Isolated flyback DC based on LT3573

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Based on LT3573 isolated flyback dc

Abstract: at present, the application of switching power supply is becoming more and more popular, and it is developing in the direction of high efficiency, energy saving, safety and environmental protection, miniaturization and portability. Based on LT3573, this paper introduces its structural characteristics, designs an isolated flyback DC-DC converter circuit, and carries out simulation experiments to verify the rationality and reliability of its application circuit design. Description: at present, the switching power supply applications become increasingly popular, is moving in high efficiency, energy saving, security, environmental protection, small size and light Based on the LT3573, introduction of its structural features, and a flyback isolated DC-DC converter circuit is designed, simulation experiments are carried out to verify the reasonable and reliability of the application circuit design. Keyword:switch mode power supply, flyback, isolated, lt3573

1 introduction

since the advent of single-chip switching power supply in 1994, conditions have been created for the promotion and popularization of switching power supply. The application of switching power supply involves various electronic and electrical equipment fields, such as program-controlled switch, communication, electronic detection equipment power supply, control equipment power supply, etc. switching power supply has been widely used, which has promoted the rapid development of switching power supply technology. A variety of new technologies, new processes and new devices have sprung up, making the application of switching power supply more and more popular. High frequency switching power supply is the direction of its development. From the initial 20kHz to the present hundreds of kHz or even several megahertz, high frequency brings the miniaturization of switching power supply. At present, switching power supply is developing towards high efficiency, energy saving, safety and environmental protection, miniaturization and portability

2 introduction to LT3573

lt3573 is a Single-chip Switching Regulator designed for isolated counterattack topology. In the isolated flyback topology, the primary circuit of the transformer needs to sense the change of the secondary output voltage at all times in order to maintain the stability of the output voltage. In the previous circuit topology, photoelectric coupling devices or additional transformer windings are often used to obtain the output voltage feedback information. The problems of optocoupler devices are: ① consumption of output power; ② As the cost increases, the circuit structure becomes complex; ③ Limited dynamic response, device nonlinearity, aging, etc., will bring trouble

in addition, if a new transformer or transformer winding is added, the physical size of the transformer will become larger, the cost will increase, and its dynamic response will not be very good. The LT3573 does not need an external optocoupler or the third winding. Its unique built-in flyback error amplifier starts to sample the output voltage information when the secondary winding current is zero. It directly detects the change information of the output voltage from the flyback voltage waveform on the primary side of the transformer, and automatically maintains the stability of the output voltage. This is also the highlight of this IC design. Flyback at present, some lead rods of electronic metal tensile testing machines in the market use T-shaped ordinary lead rod voltage to convert into current due to the action of RFB and Q2. Almost all of this current flows through the resistor rref to form a feedback voltage, which enters the flyback error amplifier and is compared with its 1.22V reference voltage, so that the follow-up circuit can adjust the duty cycle of the switch to achieve the purpose of stabilizing the output voltage, As shown in Figure 1, the company is a super large building profile enterprise in Southwest China

a 1.25A, 60V NPN power switch and all control logic units are integrated into a 16 pin MSOP encapsulated LT3573. It greatly simplifies the peripheral circuit design of the IC application. The maximum output power can reach 7W in the input voltage range of 3v~40v. It can be used in many fields requiring isolated power supply, such as industry, medical treatment, data communication, automotive applications, low-power Poe and VoIP interfaces

lt3573 operates in boundary mode. Compared with equivalent continuous conduction mode design, boundary mode operation allows the use of smaller transformers

Figure 1 LT3573 internal topology block diagram

3 clamp circuit design

transformer leakage inductance LSL (either primary or secondary side), as shown in Figure 2, will cause a voltage spike on the primary side. When the output switch is turned off, the peak becomes sharper and sharper with the higher load current, so it is necessary to select the energy absorption network to consume the energy stored in the leakage inductance. In most cases, a buffer circuit is needed to avoid over-voltage breakdown of the output switch node. Therefore, transformer leakage induction shall be minimized

select the peak voltage of the flyback switch clamped by the absorption network. Due to the voltage spike caused by the leakage inductance of the switching transformer, the flyback voltage can be calculated by the following formula:


where: the forward voltage drop of rectifier diode D2 at the secondary side of VF transformer

isec transformer secondary side current

esr total impedance of secondary circuit

nps transformer effective primary and secondary side turns ratio

vout output voltage

the sum of this voltage and input voltage (VIN +vflbk) is directly added to the collector of power switch Q1, which is easy to cause secondary breakdown of power switch Q1 and damage. In order to clamp the peak value of the voltage spike within the rated value of the switch, the RCD absorption circuit is most commonly used, so that the energy stored in the leakage inductance is transferred to the absorption capacitance C1 during the switching off of the switch, and finally consumed on the resistance R1, as shown in Figure 2

the switching speed of clamp diode D1 here should be fast enough, otherwise, the leakage inductance peak cannot be transmitted to capacitor C1 in time at the moment of switching off of the switch tube, and an instantaneous high voltage will be generated at the collector of switch tube Q1, as shown in Figure 3. Therefore, Schottky diode is usually the best choice

once the clamping diode D1 is turned on, the leakage inductance current will be absorbed by C1, and the absorption time shall not be longer than 150ns, as shown in Figure 3, TSP 150ns, otherwise, the stability of the output voltage will be disturbed

the absorption resistance R1 affects the voltage peak amplitude VC and the duration TSP. The TSP time should not be too long, otherwise the detection output voltage will be distorted, as shown in the figure

