Application of TOPSwitch-Ⅱ in voltage-stabilized power supply

Abstract: TOPSwitch-II device is a new type of integrated chip that combines PWM/MOSFET. Using it to make high-frequency switching power supply can not only simplify the circuit, but also improve the electromagnetic compatibility performance of the power supply and reduce the production cost. This article introduces an experimental voltage-stabilized power supply made of TOP225Y. The circuit uses PWM technology and linear voltage-stabilization technology, with novel design and certain practical value.

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

Application about TOPSwitch－ II Device in Stabile valtage Power Supply for experiment

Abstract: TOPSwitch Ⅱ device is a new type IC merging PWM and MOSFET into one.The high frequency switching power supply mode from it can not only simplify the circuit,but also can improve the EMC characteristics of power supply and reduce the cost of productioncosts. This paper introduces a kind of stabile voltage supply for experiment made of TOP225Y. The circuit used PWM technique and linear stabile voltage technique.Its design is original,and also has some practical value.

Keywods: TOPSwitch, Single ended backward topology, PWM,

EMC Chinese Library Classification Number: TN86 Document Identification Code: A Article Number: 02192713(2000)1155303

1 Introduction

When conducting circuit and single-chip microcomputer experiments, experimental voltage stabilizer is one of the necessary equipment. For this type of voltage stabilizer, the power capacity is not required to be very large, but there must be multiple sets of output voltages, among which it is best to have a set of output voltages that can be continuously adjusted to meet the needs of different occasions. There are many types of experimental voltage stabilizers on the market, but basically they all use power frequency linear voltage stabilization. Its advantages are simplicity, easy manufacturing, and low price, and its disadvantages are bulky and low efficiency. The use of high-frequency switching power supply is an effective way to overcome the above shortcomings, but the general high-frequency switching power supply circuit is complex, with more components and the production cost will also increase, so it is still rare to use high-frequency switching technology in experimental voltage stabilizer. However, the high-frequency switching power supply made of new devices such as TOPSwitch can make the circuit as simple as the power frequency linear voltage stabilizer, with excellent performance and much higher efficiency than the latter, and the cost will not increase significantly.

Figure 1 Internal structure of TOPSwitch-Ⅱ series devices

TOPSwitch device is a new type of high-frequency switching power supply chip launched by POWER Integrations Inc in the United States in the mid-1990s. It is the abbreviation of three-terminal off-line PWM switch. Its feature is that the PWM controller and MOSFET power switch tube in the high-frequency switching power supply are integrated on the same chip, which is 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 have a variety of packaging forms. They are packaged in DIP-8 and SMD-8. The middle 4 pins are empty. They can be connected to the copper foil of the printed circuit board to directly transfer the heat generated by the chip to the printed circuit board. There is no need to set up a heat sink, which saves costs. The TO220 package has only 3 pins, which is as convenient to use as a high-power triode. In addition, because the PWM controller and the MOSFET power switch tube are connected inside the tube shell, the connection line is extremely short, which eliminates the high-frequency radiation phenomenon, improves the electromagnetic compatibility performance of the power supply, and reduces the device's requirements for circuit board layout and input bus transients. TOPSwitch-II is an improved model of TOPSwitch. It increases the maximum power of single voltage input from 100W to 150W, and the electromagnetic compatibility is also enhanced. The device has a higher performance-price ratio. TOPSwitch-II devices include several models such as TOP225-TOP227, and the main difference lies in the output power. The internal circuit and performance of TOPSwitch-II devices are shown in Figure 1 and Table 1 respectively.

