Design and analysis of green switching power supply based on LD7552B

1LD7552B Introduction

1.1 Internal structure

LD7552B is a power control integrated block with green working mode launched by Leadtrend Technology Co., Ltd. It is responsible for generating switching pulses, and can also complete voltage regulation control and various protections. The internal structure of LD7552B is shown in Figure 1. It has good anti-static function, current mode control function, noise-free green mode control function and multiple protection functions (such as overvoltage protection, overload protection, etc.). The integrated block has low starting current (less than 20μA) and low power consumption (less than 0.4W). It can be used to design a 30-60W green switching power supply.

It is worth mentioning that LD7552B and LD7552D are sister chips, and the difference between them is reflected in the 4-pin external. The 4-pin external of LD7552B is generally connected to a 100kΩ resistor, while the 4-pin external of LD7552D is generally connected to a 0.047μF capacitor. This should be noted when replacing. Both LD7552B and LD7552D can drive a variety of switch field effect tubes, such as 2SK2630, 2SK2645, 2SK2649, etc., and can be selected according to actual conditions when applied.

1.2 Packaging

Both LD7552B and LD7552D adopt DIP-8 package, and their appearance and dimensions are shown in Figure 2.

2 Circuit Design

The switching power supply designed using LD7552B is shown in Figure 3. The power supply can output 5V and 14V DC voltages, with an output power of 40W. The output current of the 5V power supply can reach 2A, and the output current of the 14V power supply can reach 2.2A.

2.1 AC input and rectification and filtering circuit

This part of the circuit is mainly composed of power switch S1, fuse F1, negative temperature coefficient thermistor NR1, mutual inductance filter L1, bridge rectifier BD1, filter capacitors C2 and C3, etc. Its function is to complete the transmission and rectification and filtering of AC voltage, and finally obtain a DC voltage of about 300V on C2.

Because the power supply is large, the startup surge current is also large. The fuse F1 uses a 2A/250V specification; a mutual inductance filter is used to filter out interference in the power grid, and L1 can use a mutual inductance filter with an inductance of 4mH; the bridge stack uses a rectifier current of 2A and a withstand voltage of more than 500V (such as 2KBP06~2KBP10 series); C2 uses an electrolytic capacitor of 100~150μF and a withstand voltage of more than 400V. In order to improve the high-frequency filtering effect, a high-voltage ceramic capacitor C3 can be connected in parallel to C2. The purpose of connecting a negative temperature coefficient thermistor NR1 in series is to reduce the startup surge current. The normal temperature resistance value of NR1 can be selected between 6~9Ω/4A.

2.2 LD7552B's 3-pin and 7-pin external circuits

Pins 3 and 7 are connected to the external startup circuit and power supply circuit, whose task is to provide startup voltage for the chip at the moment of startup and to provide power supply voltage for the chip during normal operation. When the circuit starts, the current required by pin 3 of LD7552B is less than 20μA, which belongs to the micro-current startup mode, so the total resistance of the startup resistors R1, R2, and R3 can be selected to be around 1000kΩ. After the circuit works, the current required by pin 7 reaches 2mA. At this time, R1, R2, and R3 cannot provide such a large current, so the power supply circuit composed of the winding of the switching transformer, R8, D1 and other components is used to power pin 7 to meet the requirements of the internal circuit. The capacity of C6 is generally 10~47μF, D1 must use a fast recovery diode with a withstand voltage of more than 200V and a rectifier current of more than 0.8A (such as FR103~FR106, etc.), and the resistance of R8 is generally less than 10Ω. The values ​​of R1, R2, R3, and C6 cannot be too large, otherwise it will cause difficulty in starting the circuit or even fail to start. The normal operating voltage of pin 7 should be designed between 12 and 16V.

2.3 LD7552B's 4-pin external resistor

The normal oscillation frequency of LD7552B is set by the external resistor at pin 4 and can be calculated by the following formula.

f=(65/R4)×100

Where: f represents the normal oscillation frequency of LD7552B, in kHz; R4 represents the resistance value of the external resistor at pin 4, in kΩ. For example, when the external resistor at pin 4 is 100 kΩ, the oscillation frequency is 65 kHz. When selecting the resistance value of the external resistor at pin 4, be sure to ensure that f is between 50 and 130 kHz, that is, the value range of R4 is between 50 and 130 kΩ.

