1990s. Its characteristic is that the pulse width modulator, power output stage, protection circuit, etc. are integrated into one chip. When used, a power frequency transformer is required to isolate it from the power grid. It is suitable for making low-voltage output (5.1~40V), large and medium power (below 400W) ), large 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, the American Power Company was the first in the world to successfully develop a three-terminal isolated pulse width modulation monolithic switching power supply, which was hailed as the "top switching power supply". Its first-generation product is the TOPSwitch series, and its second-generation product is the TOPSwitch-II series that came out in 1997. In 1998, the company launched the TinySwitch series of high-efficiency, low-power, low-price four-terminal single-chip switching power supplies. After that, Motorola launched the MC33370 series of five-terminal monolithic switching power supplies in 1999, also known as High Voltage Power Switching Regulator. At present, single-chip switching power supplies have formed four series and nearly 70 models of products.

    2 TOPSwitch-11 series three-terminal monolithic switching power supply

    According to the packaging 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 details are shown in Table 1 . Among them, the output power of TOP227Y is the largest.

    2.1 Performance characteristics of TOPSwitch-11

 (1) TOPSwitch-II internally includes an oscillator, error amplifier, pulse width modulator, gate circuit, high-voltage power switch tube (MOSFET), bias circuit, overcurrent protection circuit, overheating protection and power-on reset circuit, shutdown Break/auto-restart circuit. It completely isolates the output end from the power grid through a high-frequency transformer, making it safe and reliable to use. It is a current-controlled switching power supply with open-drain output. Due to the use of CMOS circuits, the power consumption of the device is significantly reduced.

 (2) There are only three terminals: control terminal C, source S, and drain D. It is comparable to a three-terminal linear voltage regulator and can constitute a flyback switching power supply without a power frequency transformer in the simplest way. . In order to complete various control, bias and protection functions, C and D are multi-functional terminals, realizing one pin for multiple purposes. Taking the control terminal as an example, it has three functions: ① The voltage VC at this terminal provides bias voltage for the on-chip parallel regulator and gate driver stage; ② The current IC at this terminal can adjust the duty cycle; ③ This terminal also serves as a power branch The connection point with the automatic restart/compensation capacitor determines the frequency of automatic restart through an external bypass capacitor and compensates the control loop.

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

 (4) The typical switching frequency is 100KHz, and the duty cycle adjustment range is 1.7% to 67%. The power efficiency is about 80% and can reach up to 90%, which is nearly twice as high as the linear integrated regulated power supply. Its operating temperature range is 0~70℃, and the maximum junction temperature of the chip is Tjm=135℃.

 (5) The basic working principle of TOPSwitch-II is to use the feedback current IC to adjust the duty cycle D to achieve voltage stabilization. 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↓, keeping Vo unchanged. vice versa.

 (6) The peripheral circuit is simple and low cost. Only the external rectifier filter, high-frequency transformer, primary protection circuit, feedback circuit and output circuit are needed. The use of such chips can also reduce 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 and other fields, and can form a variety of miniaturized, high-density, low-cost switches Power module. In addition, it is also suitable for forming backup switching power supplies, non-isolated switching power supplies, constant current and constant voltage output switching power supplies, DC/DC power converters for wireless communications, constant power regulators, power factor compensators, etc.

    2.3 Circuit design requirements

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

 (2)An optocoupler whose current transfer ratio (CTR) can change linearly should be selected, such as PC817A, NEC2501, 6N137 and other models. 4N×× type ordinary optocouplers such as 4N25 and 4N35 are not recommended. The latter has poor linearity and will cause distortion when transmitting analog signals, affecting the voltage stabilization performance of the switching power supply.

 (3)The primary side of the high-frequency transformer must be equipped with a protection circuit to absorb the peak voltage caused by leakage inductance to ensure that the MOSFET is not damaged. This protection circuit should be connected in parallel on the primary. There are four specific design options: ① A clamping circuit composed of a transient voltage suppression diode (TVS) and an ultra-fast recovery diode (SRD); ② A clamping circuit composed of TVS and a silicon rectifier (VD) A clamping circuit composed of RC elements; ③ an absorption circuit composed of RC elements and SRD; ④ an absorption circuit composed of RC elements and VD. Among the above solutions, ① has the best effect and can give full play to the advantages of TVS with extremely fast response speed and ability to withstand high-energy transient pulses. Plan ② is second.

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

 (5)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, it is necessary to add a first-level electromagnetic interference filter (EMI filter) at the power incoming end. It is also called a power supply noise filter (PNF). ).

 (6)When using this type of chip, the source lead 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 end of the regulated power supply as the minimum load, or a voltage regulator tube can be connected in parallel.

    3 TinySwitch series four-terminal switching power supply

    TinySwitch is a new high-efficiency, low-power four-terminal monolithic switching power supply launched by Power Company. Because the size of the switching power supply is very small, it is called TinySwitchmicro switch series. Compared with the three-terminal single-chip switching power supply, it adds an enabling terminal and is more convenient and flexible to use. The TinySwitch series has excellent performance and low price, and the peripheral circuit is very simple. It is especially suitable for making micro switching power supplies or standby power supplies below 10W. It is an ideal product to replace low-efficiency, large-volume, low-power linear regulated power supplies.

