Application of TOPSwitchⅡ in regulated power supply

    Abstract: TOPSwitch-II device is a new PWM/MOSFET two-in-one integrated chip. Using it to make high-frequency switching power supplies not only simplifies the circuit, but also improves the electromagnetic compatibility performance of the power supply and reduces production costs. This article introduces a Experimental regulated power supply made with TOP225Y. The circuit uses PWM technology and linear voltage stabilization technology, and its design is novel and has certain practical value.

    Keywords: TOPSwitch device, single-ended flyback topology, PWM circuit, electromagnetic compatibility

1 Introduction

    When conducting circuit and microcontroller experiments, the experimental stabilized power supply is one of the necessary equipment. For this type of stabilized power supply, the power capacity is not required to be large, but there must be multiple sets of output voltages. It is best to have one set of output voltages that can Continuously adjustable to suit the needs of different occasions. There are many types of experimental stabilized power supplies on the market, but they basically adopt the power frequency linear voltage stabilization method. Its advantages are simplicity, ease of manufacture, and low price, while its disadvantages are large, bulky, and low efficiency. The use of high-frequency switching power supplies is an effective way to overcome the above shortcomings. However, the general high-frequency switching power supply circuit is complex, has many components, and the production cost will also increase. Therefore, high-frequency switching technology is currently used in experimental regulated power supplies. Still rare. However, for high-frequency switching power supplies made with new devices such as TOPSwitch, the circuit can be as simple as a power frequency linear regulated power supply. The performance is very superior, but the efficiency is much higher than the latter, and the cost will not increase significantly.

    TOPSwitch device is a new high-frequency switching power supply chip launched by the American power integration company (POWER Integrations Inc) in the mid-1990s. It is the English abbreviation of three-terminal off-line PWM switch (ThreeterminalofflinePWMSwitch). Its characteristic is that it integrates the PWM controller and MOSFET power switch tube in the high-frequency switching power supply on the same chip, making it a two-in-one device. This greatly simplifies the power supply circuit, improves reliability, and makes the design of the power supply simpler and faster. TOPSwitch devices come in a variety of packaging forms. They are packaged in DIP-8 and SMD-8. The four middle pins are empty. They can be connected to the copper foil of the printed circuit board to transfer the heat generated by the chip directly to the printed circuit board. There is no need to install a separate radiator, which saves costs. It is packaged in TO220 and has only 3 pins, making it as convenient to use as a high-power triode. In addition, since the PWM controller and MOSFET power switch tube are connected in the tube shell, the connection is extremely short, which eliminates the high-frequency radiation phenomenon, improves the electromagnetic compatibility performance of the power supply, and reduces the impact of the device on the circuit board layout and input bus transient requirements. TOPSwitch-II is an improved model of TOPSwitch. It increases the maximum power from 100W to 150W at single voltage input. The electromagnetic compatibility is also enhanced. The device has a higher performance-price ratio. TOPSwitch-II devices include several models such as TOP225-TOP227. The main difference lies in the output power. The internal circuit and performance of the TOPSwitchII device are shown in Figure 1 and Table 1 respectively.

Table 1 TOPSwitch-Ⅱ product classification and maximum output power POM

2 Working principle

    The power supply circuit topology is single-ended flyback type, and the circuit principle is shown in Figure 2. The 220V mains power is directly rectified by the bridge rectifier after passing through the power supply noise filter LF. On the one hand, the function of the power supply filter is to filter out the clutter voltage transmitted from the power grid and purify the input power supply. On the other hand, it also prevents the oscillation voltage of high-frequency switching power supply from entering the power grid and interfering with other electrical appliances. Nowadays, many power supply designs do not pay attention to the selection of power filters. Some small and medium-power high-frequency switching power supplies often do not add power filters. This not only reduces the anti-interference ability of the power supply itself and affects its working stability, but also causes damage to the public power grid. of pollution. After the mains power is rectified and capacitor filtered, it becomes a 308V DC voltage and is supplied to the TOPSwitch-II device. TOPSwitch-II forms a DC/DC converter, which converts the input DC high voltage into a high-frequency pulse voltage with adjustable pulse width. After the high-frequency transformer steps down, it performs half-wave rectification and filtering to become the required DC voltage output. R1, C1, and VD1 form a buffer and absorption circuit to absorb the voltage spike overshoot caused by the leakage inductance of the transformer during the shutdown process of the power device. The operating frequency of the circuit is 100kHz. The oscillation element has been solidified inside the device. The secondary of the high-frequency transformer There are 4 windings, of which the 5V/3A winding N3 controls the pulse width of the TOPSwitch-II device, that is, the output voltage of this group is PWM regulated, which is composed of the parallel programmable voltage regulator TL431, the photocoupler PC and the voltage dividing resistor R7 , R8 completes the sampling feedback work. The reason why this winding is selected for pulse width control is that its output voltage is low and the current is large, which can better reflect the advantages of switching power supply. In order to realize the isolated power supply to the optocoupler, the transformer is equipped with an auxiliary winding N2. The ±12V/0.5A windings N4 and N5 use integrated three-terminal voltage regulators LM7812 and LM7912 for linear voltage regulation. Since the output current is not large, the power consumption is low, and the heat dissipation problem is easy to solve. There is also an adjustable winding N6 of 1.25V~30V/1.5A in the secondary. An adjustable three-terminal voltage regulator LM317 is used for linear voltage stabilization and voltage regulation. In order to overcome the large power consumption of linear voltage stabilization at low voltage output, Due to the disadvantage of low efficiency, this winding is equipped with two taps, high and low. The tap change is automatically switched by a relay. The switching voltage is set at 15V and sampled by TL431. When the output voltage is required to be lower than 15V, the voltage at the reference input terminal of TL431 is lower than 2.5V, TL431 is cut off, relay K does not pull in, and the low-voltage winding is turned on. On the contrary, when the output voltage is required to be higher than 15V, the voltage at the reference input terminal of TL431 is higher than 2.5V, TL431 is turned on, the relay is closed, and the high-voltage winding is turned on. The voltmeter PV indicates the value of the output voltage.