2.4 LD7552B 6-pin external circuit

Pin 6 is used to detect the current of the switch tube to achieve overcurrent protection. An RC filter (composed of R5 and C8) needs to be connected to the outside of pin 6. First, it can prevent the pulse front from damaging the internal circuit of pin 6, and second, it can avoid circuit misprotection. The time constant of the RC filter should not be too large, as long as the pulse front has a delay of 350ns. The switch tube source resistance R7 has a great influence on the output power and overcurrent protection sensitivity of the circuit. The larger R7 is, the lower the overcurrent protection point is, and the smaller the output power of the circuit is; if R7 is smaller, the overcurrent protection point is higher, and the output power of the circuit is greater. Therefore, when selecting the resistance value of R7, the above two aspects must be taken into account. Experiments show that when the switch tube source resistance is selected as 0.47Ω/2W, the circuit has a load capacity greater than 40W, and the overcurrent protection point is about 2A. In order to leave some room for the design, R7 can be selected as 0.43Ω/2W.

2.5 Voltage stabilization circuit

The precision three-terminal comparator IC3, the photocoupler IC2 and the three sampling resistors (R14, R16 and R15) are all key components in the voltage stabilization circuit. The precision three-terminal comparator can be KIA431A or TL431A, and the photocoupler can be PC123 or PC817. The output voltage is closely related to the resistance of the three sampling resistors (R14, R16 and R15). When the output voltage is 5V and 14V, according to Kirchhoff's law, it is easy to deduce that there is the following relationship between R14, R16 and R15.

Where: R15, R14 and R16 represent the resistance values ​​of resistors R15, R14 and R16 respectively. If R14 and R16 are 3.6kΩ and 33kΩ respectively, the resistance value of R15 can be calculated to be 2.4kΩ. In the experiment, it was found that when R15 is 2.43kΩ, the output voltage of 5V and 14V is most accurate. It is worth noting that R14, R15 and R16 must be precision resistors with an error within 1%, otherwise it will affect the voltage regulation accuracy and increase the error of the output voltage.

2.6 Switching tube and anti-peak absorption circuit

The switch tube VT1 should use a field effect switch tube with UDSS ≥ 600V, IDM ≥ 6A, PDM ≥ 50W, and RDS < 1.5Ω, such as 2SK2630, 2SK2645, 2SK2649, 2SK2677, 2SK2761, etc.

At the moment when the switch tube is turned off, the primary winding of the switch transformer will generate a reverse peak voltage. In order to prevent the reverse peak voltage from breaking through the switch tube, a reverse peak absorption circuit must be connected in parallel to the primary winding of the switch transformer, namely R9, C5 and D3. D3 must use a high-voltage fast recovery tube, such as RF107, RGF10M, etc. The withstand voltage of C5 must be above 1.5kV and the capacity must be 1~2nF; the power of R9 must be above 2W and the resistance must be about 100kΩ.

2.7 DC output circuit

This power supply outputs two DC voltages of 5V and 14V, both of which can provide more than 2A current to the load. For 14V rectification, dual diodes with a withstand voltage of more than 100V and an average rectification current of more than 6A are generally used for rectification, such as FCH10U10, FCH10A15, SP10100, etc. The total capacity of the filter capacitor should be more than 1000μF, and it is best to use LC filtering. For 5V rectification, fast recovery diodes with a withstand voltage of more than 50V and an average rectification current of more than 3A are generally used for rectification, such as 31DQ06 series, 31DQ09 series, etc. The total capacity of the filter capacitor should be more than 2000μF, and it is best to use LC filtering.

3 Circuit Working Principle

3.1 Startup process

The 220V AC mains is rectified by BD1 and filtered by C2 to obtain a DC voltage of about 300V. This voltage is sent to pin 3 of IC1 through the starting resistors R1, R2, and R3, and the external capacitor (C6 and C4) of pin 7 is charged through the internal circuit, so that the voltage of pin 7 rises. When the voltage of pin 7 reaches 16V, the internal circuit starts and outputs a switch pulse from pin 8, and the switch tube VT1 enters the switch working state. After the circuit works, the pulse voltage on the windings 1 to 3 of the switching transformer is limited by R8, rectified by D1, and filtered by C6 and C4 to obtain a DC voltage of about 12V and provide it to pin 7 as the power supply voltage of IC1. LD755-2B has a green working mode. When running under light load (such as standby), the internal green mode controller works and the oscillation frequency becomes about 20kHz. At this time, the power supply works in the green mode and its power consumption is only about 0.4W. When the load increases, the chip switches to the normal working mode. At this time, the oscillation frequency is not controlled by the green mode controller and the frequency increases to 65kHz. The mode conversion is achieved by the internal circuit detecting the voltage of pin 2. When the voltage of pin 2 is lower than 2.35V, the circuit works in green mode.

3.2 Voltage output process of each channel

The switching transformer has two windings with center taps, which are connected in parallel. Their upper ends are grounded, and their lower ends are connected in parallel as the output of the 14V winding, and the center taps are connected in parallel as the output of the 5V winding. After the circuit is working, the pulse voltage output by the 14V winding is rectified by D4 (two parallel diodes), filtered by C13, L2, and C14, and a 14V DC voltage is generated. The pulse voltage output by the 5V winding is rectified by D5, filtered by C10, L3, and C11, and a 5V DC voltage is generated.

