Application and development of single chip switching power supply

The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, simplest peripheral circuit, best performance index, and can form a high-efficiency isolated switching power supply without power frequency transformer. After it came out in the mid- and late 1990s, it showed strong vitality. At present, 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 formed by it is equivalent to the linear voltage-stabilized power supply of the same power in cost, while the power supply 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 Brief introduction to the development of integrated switching power supplies

The switching power supply is known as a high-efficiency and energy-saving power supply. It represents the development direction of the voltage-stabilized power supply and has now become the mainstream product of the voltage-stabilized power supply. In the past 20 years, the integrated switching power supply has been developing 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 pulse width modulation (PWM) controller integrated circuits. Motorola, Silicon General, Unitrode and other companies in the United States have successively launched a batch of PWM chips, with typical products including MC3520, SG3524 and UC3842. Since the 1990s, foreign countries have developed high-speed PWM and PFM (pulse frequency modulation) chips with a switching frequency of 1MHz, with typical products such as UC1825 and UC1864. The second direction is to achieve monolithic integration of small power switching power supplies. This can be roughly divided into two stages: In the early 1980s, SGS-Thomson first launched 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, it needs to be equipped with an industrial frequency transformer to isolate it from the power grid. It is suitable for making low-voltage output (5.1~40V), high and medium power (below 400W), large current (1.5A~10A), and high efficiency (can exceed 90%) switching power supplies. But in essence, it is still a DC/DC power converter.

In 1994, the American Power Company successfully developed the world's first three-terminal isolated pulse width modulation monolithic switching power supply, which was hailed as the "top switching power supply". Its first generation of products is the TOPSwitch series, and the second generation of products is the TOPSwitch-II series that came out in 1997. In 1998, the company launched the high-efficiency, low-power, low-cost four-terminal monolithic switching power supply TinySwitch series. After that, Motorola launched the MC33370 series of five-terminal monolithic switching power supplies, also known as high-voltage power switching regulators (High Voltage Power Switching Regulators) in 1999. At present, monolithic switching power supplies have formed four major series and nearly 70 types of products.

2. TOPSwitch-11 series three-terminal monolithic switching power supplies

Based on 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 is detailed in Table 1.

Among them, TOP227Y has the highest output power.

2.1 Performance characteristics of TOPSwitch-11

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

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

(3) The input AC voltage range is extremely wide. When used as a fixed voltage input, 220V±15% AC can be selected. If it is equipped with 85~265V wide-range AC, 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%~67%. The power supply efficiency is about 80%, and can reach up to 90%, which is nearly double that of a linear integrated voltage regulator. Its operating temperature range is 0~70℃, and the chip's maximum junction temperature Tjm=135℃.
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(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 by some reason, the optocoupler feedback circuit will make Ic↑→error voltage Vrt→D↓→Vo↓, so that Vo remains unchanged. Vice versa.
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(6) The peripheral circuit is simple and low cost. The external circuit only needs to be connected to the rectifier filter, high-frequency transformer, primary protection circuit, feedback circuit and output circuit. The use of such chips can also reduce the electromagnetic interference generated by the switching power supply.

2.2 Typical Applications

TOPSwitch-II can be widely used in instrumentation, laptop computers, mobile phones, televisions, VCDs and DVDs, camcorders, battery chargers, power amplifiers and other fields, and can form various miniaturized, high-density, low-cost switching power supply modules. 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
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1) The feedback circuit of TOPSwitch-II needs to be equipped with an optocoupler to isolate it from the output circuit. When designing a precision switching power supply, a TL431 type adjustable precision correlated voltage regulator should also 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 Sl of the precision switching power supply can reach about ±0.2%, which is close to the indicators of linear integrated voltage regulators. (2
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Optocouplers with linear current transfer ratio (CTR) should be selected, such as PC817A, NEC2501, 6N137 and other models. It is not recommended to use 4N25, 4N35 and other 4N×× type ordinary optocouplers. The latter has poor linearity and will cause distortion when transmitting analog signals, affecting the voltage regulation performance of the switching power supply. (3) The
primary
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 to the primary. There are four specific design schemes: ① A clamping circuit composed of a transient voltage suppression diode (TVS) and an ultrafast recovery diode (SRD); ② A clamping circuit composed of a TVS and a silicon rectifier (VD); ③ An absorption circuit composed of a resistor and capacitor and SRD; ④ An absorption circuit composed of a resistor and capacitor and VD. Among the above schemes, scheme ① has the best effect and can give full play to the advantages of TVS's extremely fast response speed and ability to withstand high-energy transient pulses. Scheme ② is second.
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(4) When using the chip, a suitable heat sink must be added. For TO-220 packages, it can be directly installed on a small loose board. For DIP-8 and SMD-8 packages, the four source electrodes can be soldered on a copper foil of a printed circuit board with an area of ​​2.3 instead of a heat sink.
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(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 (EMIfilter) at the power supply input end, also known as 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 when no load or light load is applied, a resistor of several hundred ohms should be connected to the output end of the voltage regulator 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. It has an additional enable terminal compared to the three-terminal monolithic switching power supply, making it more convenient and flexible to use. The TinySwitch series has excellent performance and low price, and the peripheral circuit is very simple. It is particularly 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 voltage regulators. 3.1 Performance characteristics of TinySwitch ( 1) TinySwitch has two packaging forms, DIP-8 and SMD-8, and 6 models. See Table 2 for product classification.

