The hottest non isolated step-down LED driver for

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A non isolated step-down LED driver for general lighting

with the improvement of people's living standards, energy consumption is increasing, and energy conservation and environmental protection have become an important topic of global sustainable development. According to the statistics of the world energy organization, the power consumption of general lighting accounts for 19% of the total power consumption in the world, and the ratio is increasing. The best way to solve this power demand and ensure energy conservation and environmental protection is to improve energy efficiency. LED has the characteristics of high energy efficiency (>120 lumens/watt), long life (more than 50000 hours of working life), environmental protection (no radiation, no mercury) and so on. With the decline of LED production cost, the day for led to enter general lighting is approaching

the driving mode of LED is different from that of traditional halogen lamp and fluorescent lamp. It needs to maintain constant current driving, so it needs special driving circuit. As general lighting, because most of them are high-voltage mains input and the output is limited by SELV (safety extra low voltage), buck topology is often used because of its simple structure, high efficiency and small current ripple. Pt4207 is an LED driver chip designed based on buck topology

pt4207 chip structure features

pt4207 adopts an innovative architecture, which can ensure that the system can work reliably under the DC voltage of 8V to 450V after AC input rectification. The built-in 350ma/20v MOSFET can provide 350mA LED output current. In addition, it is equipped with an external MOSFET switch drive port, which can realize the LED output current of up to 1a and work stably. The system efficiency can reach 96%, and the current accuracy can reach ± 5% (including the input voltage adjustment rate and component differences). Through the multifunctional dimming pin, the LED current can be adjusted linearly by using resistance or DC voltage, or PWM dimming can be selected by using digital pulse signal. In addition, the chip also has soft start function, load short-circuit protection and over temperature protection functions. The internal structure block diagram of pt4207 is shown in Figure 1

Figure 1 pt4207 internal structure block diagram

constant current working principle: pt4207 uses fixed off time mode to control output current. After the internal MOSFET is turned on, the current flows through the load, inductance, MOSFET and sampling resistance, and rises linearly with time, generating a voltage at the CS pin. When the voltage reaches the internal reference value, the internal control circuit of the chip turns off the MOSFET and enters the turn off cycle. The off time is fixed through the external resistance. After that, the MOSFET will reopen and enter the next working cycle. The way of Buck structure is shown in Figure 2

Figure 2 two forms of Buck structure

in the MOSFET off cycle, the energy in the inductance L is released into the load led through the freewheeling diode D to form a loop, as shown in Figure 3

Figure 3 buck structure turn off cycle current circuit

can be obtained from the inductance formula

, where VL is the voltage at both ends of the inductance, l is the inductance, and toff is the settable fixed turn off time, Δ Il is the amount of current that changes in the inductance

Figure 4 inductance current waveform under CCM

if the system works in CCM (continuous working mode), the current waveform in the inductance is shown in Figure 4. Where iled is the average current of LED, ipeak is the peak current in the inductance, that is, the peak current through MOSFET or freewheeling diode, and iled=ipeak-0.5 is obtained* Δ Il, substituting into the inductance formula, it is concluded that

ipeak can be set through the sampling resistance. Therefore, once the output LED scheme is determined, the output current is independent of the input voltage, so as to achieve constant current control

short circuit protection principle: the chip detects the CS pin voltage in each opening cycle. Once it detects that the CS voltage rises too fast, the chip will turn off m Jinan experimental machine factory, notify osfet of the holiday, and turn it on again after a period of time, so as to realize short circuit protection

over temperature protection principle: the chip has built-in over temperature protection function. When the chip junction temperature exceeds 135 ℃, the output current will be automatically reduced to prevent the temperature from rising further. If the temperature exceeds 150 ℃, the output current will be reduced to 0, which can effectively protect the chip and avoid flickering at the same time. If you need to protect the LED from over temperature, you can indirectly negative temperature coefficient thermistors on the dim pin and GND pin. When the ambient temperature rises, the dim voltage will drop, and at the same time, the reference voltage of the internal CS pin will be reduced or even turned off, so as to realize the over temperature protection function

soft start function: the chip has a built-in soft start time of 4ms. When starting, gradually increase the current, so that the load current gradually reaches the set value, and effectively reduce the starting surge current

Figure 5 pt4207 typical application circuit (output: 24 string LED array, 250mA) (print)

Figure 6 efficiency and constant current characteristics of pt4207 typical application circuit

Figure 7 pt4207 high current application (output 12 string LED array, 1000mA)

Figure 5 is pt4207 typical application circuit. The efficiency and constant current characteristics of pt4207 typical application circuit are shown in Figure 6. Other application schemes of pt4207 are shown in Figure 7 and figure 8. Among them, Figure 7 shows the high current application of pt4207 (output 12 string LED array, 1000mA); Figure 8 shows pt4207 DC low voltage application (output 1 3wled, 700mA)

figure 8 pt4207 DC low voltage application (output 1 3wled, 700mA)

system parameter design

refer to figure 5 for typical application circuit. Determination of output current: the appropriate R4, R5, R6 and l can be selected according to the formula

. Refer to the pt4207 data sheet for specific calculation steps

input capacitance selection: the input capacitance provides a stable power supply voltage for the system, which can be selected according to the output power and the capacitance according to uf/w. Lighting applications are in high temperature environment, so the temperature resistance of capacitor is recommended to be above 105 ℃

mosfet selection: the drain source withstand voltage VDS is selected according to the actual input situation, and the drain current ID is more than 4 times of iled

output capacitance selection: the capacitance in parallel with the LED can absorb the LED ripple current. Ideally, the inductance ripple current is less than that of the domestic 3-yuan cathode material enterprises, and the technology is still far behind. All of them are absorbed by the output capacitance. To a certain extent, the industry is the carrier to solve the employment problem, and the long service life of LED. Usually select UF

freewheeling diode selection: Schottky diode or ultra fast recovery diode is recommended. The reverse recovery time TR is recommended to be less than 100ns, and the current capacity should be greater than ipeak

inductance selection: I-shaped inductance or transformer inductance with closed magnetic circuit can be selected. I-type inductors are generally cheap and simple, but the magnetic circuit is open, which is easy to cause the loss of magnetic lines of force in the metal enclosed space and make the system work abnormally, so they are generally used in lamps with non-metallic shells. No matter which kind of inductor is used, the saturation current of the inductor is required to be greater than 1.2 times of iled, and the Curie temperature of the core material is recommended to be greater than 150 ℃

layout design points

refer to figure 5 typical application circuit. Among them, filter capacitors C3, C4, C5, and resistor R4 should be as close to the chip pin as possible. Input capacitance C1, load, inductance L4, MOSFET, chip s pin, sampling resistance R5, do you know how to install and operate the concrete pressure tester? R6 is a high current path, the wiring should be as thick and short as possible, and the enclosed area should be as small as possible. The sampling resistors R5 and R6 are connected to the high-frequency and high current ground, which belongs to the interference source and should be connected to the negative pole of the input filter capacitor C1 through the shortest path. The third pin of the chip and the ground of C3, C4, C5, R4 need stable reference ground, which can be led out from C1 alone

summary of this paper

the above introduces the basic structure, working principle, typical application scheme and design considerations of pt4207 chip. With its wide voltage input range, high constant current accuracy and reliability, the chip has a wide application prospect in general lighting such as fluorescent lamps, spotlights, downlights, street lamps, automotive equipment and other fields. (end)

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