Application of Multisim in pulse width modulation high brightness LED drive circuit simulation

The component library of the simulation software Multisim 8 has thousands of circuit components and modules available for experiment selection. However, with the rapid development of electronic technology, new products from component manufacturers continue to emerge, and the original Multisim component library cannot meet the needs of users. This requires editing Spice models for new components. If you use the simulation models of the corresponding devices that the component manufacturers have published on their websites, you can quickly create new components in the Multisim 8 software. Maxim (Maxim) of the United States provides Spice models of a large number of components such as analog switches and multiplexers, comparators, current detection/instrumentation amplifiers, high-frequency/fiber-optic communication products, LED Drivers, operational amplifiers, power supplies and battery management on its website http://www.maxim-ic.com.cn/tools/Spice. The following takes Maxim's high-voltage, 3-channel, high-brightness LED driver MAXl6823 as an example to explain how to use Multisim simulation software to create a new component MAXl6823 in its component library and simulate and analyze the circuit.

l Creation of new MAXl6823 components
Go to the website of Maxim Corporation of the United States, open the macro model file MAXl6823.LIB of the LED driver MAXl6823, and save it as a text file format (*.txt), such as MAXl6823.txt, and you will get the Spice model file of MAXl6823. Go to the main menu of Multi-sire 8, select Tools, /Component wizard, and the component editing dialog box will appear. It takes a total of 8 steps to create a new component. In the sixth step, select the "Load from File" button in the simulation model dialog box in the component editor and load the MAXl6823.txt file; in the seventh step, map the component schematic symbol pins with the component simulation model pins, so that the predetermined circuit function can be realized during simulation, and finally store the edited components in the "User" library. For detailed usage of the component editor, please refer to the "Help" in the main menu of Multisim 8.

2 Simulation Analysis of Pulse Width Modulation High-Brightness LED Driving Circuit
2.1 High-Brightness
LED High-brightness LED refers to a diode with a luminous intensity of several hundred to several thousand med (millicandela). Generally, the maximum operating current of a low-power LED is only 30-70 mA. The luminous intensity of an LED is proportional to its operating current IF. If the maximum operating current is exceeded, it will over-current and burn out.
The following LED products, such as NS-SWl00DT (white), NSSRl00BT (red), NSSGl00BT (green) and NSSBl00BT (blue), produced by Nichia Chemical Industry Co., Ltd. (website http://www.nichia.corn,/cn/product/index.html), are all surface mounted type LEDs (SMD Type LEDs). Their main performance is shown in Table 1.

MAXl6823 is very suitable for automotive applications that require high voltage input and can withstand load dump voltages up to 45 V. Its main application areas are: architectural, industrial and environmental lighting, automotive combination tail lights (RCL) and fog lights, instrument panel indicators, LCD backlighting, automotive emergency warning lights, photobioreactors and crossing warning lights, etc.
2.3 Simulation analysis of pulse width modulation high-brightness LED driver circuit
When using MuItisim 8 for simulation, select the required device from the component library and drag it to the workspace, and drag the pins to connect. Figure 2 is a pulse width modulation LED driver circuit composed of a 555 timer and a MAXl6823 driver. The left half of Figure 2 is a square wave generator circuit with adjustable output pulse width (i.e., adjustable duty cycle) composed of a 555 timer; the right half is an LED driver circuit composed of an LED driver MAXl6823.

2.3.1 Duty cycle adjustable square wave generator composed of 555 timer
Build the circuit according to Figure 2. LMC555CH and R4, R5, RP, D1, D2, C4 form an astable multivibrator. By adjusting RP, the third foot outputs a duty cycle adjustable square wave signal. D1 and D2 make the charging and discharging currents of capacitor C4 flow through different paths. The charging current only flows through R4 and RP left, and the discharging current only flows through R5 and RP right. Therefore, the charging time T1 and discharging time T2 of capacitor C4 are: T1=0.693(R4+RP left)C4, T2=0.693(R5+RP right)C4, square wave period: T=T1+T2=0.693(R4+R5+RP)C4=0.693(6.8+6.8+100)×103×0. 033×10-5=2．6×10-3 s, which has nothing to do with the adjustment of RP. The square wave frequency f=1／T=385 Hz. By adjusting Rp, the duty cycle of the output square wave signal can be changed D=T1／T=(R4+Rpleft)／(R4+R5+RP).

2.3.2 Simulation Analysis of Pulse Width Modulation High Brightness LED Driving Circuit
In Figure 2, R1, R2, and R3 are the detection resistors of the three output channels. MAXl6823 uses detection resistors to set the output current of each channel. The detection resistor required for the expected value of the output current can be calculated using the following formula: Rcs=Vcs/IOUT
Where: Vcs is 203 mV; IOUT=ILED (average current); ILED is related to the duty cycle D of PWM, ILED=IFpeakD (IFpeak is the pulse current amplitude). If D=100%, ILED=IF=20 mA, it can be calculated that R1=R2=R3=Rcs=203 mV/20 mA=10.15 Ω. Set the parameters of LED1, LED4, and LED7 to the same as those of NSSBl00BT (blue); set the parameters of LED2, LED5, and LED8 to the same as those of NSSRl00BT (red); and set the parameters of LED3, LED6, and LED9 to the same as those of NSSG100BT (green). Press the simulation switch of Multisim 8, and the red, green, and blue LEDs on the right side of Figure 2 will light up, go out, light up again, and go out again. The length of their on and off time varies with the duty cycle of the square wave signal output from the third pin of LMC555CH. By pressing the A key or Shift+A key on the keyboard, the size of RP can be adjusted to change the duty cycle of the output square wave signal. Use Multisim 8 simulation oscilloscope to detect the pulse signal waveform at any output end of MAXl6823, and you can see that its peak voltage is 9.203 V, that is, VFB+VFR+VFG+Vcs=3.6+1.9+3.5+0. 203 = 9.203 V, which is consistent with the set value. This shows that the circuit function of the new MAX16823 component is correct.

3 Conclusion
From the above examples, it can be seen that Multisim 8 simulation software can be used to easily create new components and various design circuits, and to simulate and analyze circuit performance indicators by changing component parameters to meet design needs.