Figure 2 RCD absorption circuit diagram Figure 3 switch Q1 collector voltage waveform diagram

4 switch transformer design

4.1 primary minimum inductance

because the LT3573 adjusts the isolation output voltage by detecting the primary flyback pulse wave of the switch transformer. When the secondary winding conducts current, the LT3573 obtains the output voltage information from the SW pin. At this time, the sampling circuit needs a minimum time to sample the output voltage. In order to ensure sufficient sampling time, the primary side needs to maintain a minimum inductance value lpri. The inductance value is calculated as follows:


, where:

toff (min) = 350ns, the minimum switching off time of the switch tube

imin = 250mA, the minimum current limit of the converter

4.2 transformer turn ratio

generally speaking, the transformer turn ratio is selected to maximize the output power and enable the converter to have sufficient current processing capacity, but the turn ratio should not be too large. For low output voltage (3.3V or 5V), use the number of turns on the primary side times the number of turns on the secondary side (n:1) to maximize the current gain (and output power) of the transformer. At this time, the voltage of the SW pin is equal to the sum of the maximum input power supply voltage plus the output voltage multiplied by the number of turns. This value must be kept below the VSW (max) value of the SW pin to prevent the internal power switch from collapsing. Based on these conditions, for a specific application, it is necessary to set an upper limit value n of turn ratio and select a transformer with sufficiently low turn ratio


where: VSW (max) is the maximum voltage stress of the switch tube. According to the circuit simulation, the switching voltage stress and output current capacity under different transformer turn ratio are obtained as shown in Table 1

Table 1 switch voltage stress and output current capacity to turns ratio 5 actual application circuit and simulation analysis

simulation experiment circuit is shown in Figure 4. 12V input voltage is adopted, the winding turns ratio of the original and secondary sides of the switch transformer is set to 3:1, and the rref pin is externally connected to the ground reference resistance, which is generally set to 6.04k. This resistance value cannot deviate too much from 6.04k. Generally, a few percent change is acceptable, otherwise, It will cause large output voltage error. RFB is the input pin of the external feedback resistor, where the resistance value is set to 80.6k. This pin is connected to the primary side (VSW) of the transformer. The ratio of this resistance to rref resistance determines the output voltage (plus the effect of any non integrated transformer turns ratio). During flyback, the average current through this resistor is about 200 a. The relationship between RFB and rref can also be determined by the following formula:


where: ic/ie ratio of switch Q1, typical value is 0.986

VTC 0.55V;

vbg internal bandgap reference voltage


tc pin is internally connected with a positive temperature coefficient current source to the rref pin, and the output voltage temperature compensation resistance is externally connected to the almost blank pin in most places. The generated current is proportional to the absolute temperature, and the resistance value is set to 28.7k during simulation. Rilim maximum current limit adjustment pin, with a 10k resistor, the LT3573 can reach the maximum operating current capacity/UVLO is the off/undervoltage lockout pin, and the resistance voltage divider connected to VIN is fixed on this pin to provide the lowest input voltage for LT3573 operation. When the voltage is lower than about 0.7V, there is almost no quiescent current in the internal circuit. When 0.7V and 1.25V, this part will have 10 a current, but the internal circuit will continue to close. When 1.25V, the internal circuit will open and 10 a current will be input to the SS pin

Figure 4 example of practical application circuit

the waveform of each key potential of circuit simulation is shown in Figure 5. It can be verified from the waveform that the boundary mode makes the secondary side current return to zero every cycle, so that the voltage drop of parasitic resistance will not lead to load stability error. The circuit can stably output 5V DC voltage, 0.5A DC current, and the rated power can reach 2.5W. When the input voltage is 12V, the maximum voltage stress of switch Q1 is about 28V, which meets the predetermined design goal

Figure 5 waveform of key points in circuit simulation

6 conclusion

the highlight of this circuit design is that it does not use optoelectronic coupling devices, or transformers, or transformer windings, but relies on detecting the voltage waveform of the switch tube collector to stabilize the output voltage, simplifying the peripheral circuit, which not only avoids the additional power loss of the circuit, but also increases the reliability of the circuit


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