Design of a new LED shadowless lamp control system based on LM3402

Shadowless lamps are an essential and important device used to illuminate surgical sites. They are required to be able to optimally observe objects of different depths, sizes, and low contrast in the incision and body cavity. Therefore, in addition to being "shadowless", they also require uniform illumination and good light quality, and the ability to distinguish the color difference between blood and other tissues and organs in the human body. In addition, shadowless lamps must be able to work continuously for a long time without emitting excessive heat, because overheating can cause discomfort to the surgeon and dry out the tissues in the surgical area [1].

At present, surgical lamps generally use circular energy-saving lamps or halogen lamps, but with the continuous development of light-emitting diode LED technology, especially the development of high-brightness white light LED, LED shadowless lamps have completely solved the inherent defects of circular energy-saving lamps and are the upgraded products of circular energy-saving lamps. At present, the advantages of LED shadowless lamps are gradually understood and accepted by microscope users, and the cost of use is also lower than that of circular energy-saving lamps, reducing the trouble of replacing circular lamp tubes every 1 to 2 months.

At present, the technology has successfully solved the problems of high-power LED packaging, constant current drive, heat dissipation and illumination control, and can achieve multi-level regulation of the illumination of shadowless lamps, with flexible and convenient dimming. The advantages of LED shadowless lamps are [1-2]:

(1) Excellent cold light effect: The new LED cold light source is used as surgical lighting. It is a true cold light source, and there is almost no temperature rise in the doctor's head and wound area.

(2) Good light quality: White light LED has color characteristics that are different from those of ordinary surgical shadowless lamps. It can increase the color difference between blood and other tissues and organs of the human body, making the doctor's vision clearer during surgery. The various tissues and organs of the human body can be more easily distinguished in the flowing and infiltrating blood. This is something that ordinary surgical shadowless lamps do not have.

(3) Stepless brightness adjustment: The brightness of the LED is steplessly adjusted using digital methods. The operator can adjust the brightness at will according to his or her own adaptability to brightness to achieve the most ideal comfort level, so that the eyes are not easily fatigued after working for a long time.

(4) No flicker: Because LED shadowless lamps are powered by pure DC, they are flicker-free, less likely to cause eye fatigue, and will not cause harmonic interference to other equipment in the work area.

(5) Uniform illumination: A special optical system is used to evenly illuminate the object being observed at 360°, without any shadows and with high clarity.

(6) Long life: The average life of LED shadowless lamps is long (35,000 hours), which is much longer than that of circular energy-saving lamps (1,500-2,500 hours). The life is more than ten times that of energy-saving lamps.

(7) Energy saving and environmental protection: LED has high luminous efficiency, is impact-resistant, not easy to break, has no mercury pollution, and the light it emits does not contain infrared and ultraviolet radiation pollution.

1 System Structure

1.1 Overall structure

Since the luminous efficiency of a single LED cannot meet the requirements of the illumination intensity and uniformity of the surgical shadowless lamp, multiple LEDs need to be used to form an array, and the arrangement and distribution must be reasonable and uniform, so that the illumination of each target plane meets the requirements, and the number and distribution rules of LEDs are determined. Although LEDs have high luminous efficiency and generate less heat, so many LEDs in a sealed box will still generate a lot of heat, and the temperature can even reach 60℃~80℃, so the heat sink and heat dissipation channel must be reasonably designed to effectively reduce the thermal resistance of the system.

The LED surgical shadowless lamp is composed of multiple lamp heads in a petal shape, fixed on a balance arm suspension system, with stable positioning and vertical or circular movement to meet the needs of different heights and angles during surgery. The entire shadowless lamp has 144 high-brightness white LEDs, 8 of which are connected in series in a group, called a high-brightness light-emitting diode string HBLEDs (High Brightness Lighting Emitting Diode), and 18 groups are connected in parallel. Each group is independent of each other. If one group is damaged, the others can continue to work, so the impact on the surgery is small. Each group is driven by a LM3402 for constant current, and according to user needs, it is controlled by a microprocessor (CMU) P89LPC932 for PWM pulse width regulation, which can be adjusted steplessly. The current flowing through each LED is about 120~320 mA. The electrical schematic diagram of the LED shadowless lamp is shown in Figure 1.

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1.2 Operation Panel

There are 4 buttons (ambient lighting, off, brighten and dim buttons) and 8 LED indicators on the operation panel. Pressing the ambient lighting button means that only the laparoscope ambient lighting is needed, leaving only 3 LEDs on one channel for lighting, and turning off the remaining LED light-emitting diode strings; pressing the off button will extinguish all 8 LED light-emitting diode strings. Pressing this button again will return to the original illumination display state, and it can also return to the set illumination state after power failure or restart; the brighten and dim buttons are used to change the illumination of the shadowless lamp, corresponding to 8 indicator lights, indicating 8 levels of brightness. From left to right, one indicator LED lights up for each additional level. When the shadowless lamp is in the off state, the 2 indicator LEDs on the far right light up. All operation information is transmitted to the driver board MCU via the RS485 bus.