Although it uses an 8-pin package, it actually has 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 from the outside, and the output voltage will not overshoot during fast power-on, and the MOSFET will not have frequency doubling during power-off.
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(2) High efficiency and low power output. When a 220V AC power supply is selected, its no-load power consumption is less than 60mW. It is suitable for making low-power, low-cost switching power supplies with a range of 0 to 10W, and can save about 38% of power compared to linear regulated power supplies.
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(3) Use an on/off controller instead of PWM to adjust the output voltage. The on/off controller is equivalent to a pulse frequency modulator (PFM), which has a faster adjustment speed than ordinary PWM and a stronger ripple suppression capability.
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(4) Compared with TOPSwitch-II, it has a unique 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, the feedback coil and related circuits are not needed, and no loop compensation components are added; fourth, the chip has added enable detection and logic circuits.

3.2 Application of TinySwitch

This series of products is suitable for making small switching power supply modules in mobile phone battery constant voltage and constant current chargers, IC card payment electricity meters, and standby power supplies in high-end household appliances such as microcomputers, color TVs, and video cameras. For example, the large-screen color TVs currently produced all have a standby function. After the power is turned off using the remote control, it enters the standby state. At this time, the power switch tube of the switching power supply in the color TV is in the off state, and the standby circuit continues to power the CPU, so that the power consumption of the whole machine is reduced to the minimum. 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. Using a piece of TNY255P can form a 5V, 2A standby power supply for 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 within the range of 85-265V AC input voltage. In addition, TinySwitch is also suitable for making adapters for small household appliances (such as Walkman), converting 220V AC power into the required DC regulated power supply. This adapter not only has no bulky transformer, but also has high efficiency, small size, and good voltage stabilization performance, and can completely replace various plug-in AC/DC converters currently on the market.

4MC33370 series five-terminal single-chip switching power supply

MC33370 series includes five specifications and 17 models from MC33369 to MC33374. Taking the TO-220 five-pin straight-bend package as an example, its product classification is shown in Table 3.

The 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) Compared with TOPSwitch-II, it has an additional power supply terminal (Vcc) and a state controller input terminal (StateControlinput); 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
the programmable state controller and the external mode selection circuit, a variety of control methods can be realized (including manual control, operation by the microcontroller MCU, digital circuit control, prohibition of operation, etc.), and the switching between the working state and the standby state can be realized. (3) A current sensing power switch tube called "SenseFET" is integrated inside, which can detect the size of the drain current ID in real time without power loss and perform overcurrent protection. ( 4) When
the AC power supply is fixed or the change rate does not exceed ±20%, the feedback coil of the high-frequency transformer and the related high-frequency filter circuit can be removed. This helps to further simplify the peripheral circuit and reduce the cost of the switching power supply. 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 the switching power supply or power module formed by it can reach more than 80%. In the standby state, the static power consumption 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 the pulse width modulation gain is -14%/mA. The operating junction temperature of the chip is -40~150℃, and the overheat protection temperature is set to 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 electronic products to form a high-voltage isolated AC/DC power converter. When making special applications, the feedback winding, fast recovery diode and filter capacitor of the high-frequency transformer can be removed, and a voltage regulator or bipolar transistor or MOS tube can be used for series adjustment. In addition, this chip can also be used to make a high-voltage stepper power supply.