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 (5.1~40V) 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

        (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, shutdown / Automatic restart circuit. 

It uses a high-frequency transformer to completely isolate the output from the grid, making it safe and reliable 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.

         (2) 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 a 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%~67%. The power efficiency is 

about 80%, up to 90%, nearly double the linear integrated regulated power supply. The operating temperature range is 0~70°C, 

and the maximum junction temperature of the chip is 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. 

          (6) 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 a photocoupler and isolated from the 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) Optocouplers that can vary linearly with current transfer ratio (CTR), such as PC817A, NEC2501, and 6N137, 

should be selected. 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. 

           (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 the DIP-8 and SMD-8 packages, four sources can be soldered to the 2.3-pad printed 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, an EMI filter, also known as a power supply noise filter (PNF), 

is added to the power supply line end. 

            (6) When using such a 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 terminal of the regulated 

power supply as the minimum load, or a voltage regulator tube can be connected in parallel.

             3 TinySwitch Series Four-End Switching Power Supply

            TinySwitch is a new high-efficiency, low-power, four-terminal, single-chip switching power supply from Power. 

Because of the small size of the switching power supply, it is called TinySwitch micro switch series. It adds an enabler 

to the three-terminal single-chip switching power supply, making it more convenient and flexible to use. TinySwitch series 

is excellent in price and low price, and the peripheral circuit is very simple. It is especially suitable for making micro-switching 

power supply or standby power supply below 10W. It is an ideal product to replace low-efficiency and large-sized low-power 

linear regulated power supply.

             3.1 Performance Features of TinySwitch 

            (1) TinySwitch is available in DIP-8 and SMD-8 packages and six models. Product classification is shown in Table 2. 

            Although it uses an 8-pin package, there are actually only four legs: 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 has no overshoot when the power is turned on quickly, and the MOSFET has no 

frequency multiplication when the power is turned off. 

            (2) High efficiency, low power output. When the 220V AC power supply is selected, its no-load power consumption is 

less than 60mW. It is suitable for making 0~10W low-power, low-cost switching power supply, which can save 38% energy 

compared with 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), which is faster than normal PWM and has better ripple rejection. 

            (4) Compared with TOPSwitch-II, it is quite unique in circuit design. First, the EMI filter can be omitted from the AC 

input. Second, the primary protection circuit does not need to use TVS, and only the RC circuit can absorb the spike voltage. 

Third, the feedback coil and related circuits are not used, and the circuit compensation component is not added. Fourth, 

the enable detection and logic circuit is added inside the chip.

            

             3.2 TinySwitch Application

            This series of products is suitable for making mobile phone battery constant voltage constant current charger, small 

switching power supply module in IC card payment meter, and standby power supply in high-end household appliances such 

as microcomputer, color TV and camcorder. For example, the large-screen color TVs currently produced have a standby 

function, and after the power is turned off using the remote controller, the standby state is entered. 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, 

so that the power consumption of the whole machine is minimized. The 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, and 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 consisting of 

TNY254P can charge 6V NiMH battery in the range of 85~265V AC input voltage. In addition, TinySwitch is also suitable for making 

adapters for small appliances (such as Walkman), turning 220V AC power into the required DC regulated power supply. This kind 

of adapter not only has no bulky transformer, but also has high efficiency, small size and good voltage regulation performance, 

and can completely replace various plug-type AC/DC converters currently available on the market.

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

            The MC33370 series includes five specifications and 17 models of MC33369~MC33374. Take the TO-220 five-leg 

straight-bend package as an example. The product classification is shown in Table 3. The RDS(ON) in the table indicates the 

drain-source on-resistance, and the ID (PK) represents the drain peak current.

            1 MC33370 performance characteristics

            (1) It adds a power supply (Vcc) and a state controller input (State Control input) to TOPSwitch-II; an undervoltage 

lockout comparator, an external shutdown circuit, and a programmable state controller are added to the chip. Its cost 

performance is better than TOPSwitch-II, and the peripheral circuits are much simpler. 

            (2) Using the programmable state controller and the external mode selection circuit, various control modes 

(including manual control, MCU operation by the microcontroller, digital circuit control, prohibition operation, etc.) can be 

realized to realize the mutual interaction between the working state and the standby state. Switch.

            (3) internally integrates a current-sensing power switch called “Sense FET”, which can detect the drain current ID 

in real time without power loss. protection. 

            (4) When the AC power supply is fixed or the rate of change does not exceed ±20%, the feedback coil of the 

high-frequency transformer and the associated high-frequency filter circuit are allowed to be removed. This helps to further 

simplify peripheral circuits and reduce the cost of switching power supplies. To meet the needs of special applications, soft 

start functions can also be added to the switching power supply.

            (5) High power efficiency. The efficiency of the switching power supply or power module is up to 80%. Static power 

consumption is as low as tens to hundreds of milliwatts in the standby state. 

            (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°C, and the overheat protection temperature is set at 157°C 

(TOPSwitch-II is only 135°C).

            4.2 Application of MC33370

            The MC33370 series can be widely used in office automation equipment, instrumentation, wireless communication 

equipment and consumer electronics to form a high-voltage isolated AC/DC power converter. In special applications, the 

feedback winding of the high-frequency transformer, the fast recovery diode, and the filter capacitor can be removed, and the 

Zener diode or the bipolar transistor or the MOS transistor can be used for series adjustment. In addition, high-voltage 

stepping power supplies can be fabricated using this chip.

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