Design of multi-channel switching power supply based on TOP247Y

Overview

In the switching power supply for three-phase inverter, there are two working modes of the power supply, one is to use the power frequency transformer for power supply, and the other is to use the switching voltage regulator for power supply. With the development of microelectronics and power electronics technology, they all use switching power supplies without exception. The switching power supply has the advantages of light weight, small size, high efficiency, and wide voltage regulation range. It is developing in the direction of short, small, light, thin, monolithic integration, and intelligence. The monolithic switching power supply integrated chip TOP247Y developed by PowerIntegrations of the United States in early 2001 belongs to the company's fourth-generation monolithic switching power supply integrated circuit TOPSwitch-GX series. In addition to all the advantages of the TOPSwitch-FX series, this series of products also expands the maximum output power from 75W to 250W, which is suitable for forming high-efficiency, isolated switching power supplies with large and medium power. Its switching frequency is as high as 132kHz, which helps to reduce the volume of high-frequency transformers and the entire switching power supply. This article introduces a multi-channel switching voltage regulator based on TOP247Y, which has a simple structure, low cost, and convenient production and debugging, and can basically meet the required conditions.

Working principle of TOPSwitch-GX series chips

Figure 1 shows the internal structure of the TOP247Y chip. There are 6 pins in total, which are the control terminal C, the line detection terminal L, the limit current setting terminal X, the source terminal S, the switching frequency selection terminal F and the drain terminal.

The line detection terminal (L) can realize four functions: overvoltage (OV) protection; undervoltage (UV) protection; voltage feedforward (used to reduce the maximum duty cycle when the grid voltage is too low); remote on/off (ON/OFF) and synchronization. The limit current setting terminal can be used to set the limit current of the chip from the outside. In each switching cycle, the drain peak current ID (PK) on the drain-source on-resistance Ros (on) of the power MOSFET is detected. When ID (PK)> ILIMIT, the overcurrent comparator outputs a high level, which passes through the trigger, the main control gate and the driver stage in turn, and turns off the MOSFET to play an overcurrent protection role.

When the power supply starts, the internal high-voltage current source connected between the drain and the source charges the control electrode, generating a voltage drop across RE. After RC filtering, it is input to the in-phase terminal of the PWM comparator, compared with the sawtooth voltage generated by the oscillator, and a pulse width modulation signal is generated to drive the MOSFET tube. Therefore, the circuit can be soft-started through the charging process of the capacitor connected to the control electrode. When the control electrode voltage Uc reaches 5.8V, the internal high-voltage current source is turned off, and the feedback control current supplies power to Uc. In the normal working stage, the external circuit forms a voltage negative feedback control loop to adjust the duty cycle of the output stage MOSFET to achieve voltage regulation. When the output voltage increases, Uc increases, and the error voltage on the sampling resistor RE also increases. After comparison with the sawtooth wave, the duty cycle of the output voltage will be reduced, thereby reducing the voltage of the switching power supply. When the control electrode voltage is lower than 4.8V, the MOSFET tube is turned off, the control circuit is in a low current waiting state, the internal high-voltage current source is reconnected and charges Uc, and its shutdown/automatic reset hysteresis comparator can keep Uc between 4.8 and 5.8V. When the load of the switching power supply is very light, it can automatically reduce the switching frequency from 132kHz to 30kHz (from 66kHz to 15kHz in half-frequency mode), which can reduce switching losses and further improve power supply efficiency.

Design of switching power supply with multiple outputs

The schematic diagram of the multi-channel switching power supply composed of TOP247Y is shown in Figure 2, which outputs three 200mA, 15V DC, one 400mA, 15V DC, and 1A, 5V DC. The multi-channel power supply uses a high-frequency transformer to obtain multiple groups of voltage outputs, and obtains multiple DC power supplies after fast recovery diodes and capacitor filtering.

When the power input is AC 85-265V, the AC voltage U passes through the electromagnetic interference (EMI) filter (C1, L1) and the input rectifier filter (KBL406G, C2) to obtain the DC high voltage UI. UI is connected to the L terminal through R1, which can reduce the limiting current as UI increases. It uses C3, VD type drain clamping diode P6KE200A and blocking diode D1 to replace the more expensive TVS (transient voltage suppressor) to absorb the peak voltage generated by the leakage inductance of the high-frequency transformer when TOP247Y is turned off, and protect the drain. The secondary voltage is rectified and filtered to obtain multiple outputs. Among them, the 15V power output uses a fast recovery diode, and the diodes used for other outputs are Schottky diodes, the purpose of which is to reduce the loss of the rectifier tube.