  3.1TinySwitch performance characteristics

 (1) TinySwitch has two packaging forms, DIP-8 and SMD-8, and 6 models. Product classification is shown in Table 2. Although it uses an 8-pin package, there are actually only four pins: S, D, BP (equivalent to the control terminal), and EN (enable terminal), so it is equivalent to a four-terminal device. The enable terminal can be used to turn off the MOSFET externally, and there will be no overshoot in the output voltage during fast power-up and no frequency doubling of the MOSFET during power-down.

 (2)High efficiency, low power output. When choosing 220V AC power supply, its no-load power consumption is less than 60mW. It is suitable for making 0~10W low-power, low-cost switching power supply, and can save about 38% of electricity than a linear regulated power supply.

 (3)Use an on/off controller instead of PWM to adjust the output voltage. The on/off controller can be equivalent to a pulse frequency modulator (PFM), whose adjustment speed is faster than ordinary PWM and has stronger ripple suppression capability.

 (4)Compared with TOPSwitch-II, it is quite distinctive in circuit design. First, the EMI filter can be omitted at the AC input end; second, the primary protection circuit does not need to use TVS, and only the RC circuit can absorb the peak voltage; third, no feedback coil and related circuits are needed, and no loop compensation components are added ; Fourth, enable detection and logic circuits are added inside the chip.

   3.2Application of TinySwitch

    This series of products is suitable for making constant voltage and constant current chargers for mobile phone batteries, small switching power supply modules in IC card payment meters, and standby power supplies in high-end household appliances such as microcomputers, color TVs, and video cameras. For example, all large-screen color TVs currently produced have a standby function. After turning off the power using the remote control, the TV enters the standby state. At this time, the power switch tube of the switching power supply in the color TV is turned off, and the standby circuit continues to supply power to the CPU, thereby minimizing the power consumption of the entire machine. TNY253P can form a 5V, 1.3W color TV standby power supply. It uses the DC high voltage generated by the main power supply of the color TV as the input voltage

, the allowable range is 120~375V (depending on the color TV model), and Vo=+5V. A piece of TNY255P can be used to form a 5V, 2A standby power supply for a PC. The +6.7V, 3.6W mobile phone battery constant voltage and constant current charger composed of TNY254P can charge 6V nickel metal hydride (NIMH) batteries in the AC input voltage range of 85~265V. In addition, TinySwitch is also suitable for making adapters for small home appliances (such as Walkmans), converting 220V AC power into the required DC regulated power supply. This adapter not only does not have a bulky transformer, but also has high efficiency, small size, and good voltage stabilization performance. It can completely replace various plug-type AC/DC converters currently on the market.

    4 MC33370 series five-terminal monolithic switching power supply

      The MC33370 series includes five specifications and 17 models from MC33369 to MC33374. Taking the TO-220 five-pin straight and bent package as an example, its product classification is shown in Table 3. RDS (ON) in the table represents the drain-source on-resistance, and ID (PK) represents the drain peak current.

   4.1 Performance characteristics of MC33370

 (1)It adds a power supply terminal (Vcc) and a state controller input terminal (State Control input) compared to TOPSwitch-II; an undervoltage lockout comparator, an external shutdown circuit and a programmable state controller are added inside the chip. . Its cost performance is better than TOPSwitch-II, and the peripheral circuit is simpler.

 (2)Using programmable state controller and external mode selection circuit, a variety of control methods can be realized (including manual control, operation by microcontroller MCU, digital circuit control, prohibited operation, etc.) to realize working state and standby state switch between each other.

 (3)A current sensing power switch tube called a "Sense FET" is integrated inside, which can be used to detect the size of the drain current ID in real time without power loss. Stream protection.

 (4)When the AC power supply is at a fixed value or the change rate does not exceed ±20%, the feedback coil of the high-frequency transformer and the related high-frequency filter circuit are allowed to be removed. This helps further simplify the peripheral circuit and reduce the cost of the switching power supply. In order to meet the needs of special applications, a soft-start function can also be added to the switching power supply.

       (5)High power efficiency. The efficiency of switching power supply or power module composed of it can reach more than 80%. Static power consumption in standby mode is as low as tens to hundreds of milliwatts.

 (6)The duty cycle adjustment range is wider, up to 0.1~74%. The typical value of pulse width modulation gain is -14%/mA. The working junction temperature of the chip is -40~150℃, and the overheat protection temperature is set at 157℃ (TOPSwitch-II is only 135℃).

    4.2 Application of MC33370

      The MC33370 series can be widely used in office automation equipment, instrumentation, wireless communication equipment and consumer electronics products to form a high-voltage isolated AC/DC power converter. For special applications, the feedback winding, fast recovery diode, and filter capacitor of the high-frequency transformer can also be removed, and a voltage regulator tube, bipolar transistor, or MOS tube can be used instead for series adjustment. In addition, this chip can also be used to create high-voltage stepper power supplies.

    references

    1 Power. Supplemental Data Book and Design Guide,1998

    2Motorola Product Manual, 1999

    3 Sha Zhanyou, new special integrated power supply and its application. Beijing: People's Posts and Telecommunications Press, 1998.3

    4. Sha Zhanyou et al., newly compiled practical digital measurement technology. Beijing: National Defense Industry Press, 1998.1

    (Date of receipt: 1999-11-15)