Design of switching power supply based on TopswitchⅡ switch chip

introduction

There are many types of switching power supplies, and the design methods are also complex and varied. Therefore, it is necessary to study a simple method to quickly design the required general-purpose, high-efficiency, and low-cost switching power supply.

1 Working principle of switching power supply

The switching DC regulated power supply is formed based on the average value of the square wave voltage being proportional to its duty cycle and the integral characteristics of the inductor and capacitor circuits. Its basic working principle is to first rectify the input AC voltage to form a pulsating DC voltage, transform it through the DC-DC conversion circuit, and then form a high-frequency AC with different pulse widths through the chopper circuit, and then rectify and filter it to output the required voltage and current waveform. If the output voltage waveform deviates from the required value, a current or voltage sampling circuit will perform sampling feedback, and after comparing the parameters with the voltage value of the comparison circuit, the difference signal will be amplified to control the pulse frequency f and duty cycle D of the switching circuit, thereby controlling the conduction state of the output end. Therefore, the output end can obtain the required voltage and current values.

As shown in Figure 1, the switching power supply module is divided into the following parts.

According to the actual needs of the power system, the corresponding switching power supply products can be designed by analyzing each part.

Figure 1: Block diagram of a switching power supply.

2 Introduction to TopswitchⅡ

TOPSwitchⅡ is a highly integrated special chip for switching power supply produced by POWER. It integrates the power switch tube and its control circuit into one chip, and has functions such as automatic reset, overheat protection and overcurrent protection. Its functional schematic diagram is shown in Figure 2. When the system is powered on, the D pin becomes high potential, the internal current source starts to work and the on-chip switch is in position 0. TOPSwitch charges the capacitor C5 (see Figure 2) connected in parallel to the C pin. When the voltage at the C5 terminal reaches 5.7 V, the automatic restart circuit is turned off and the on-chip switch jumps to position 1. On the one hand, C5 provides power for the internal control circuit of TOPSwitch to start the error amplifier, and on the other hand, it provides a feedback current to control the duty cycle of the switch tube. The drive signal of the MOSFET switch tube is generated by the internal oscillation circuit, protection circuit and error amplifier circuit. The higher the voltage across C5, the smaller the duty cycle of the MOSFET switch tube drive pulse.

3. Selection of TOPSwitch Chip

When designing a switching power supply, the first thing to face is how to choose a suitable switching power supply control chip. When selecting a chip, it is necessary to meet the requirements and not waste resources due to the selection. The following introduces the relationship curve between the power loss (PD) and power efficiency (η) and output power (Po) of the TopswitchⅡ series switching power supply to quickly select the chip model, thereby completing the design of a universal switching power supply with a wide input range.

Figure 2 Internal schematic diagram of TOPSwitch chip

3.1 PD,η, Po relationship curve

The wide range input AC voltage is 85~265V. Under this condition, the PD,η,Po relationship curve of TOP221~TOP227 series single-chip switching power supply is as follows, see Figure 3 and Figure 4.

Figure 3 PD, η, Po relationship curve when wide range input and output is 5 V.

Figure 4 PD,η, Po relationship curve for wide input range and 12 V output.

Note that it is assumed here that the minimum AC input voltage umin = 85 V, and the maximum input voltage umax = 265 V. The horizontal axis represents the output power, and the 15 dotted lines are the contour lines of chip power consumption.

First, determine the applicable curve. For example, when u= 85~ 265 V, Uo= + 5 V, Figure 3 should be selected; when u= 220 V (i.e. 230 V-230 V× 4.3%), Uo= + 12 V, Figure 4 should be selected; then find the power output point P o to be designed on the horizontal axis; move vertically upward from the output power point until you know which curve the appropriate chip is. If it is not applicable, you can continue to look up for another solid line; then read the power consumption PD of the chip from the isoline (dashed line), and then you can also find the junction temperature (Tj) of the chip to determine the size of the heat sink.

For example, when designing a universal switching power supply with an output of 5 V and 30 W, you should choose Figure 3. This is because the input AC voltage range of the universal switching power supply is 85~265 V. First, find the output power point of Po = 30 W on the horizontal axis, then move vertically upward and intersect with the solid line of TOP224 at a point. From the vertical axis, find out that η = 71.2% of this point, and finally find PD = 2.5 W from the contour line passing through this point. This shows that if you choose TOP224, you can output 30 W of power, and the expected power efficiency is 71.2%, and the chip power consumption is 2.5 W. If you think the indicator efficiency is low, you can continue to check the solid line of TOP225. Similarly, if you choose TOP225, you can also output 30 W of power, and the expected power efficiency can be increased to 75%, and the chip power consumption can be reduced by 1.7 W. Then, according to the obtained PD value, you can also complete the heat sink design.

3.2 Correction of equivalent output power

The PD, η, Po relationship curves all limit the minimum value of the AC input voltage, umin = 85 V. If the minimum value of the AC input voltage does not meet the above requirements, it will directly affect the correct selection of the chip. At this time, the power P'o corresponding to the actual AC input voltage u? min minimum value must be converted into the equivalent power Po when umin is the specified value before the above figure can be used. The power correction method is as follows: Select the characteristic curve to be used, and then find the conversion factor K based on the known u'min value; convert P'o into the equivalent power Po when umin is the specified value, and express the formula P o=P'o / K; then select the appropriate relationship curve from Figures 3 and 4.

Fig. 5 Relationship between K and u'min for wide input range.