Schematic diagram of multi-way switching power supply composed of TOP247Y

The power supply uses three chips, including TOP247Y (U1), optocoupler LTV817A, and adjustable precision parallel voltage regulator LM431. In order to reduce the volume of the high-frequency transformer and enhance the degree of magnetic field coupling, the secondary winding adopts a stacked winding method. The voltage regulation principle is U=UR4+UZ+ULM431. When U changes, such as when U increases, the current flowing through the optocoupler increases, and the current output by the optocoupler increases. The current flowing through the control end of TOP247Y increases, and the duty cycle decreases, so that U decreases, thus achieving the purpose of voltage regulation. Conversely, the same principle applies when U decreases.

The internal reference voltage of the adjustable precision voltage regulator LM431 is 2.495V. The output voltage is divided by the potentiometer and R7, and the adjustable voltage is between 2.5V (reference value) and 37V (maximum value). R6 and C18 form the frequency compensation network of LM431. C19 is a soft start capacitor. Except for the 5V voltage, the other outputs are not fed back, and the output voltage is determined by the turns ratio of the high-frequency transformer. R9~R12 is a dummy load for the 15V output, which can reduce the no-load and light-load voltage of the circuit.

In addition, in order to reduce electromagnetic interference as much as possible, a common mode choke is connected to the input side of the switching power supply to significantly improve electromagnetic noise. The safety capacitor C6 can filter out the common mode interference generated by the coupling capacitors of the primary and secondary windings, and the capacitor C1 can filter out the series mode interference between the power grid lines.

Design of high frequency transformer

The switching power supply is a DC power supply with multiple outputs, which is obtained by rectifying and filtering N secondary windings of a high-frequency transformer. Therefore, the performance of the switching power supply is largely determined by the design of the transformer.

● Power calculation

The secondary winding of the high-frequency transformer has three 15V DC outputs that are exactly the same. The current of another 15V voltage is 400mA. The 5V voltage is provided to other chips, plus the feedback winding. Therefore, according to the above setting conditions, the output power of the high-frequency transformer is:

P0=15×0.2×3+15×1×0.4+5×1＝20W

Considering the feedback winding and margin, the actual power selected is 25W.

● Selection of magnetic core

According to the relationship between output power and core size given in reference [3], this switching power supply uses the EI-33 core, whose rated output frequencies are 25kHz, 50kHz and 100kHz. One of them can be selected. The effective cross-sectional area of ​​the core is Ae=119.3, Le=67.6, Ve=8067.4.

● Calculation of winding turns

According to the characteristics of the switching power supply design, when determining the operating frequency of the power switching element MOS tube, if the operating frequency is low

, the noise is large; if the operating frequency is high, the switching loss will increase, but the transformer, capacitor, etc. can be miniaturized. Therefore, a compromise should be considered when determining the switching frequency. Set the operating frequency to 25kHz, ρ＝0.5, η＝0.94. ρ is the duty cycle of PWM modulation, and η is the efficiency of the transformer.

Then the primary inductance Lp is:

Where P0 is the output power, η is the efficiency of the transformer, Z is the loss distribution factor (usually set Z=0.5), and fs is the switching frequency. Transformer primary winding current:

Transformer turns ratio:

Transformer secondary winding turns:

In practice, N=9 (turns). Number of turns of primary winding:

Number of winding turns on the 5V DC output side:

Precautions for using TOP247Y

● The negative pole of the input filter capacitor C2 is directly connected to the feedback winding so that the surge current on the feedback winding can be directly returned to the input filter capacitor to improve the ability to suppress surge interference.

● The capacitor near the control terminal of TOP247Y should be as close to the source and control terminals as possible. The S pole is connected to the C, L, and X terminals through an independent branch and cannot share a branch.

● The distance between the leads of the S, L, and X terminals and the peripheral related components should also be as short as possible, and the branches away from the drain should prevent noise coupling.

● The line detection resistor R1 should be as close to the L pin as possible.

Conclusion

In summary, the low-power switching power supply designed with TOPSwitch-GX series chips has a simple circuit structure, high efficiency and low cost. The output voltage regulation rate and load regulation rate are both within the set range through experimental measurement. The experimental results prove that the switching power supply is reliable.