Figure 2 Schematic diagram of voltage-stabilized power supply

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

2 Working Principle

The power supply circuit topology is single-ended flyback, 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 noise filter LF. The function of the power 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 the high-frequency switching power supply from entering the power grid and interfering with other electrical appliances. Now many power supply designs do not pay attention to the selection of power supply filters. Some small and medium-power high-frequency switching power supplies often do not add power supply filters. This not only reduces the anti-interference ability of the power supply itself and affects its working stability, but also causes pollution to the public power grid. After rectification and capacitor filtering, the mains power becomes a 308V DC voltage to supply the TOPSwitch-II device. TOPSwitch-II constitutes a DC/DC converter, which converts the input DC high voltage into a high-frequency pulse voltage with adjustable pulse width, and then performs half-wave rectification and filtering after the high-frequency transformer steps down the voltage to become the required DC voltage output. R1, C1, and VD1 form a buffer absorption circuit to absorb the voltage spike overshoot caused by the transformer leakage inductance during the shutdown process of the power device. The operating frequency of the circuit is 100kHz, and the oscillation element has been solidified inside the device. The secondary of the high-frequency transformer has 4 windings, among which the 5V/3A winding N3 controls the pulse width of the TOPSwitch-II device, that is, this group of output voltages is PWM regulated, and the sampling feedback work is completed by the parallel programmable regulator TL431 and the optocoupler PC and the voltage divider resistors R7 and R8. The reason for choosing this winding for pulse width control is that its output voltage is low and the current is large, which can better reflect the superiority of the switching power supply. In order to realize the isolated power supply for the optocoupler, the transformer has a single auxiliary winding N2. The ±12V/0.5A windings N4 and N5 use integrated three-terminal regulators LM7812 and LM7912 for linear stabilization. Because 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, which uses an adjustable three-terminal voltage regulator LM317 for linear voltage regulation and voltage regulation. In order to overcome the shortcomings of high power consumption and low efficiency of linear voltage regulation at low voltage output, this winding is set with high and low taps. The tap change is automatically switched by the 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 of TL431 is lower than 2.5V, TL431 is cut off, relay K is not energized, and the low-voltage winding is connected. Conversely, when the output voltage is required to be higher than 15V, the voltage at the reference input of TL431 is higher than 2.5V, TL431 is turned on, the relay is energized, and the high-voltage winding is connected. The voltmeter PV indicates the value of the output voltage.

Selection of TOPSwitch-II Device Models

The total output power of this power supply is the sum of the output power 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 of the power supply should be:

Pi=PO/80％=72/0.8=90W

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

Figure 2 Schematic diagram of voltage-stabilized power supply

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 is crucial and the workload is relatively large.

Assuming 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 K=Uimax/Uimin=364/230=1.58

Dmin=Dmax/[(1－Dmax)K＋Dmax]

=0.5/[(1－0.5)×1.58＋0.5]=0.38

Ip=2PO/(Uimin×Dmax)

=2×90/(230×0.5)=1.57A

(1) Transformer primary inductance

Lp=Uimin×Dmax/(Ip×f)

=230×0.5/(1.57×100×103)

=0.73mH

Select PQ32/30 core, LP3 material, AL=1.61, magnetic flux density B=0.39T from Nanjing CONDA Company. The transformer works in single-ended state. The maximum working flux

Bmax=B/2=0.39/2=0.19T

To avoid magnetic saturation, an air gap should be left in the transformer core.

Lg=0.4×π×Lp×Ip2/(AL×B2max)

=0.4×3.14×0.73×10－3×1.572

/(1.61×0.192)

=0.04cm

(2) Transformer primary turns N1

N1=Lp×Ip×104/(AL×Bmax)

=0.73×10－3×1.57×104/1.61×0.19

=38 turns (rounded)

(3) 5V winding turns N3

N3=N1（UO＋UD)(1－Dmax）/(Uimin×Dmax)

=38×(8＋1)(1－0.5)/(230×0.5)

=1 turn (rounded)

(4) ±12V winding turns N4, N5

N4=N5=N1（UO＋UD)(1－Dmax）/(Uimin×Dmax)

=38×(15＋1)×(1－0.5)/(230×0.5)

=3 turns (rounded)

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

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

N6=N1（UO＋UD)(1－Dmax）/(Uimin×Dmax)

=38×(33＋1)(1－0.5)/(230×0.5)

=6 turns (rounded)

Tap at 3 turns

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

High-strength enameled wire is used for winding, and the wire diameter of the enameled wire used in each winding is as follows:

N1 size: 0.71mm

N2, N4, N5 for 0.42mm

N3 size: 0.99mm

N6 size: 0.72mm

The output filter inductor coil L1 of the 5V winding is a 4cm magnetic ring, and is wound with 15 turns of 1.0mm enameled wire.

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 a large number of interrelated design variables must be considered, which is laborious and time-consuming. In order to reduce the burden on designers, the American Power Integrations Corporation has designed a set of EXCEL spreadsheets for TOPSwitch devices, which can be run on PCs. Using spreadsheets to design power supplies is simple and quick, and generally 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 the paper and will not repeat it here.