    TOPSwitch-II device model selection

    The total output power of this power supply is the sum of the output powers of each group:

    PO=5×3+12×2×0.5+30×1.5=72W

    If the total efficiency of the power supply is 80%, the total power input by the power supply should be:

    Pi=PO/80%=72/0.8=90W

    As can be seen from Table 1, under the condition of a single voltage of 220V, the maximum output power of TOP225Y is 100W, which can meet the requirements of this circuit.

3 Design of high frequency transformer

    When making a single-ended flyback high-frequency switching power supply, the design and calculation of the high-frequency transformer are crucial, and the workload is also relatively large.

    Assume that the AC input voltage range is 180V ~ 260V, the rectifier voltage drop and output voltage ripple are 22V, and the maximum duty cycle is Dmax=0.5, then

    U imax = root 2×260 = 364V (no load)

    U imin = root No. 2×18-22=230V (no load)

    K=U imax =U imax /U imin =364/230=1.58

    D min =D max /[(1-D max )K＋D max ]

    =0.5/[(1-0.5)×1.58+0.5]=0.38

    Ip=2PO/(U imin ×D max )

    =2×90/(230×0.5)=1.57A

    (1) Transformer primary inductance

    Lp=U imin ×D max /(Ip×f)

    =230×0.5/(1.57×100×103)

    =0.73mH

    Choose Nanjing CONDA Company's PQ32/30 magnetic core, LP3 material, AL=1.61, magnetic flux density B=0.39T, the transformer works in single-ended state, and the maximum operating magnetic flux

    B max =B/2=0.39/2=0.19T

    In order to avoid magnetic saturation, the transformer core must have an air gap whose length

    =0.4×3.14×0.73×10－3×1.572

    /(1.61×0.192)

    =0.04cm

    (2)Number of primary turns of transformer N1

    N 1 =Lp×Ip×104/(AL×B max )

    =0.73×10－3×1.57×104/1.61×0.19

    =38 turns (rounded)

    (3) 5V winding turns N3

    N 3 =N 1 (U O + U D ) (1-D max )/(U imin ×D max )

    =38×(8+1)(1-0.5)/(230×0.5)

    =1 turn (rounded)

    (4) ±12V winding turns N4, N5

    N 4 =N 5 =N 1 (U O + U D ) (1-D max )/(U imin ×D max )

    =38×(15+1)×(1-0.5)/(230×0.5)

    =3 turns (rounded)

    (5) Number of feedback winding turns N2=N4=N5

    (6) 1.25V~30V adjustable winding turns N6

    N 6 =N 1 (U O + U D ) (1-D max )/(U imin ×D max )

    =38×(33+1)(1-0.5)/(230×0.5)

    =6 turns (rounded)

    Tap at turn 3

    In the above formulas for calculating the number of turns, UO is the output voltage (take rated output voltage + regulator voltage drop 3V), UD is the fast recovery diode voltage drop, take 1V.

    It is wound with high-strength enameled wire. The diameter of the enameled wire used in each winding is as follows:

    N1 uses φ0.71mm

    N2, N4, N5 use φ0.42mm

    N3 uses φ0.99mm

    N6 uses φ0.72mm

    For the output filter inductor coil L1 of the 5V winding, select a φ4cm magnetic ring, and use 1.0mm enameled wire to wind 15 turns.

    The high and low voltage switching relay uses JZC22F, DC12V/10A, the voltmeter model is 85C1, and the dial scale is 0~30V.

    The calculation process of high-frequency transformers is very cumbersome and requires a large number of interrelated design variables to be considered, which is labor-intensive and time-consuming. In order to reduce the burden on designers, American Power Integration Company has specially designed a set of EXCEL spreadsheets for TOPSwitch devices that can be run on a PC. Using a spreadsheet to design a power supply is simple and quick, and usually only takes about 10 minutes to complete. At the 13th Annual Meeting of the China Power Supply Society in 1999, there was a paper that specifically introduced EXCEL spreadsheets. Readers can refer to that article and will not go into details here.