3.3 Voltage stabilization process

The power supply realizes voltage regulation control by adjusting the duty cycle of the switch pulse. The main sampling point of voltage regulation is set at the 5V output end, and R14 and R15 sample the 5V voltage; the auxiliary sampling point is set at the 14V output end, and R16 and R15 sample the 14V voltage. When the output voltage of each channel rises due to some reason (such as the grid voltage rises, the load becomes lighter, etc.), the sampling voltage sent to the control pin of IC3 also rises, thereby strengthening the conduction of IC3, the conduction of the light-emitting diode and the phototransistor in IC2 is also enhanced, and the voltage of the 2nd pin of IC1 decreases. After adjustment by the internal circuit, the pulse width of its 8th pin output becomes narrower (the duty cycle is reduced), the saturation time of the switch tube VT1 is shortened, and the output voltage of each channel decreases. When the output voltage of each channel decreases due to some reason, the voltage regulation process is the opposite of the above.

3.4 Protection process

(1) Overcurrent protection. When the current of the switch tube VT1 increases due to some reason (such as excessive load, etc.), the voltage on R7 will increase. This voltage is sent to the 6th pin of IC1 through R5. As long as the voltage of the 6th pin reaches 0.85V and lasts for 350ns, the internal overcurrent protection circuit will be activated, causing the 8th pin to output a low level in advance, and VT1 will be cut off in advance, thereby effectively suppressing the further increase of the current and preventing VT1 from being damaged by overcurrent.

(2) Overload protection. When the load is short-circuited, the 14V and 5V voltages are close to 0V. At this time, IC3 and IC2 are cut off, and the voltage at pin 2 of IC1 will rise. As long as the voltage at pin 2 rises to 5V and lasts for 60ms, the internal circuit will execute overload protection and pin 8 will stop pulse output.

(3) Undervoltage protection. Undervoltage protection is accomplished by the internal circuit of pin 7 of IC1. After power on, C6 is charged. If the voltage on C6 cannot reach 16V, IC1 will not work and will be in undervoltage protection state. If the voltage on C6 can reach 16V, IC1 will work. Once the circuit is working, the voltage on pin 7 only needs to be maintained between 10 and 16V. If for some reason the voltage across C6 drops below 10V, IC1 will stop oscillating immediately, and pin 8 will stop pulse output, entering undervoltage protection state.

(4) Overvoltage protection: When the voltage regulation loop is open, the output voltage of each channel will rise sharply, and the voltage of C6 will also rise. As long as the voltage reaches 28V, the internal circuit of IC1 will immediately perform overvoltage protection and the pulse output of pin 8 will stop.

It is worth noting that when the LD7552B enters the protection state, its state cannot be automatically latched. When the protection conditions are not met, the circuit will automatically release the protection state and work again. After working, if the protection conditions are met again, it will enter the protection state again, and so on. In other words, when the circuit enters the protection state (except for overcurrent protection), the circuit will work intermittently (the so-called "hiccups"). At this time, the output voltage and the voltage of the relevant pins of the LD7552B will fluctuate.

4 key inspection points

The power supply has one eyewitness detection point and two key detection points. When the power supply fails, the fault can be quickly found by detecting these points. Fuse F1 is an eyewitness detection point. By observing or measuring whether F1 is burned out, the nature of the fault can be determined. When F1 is burned out, it means that there is a short circuit fault in the power supply. The short circuit location generally occurs in the rectifier bridge stack (BD1), filter capacitors C2, C3 or switch tube VT1. The voltage on C2 is the first key detection point. By detecting the voltage at this point, the fault location can be determined. For example, when the power supply does not work, if the voltage on C2 is 0V, it means that the fault is in the AC input circuit or the rectifier circuit; if there is a 300V voltage on C2, it means that the fault location is in the circuit after C2. Pin 7 of LD7552B is the second key detection point. When the power supply does not work, the fault location can be narrowed down by measuring the voltage at this point. For example, after power is turned on, the voltage on pin 7 does not reach 16V, which means that the circuit does not work because the starting voltage is too low; if the voltage on pin 7 swings greatly, it means that the circuit enters the protection state.

5 Conclusion

LD7552B is a very perfect new green power chip, with strong voltage regulation function and perfect protection function, so the stability and reliability are very high, and it can automatically change the working mode according to the load size. Practice shows that the switching power supply based on LD7552B can work stably in 90-240V AC environment. When outputting 40W power, the power supply itself loses only about 3W, and the efficiency is more than 90%; in green mode, the power supply itself loses only about 0.4W. LD7552B is the preferred chip for high-power switching power supply design. It will surely be favored by the majority of electronic workers and has broad application prospects.