1.3 Driver Board

The driver board is the core of the entire shadowless lamp controller, which is mainly composed of an MCU P89LPC932 and 18-channel LM2402 constant current voltage regulator circuits, and also includes RS485 communication circuits.

The P89LPC932 is a low-power MCU produced by Philips. It has a power supply voltage of 3.3 V and can run at low power consumption. It is suitable for many occasions that require high integration and low cost, and can meet many performance requirements. The P89LPC932 uses a high-performance processor structure, and the instruction execution time only takes 2 to 4 clock cycles, which is 6 times that of the standard 80C51 device. In addition, the P89LPC932 also integrates many system-level functions, which can greatly reduce the number of components, circuit board area and system cost. It has 2 internal timers and can be used as a PWM generator with 256 timer clock cycles [3].

LM3402 is a step-down regulator derived from a controllable current source. It can drive a series of high-power, high-brightness LED strings and can accept an input voltage range of 6 V to 42 V. When using the pin-compatible LM3402HV, the upper limit of the input voltage can reach 75 V. According to the needs of surgical illumination, the output voltage of the converter can be adjusted to maintain a constant current level through the LED array. Figure 2 is a typical application circuit diagram of LM3402, where RSNS is the current setting resistor, the average current IF≈0.2/RSNS, and the RON value is related to the number of LEDs in the LED string. When there are 8 LEDs, it can be 300 kΩ. After testing, when the constant current nominal value is 250 mA (RSNS=0.8 Ω), the current fluctuation is within ±20 mA, which can fully meet the surgical light sensitivity requirements [4].

1.4 Security Measures

Considering the particularity of medical instrument safety requirements, corresponding safety measures should be considered in every link of the system. First of all, the operating room is an environment with strong electromagnetic interference. It is very important to prevent the MCU from crashing. Therefore, the following measures must be taken: (1) The hardware reset circuit design and internal reset program must be carefully handled; (2) The erroneous interference signal must be eliminated, so the entire system adopts complete electrical isolation to prevent the mutual influence of various parts of the circuit. In addition, the Modbus redundancy check method is also used; (3) The price of high-brightness white LEDs is relatively high. In order to avoid damage, the influence of power grid and power supply damage on the system must be eliminated. Therefore, this article adopts overvoltage and overcurrent automatic protection circuits. When the voltage or current exceeds 20% of the set value, the system automatically cuts off the power supply to ensure the safety of the system circuit and high-brightness LED.

2 Programming

2.1 Program Structure

The program mainly includes two independent programs: the panel controller and the driver board. The panel controller program sends switch or illumination adjustment commands to the driver board according to the input status of the four buttons, and displays the command status on the eight LEDs. After the driver board program is initialized, it mainly receives operation information through the serial port interrupt, performs Modbus data redundancy check, and then sends different PWM signals according to the command instructions. The driver board program flow chart is shown in Figure 3.

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2.2 PWM generation

The current of the high-brightness LED string is mainly regulated by PWM on the DIM port of LM3402. The proportion of the actual current to the set current value depends on the duty cycle of PWM. The PWM output of the internal timer T0/T1 of P89LPC932 occupies the same pin as the count input and timer trigger output, and the port output is automatically triggered when a timer overflow occurs. This function enables timers 0 and 1 respectively through the control bits ENT0 and ENT1 in the AUXR1 register. When this mode is turned on, the output of the port is logic 1 before the first timer overflows. In order for this mode to take effect, the C/T bit must be cleared to select PCLK as the clock source for the timer. The reference procedure for the timer initialization setting is as follows:

void Timer1_init(void)

{

TMOD|=0x20; //Timer works in mode 6

TAMOD|=0x10;

TH1=256-n; //n is the duty cycle

AUXR1|=0x20; //Timer enable

TR1=1; //Start the timer

}

The duty cycle is 256-TH1. The overflow of timer 1 will cause the P1.2 or P0.7 port to flip, so the output frequency is 1/2 of the overflow rate of timer 1.

2.3 Energy saving mode

Energy consumption control is of great significance in the entire shadowless lamp control system. Most of the time, HBLEDs are off and the control system is in standby mode, which reduces power consumption to a minimum. When the power control register PCONA is set to 0xFF, the external functional module is powered off; when the power control register PCON is set to 03H, the MCU can be completely powered off. It can only be awakened when an interrupt is triggered, and then the external functional module is powered on and the MCU starts working. The MCU on the driver board is awakened by the serial port receiving interrupt, and the MCU on the panel is awakened by the keyboard interrupt. The keyboard interrupt reference program is as follows:

void KEY_ISR( ) interrupt 7

{

key_push=1; //A key is pressed

PCONA=0x00; //Power on the external function module

KBCON=0x00; //Clear keyboard interrupt flag

}

The market demand for shadowless lamps in China is more than 20,000 units per year. At the same time, due to the need for low-carbon economic development, energy-saving transformation is needed, so the market promotion prospects of LED shadowless lamps are very broad. The LED shadowless lamp technology introduced in this article has been put into mass production in a well-known medical device company in Jiangsu, and has achieved good social benefits.