For example, when designing a 12 V, 35 W general switching power supply, it is known that umin = 90% × 115 V = 103.5 V. From Figure 5, we can find out that K = 1.15. Substitute P 'o = 3.5 W, K = 1.15 into P o = P 'o / K, and calculate Po = 30.4 W; and then according to the value of Po, from Figure 4, we can find out that the best model to choose is the T OP224 chip, at which time η = 81.6%, PD = 2 W. If T OP223 is selected, η drops to 73.5%, and PD increases to 5 W, which is obviously not suitable. If T OP225 is selected, it will cause a waste of resources, because it is more expensive than TOP224 and is suitable for outputting a larger power of 40~ 60 W.

4 Calculation of main component parameters

4.1 Transformer ratio design

The transformation ratio of the switching transformer is related to the specific form of the switching conversion circuit. The transformation ratio formula of the switching transformer in the forward and half-bridge conversion circuits is:

Where, Uin and Uout are the input and output voltages of the switching transformer respectively; Nin and Nout are the turns of the primary and secondary coils of the switching transformer respectively.

When the input voltage is the lowest, the lowest input voltage should be substituted into the calculation during actual design.

The relationship between the output voltage and input voltage of the push-pull circuit is:

Uout= 2DUin/n

Therefore, we get the relationship: n = 2D Uin / Uout = N 1 / N 2.

When the input voltage is the lowest, the duty cycle D is the largest. At this time, the output voltage required by the design can still be maintained. Therefore, D in the above formula should take the maximum value and Uin should take the minimum value.

4.2 Selection of input filter capacitor

The capacity of the input filter capacitor C is closely related to the power supply efficiency and output power. For a switching power supply with a wide input range, when the capacity of C is in μF, it can be selected according to the proportional coefficient 3μF/W. For example, when Po=30 W, C=(3μF/W)×30 W=90μF, and so on. When the input is fixed, the proportional coefficient becomes 1μF/W, and C in the above example becomes 30μF. When designing a switching power supply, it is also necessary to pay attention to the small capacity error of C to avoid affecting the performance of the switching power supply. When the capacity of C is too small, the available power of TopswitchⅡ will be reduced. If 30μF is changed to 20μF, the output power will be reduced by 15%; when C<20μF, it will cause a significant decrease in available power.

In addition, the size of C capacity also determines the value of DC high voltage Ui. Figures 3 and 4 are actually drawn under the condition of Ui = 105 V, which fully reflects the influence of C on Ui.

4.3 Switching Tube Protection Circuit

On the drain D side of the switch chip, two diodes, VDZ and VD, can be used to clamp the peak voltage generated by the leakage inductance of the high-frequency transformer, which can protect the DS inter-electrode of μ from breakdown. For example, VDZ can use the transient voltage suppressor P6K200, whose reverse breakdown voltage is 200 V. VD uses the UF4005 ultra-fast recovery diode with a reverse withstand voltage of 600 V, also known as a blocking diode.

5 Application Circuit and Simulation

Figure 6 shows the schematic diagram of the flyback power supply composed of TOPSwitch. Its working process is as follows: The input AC power is rectified by the rectifier bridge BR1 and then filtered by the capacitor C1 to become a pulsating DC power.

The flyback transformer and TOPSwitch transfer the energy stored in capacitor C1 to the load. When the TOPswitch switch is turned on, the voltage across capacitor C1 is added to the primary side of the flyback transformer, and the current flowing through the primary winding increases linearly (if the transformer secondary current is not zero at the moment the MOSFET switch is turned on, the secondary induced potential is reversed, the diode D2 is cut off, and the secondary current becomes zero. However, the energy in the magnetic core cannot change suddenly, so the primary current jumps to 1/K of the secondary current, K is the transformer ratio), and the transformer stores energy; when the MOSFET switch is turned off, the primary current of the inductor suddenly changes to zero due to the lack of a loop (at this time, the breakdown voltage of the voltage regulator VR1 is cut off due to being higher than the induced potential of the original transformer), and the transformer continues to flow through the secondary side, and the secondary current is K times the primary current when the TOPswitch switch is turned off. The secondary winding charges capacitor C2 through diode D2, and thereafter, the current flowing through the secondary side of the transformer decreases linearly. Diode D1 and voltage regulator VR1 are connected in parallel to the primary side of the transformer to absorb the high voltage spikes generated by the leakage inductance of the primary side of the transformer. Resistor R1, voltage regulator VR2, optocoupler U2 and capacitor C5 form a voltage feedback circuit to ensure the output voltage is stable. Resistor R2 and VR2 form a dummy load to ensure the output voltage is stable when the power supply is unloaded or lightly loaded. Inductor L1 and capacitor C3 form an LC filter to prevent excessive output voltage pulsation. Diode D3 and capacitor C4 form a rectifier circuit to provide the bias voltage of the optocoupler U2 phototransistor. Inductor L2, capacitors C6 and C7 are used to reduce the electromagnetic interference (EMI) of the system.

Figure 6 Application schematic of flyback power supply.

FIG7 shows the output voltage waveforms when the input voltage is 220 V (AC) and the output power is 40 W; the input voltage is 85 V (AC) and the output power is 24 W; and the input voltage is 85 V (AC) and the output power is 40 W.

Figure 7 Voltage simulation output waveform under different voltage input conditions

6 Conclusion

Finally, the design process of the power supply was verified through simulation tests. The results show that the switching power supply designed based on the topswitch chip has a relatively stable output waveform, good electromagnetic compatibility, strong anti-interference ability, and is suitable for the design and manufacture of low-power switching power supplies. DC regulated power supply is an important part of modern power electronic systems, and a good DC power supply system is an important guarantee for high-quality modern electronic systems.