Design of wide voltage intelligent flashing light based on MK6A11P single chip microcomputer

The MK6A11P microcontroller has strong anti-interference ability, contains RC oscillator, watchdog and reset circuit. Compared with other series of microcontrollers, it saves a lot of peripheral components and is low-cost, suitable for the design of various small industrial products.

The intelligent flashing light designed by combining this chip with the NE555 chip has a simple overall circuit, stable operation, and good product consistency, which greatly improves mass production capacity and competitiveness.

1.1 Overall circuit diagram

The overall circuit block diagram is shown in Figure 1, which consists of temperature detection, overvoltage detection, working mode setting, energy regulation and drive, high voltage detection and pulse triggering parts, so that the flash light has complete protection and adaptive functions.

1.2 Characteristics of MK6A11P MCU

MK6A11P is a RISC high-performance 8-bit microcontroller, which contains 1 kbit×14 bit OTP ROM, 48 8-bit RAM and an 8-bit timer/counter. It contains power-on reset, low voltage reset, external reset and WDT reset. It has external RC, LS crystal oscillator, NS crystal oscillator, HS crystal oscillator and internal 4 MHz RC oscillator, and has 8-pin and 14-pin packages. The I/O port can be set to pull-up resistor or pull-down resistor mode in the input state, which can save the external pull-up resistor.

1.3 Synchronous Flyback Boost Circuit Design

The key to the design is to design an energy-controllable flyback booster using a simple circuit. The output voltage of the MK6A11P microcontroller I/O is about 4.5 V, and the drive current is small, so it cannot directly drive the MOSFET. If the level conversion method is used for driving, it is also possible, but once the program enters a "dead loop", the state of the I/O is uncertain, so the MOSFET switch tube cannot be turned off in time (the internal watchdog can be automatically reset during the dead loop, but the time is in the order of ms and cannot protect the switch tube), which can damage the switch tube.

Therefore, a driver is needed that has a driving capability of more than 10 V/300 mA, can set the energy storage value, can quickly turn off the switch tube, and is low in price.

NE555 is a universal timer chip with an output voltage of 5 to 15 V and an output current of up to 500 mA. It has two comparators, and the comparison point voltages are UF and UF/2, as shown in Figure 2. When the voltage at pin 2 is lower than UF/2, the output is high, and when the voltage at pin 6 is higher than UF, the output is low. This feature is used to design a synchronous flyback converter with variable conversion energy.

The energy stored in the primary of transformer T1 is determined by equation (1).

In the circuit of Figure 2, UH=UD+UZ, where UZ is the voltage regulation value of the voltage regulator D3. When UH>UF, NE555 outputs a low level and the switch is turned off, so equation (2) holds true.

It can be seen that the energy stored in the primary inductance of the transformer depends on the size of UF, and the size of UF depends on the duty cycle of the PWM signal at point K. Therefore, changing the duty cycle can change the size of UF, thereby adjusting the charging rate.

The MK6A11P microcontroller sends out a 30 kHz synchronization pulse, which is added to pin 2 through C5 to trigger the output, making the output high, the switch tube Q1 is turned on, the current of the primary of the transformer increases linearly, and the voltage at point D also increases linearly. When UD>UF-UZ, the NE555 outputs a low level, quickly turns off the switch tube, and the energy stored in the primary of the transformer is released to the high-voltage capacitor through the secondary, and the capacitor is charged. Since the synchronization pulse is sent by the microcontroller, the operating frequency is stable and harmonics can be reduced.

1.4 Intelligent control principle of charging rate

Figure 3 shows the charging curves of normal, slow and fast charging when working in 2-time strobe mode. The ideal charging rate is that when the voltage just reaches the set value, the strobe tube is triggered and the voltage of the high-voltage capacitor is discharged. As shown in Figure 3, charging too slowly will not reach the set value, affecting the brightness of the strobe. Overcharging voltage will increase the burden on the switch tube, transformer and high-voltage capacitor, which is more serious when working in 3-time or 4-time strobe mode.

Table 1 is the relationship between the charging voltage and the stored energy when the capacitor C1 is 100μF. When the primary discharge voltage is 350V and the secondary discharge voltage is 200V in normal operation, the primary discharge voltage is 350V and the secondary discharge voltage is 240V in over-fast operation, and the over-charge voltage is 40V. The voltage change is 20%, and the energy change is up to 44%, because the energy stored in the capacitor is in a square relationship with the charging voltage.

During operation, the MK6A11P microcontroller detects the voltage at point E in Figure 7. When the voltage reaches the set value too early, the duty cycle of the PWM signal at point K is reduced, thereby reducing UF and the charging rate, and vice versa.

FIG4 shows the waveforms of points B, D, F, and G when the boost circuit is working. Point G is at a high level after a delay of 5 to 6 μs each time it is triggered. It can be seen that the synchronization effect and the switching waveform are good, and the voltage ripple at the energy control point (point F) is very small.

Figure 5 shows the measured working waveform when working in the three-shot flash mode. It can be seen that after the high voltage just reaches the set value, it is triggered to discharge by the trigger pulse, and there is no slow or fast charging phenomenon.

1.5 Pulse sequence generator design

A flashing signal light requires multiple operating modes, and some of the different operating modes are listed in FIG6 .

In order to generate a 30 kHz synchronization pulse, the microcontroller's timer T0 requests an interrupt at a 30 kHz rate. After N times of frequency division, the required pulse sequence can be obtained. The mode selection switch S1 is used to select different working modes.
1.6 Overvoltage and overtemperature protection

Figure 7 shows the overall working circuit. The input voltage of the power supply is divided and detected by R1 and R5. If it is greater than 50 V, the synchronous pulse output is stopped and the circuit stops working.

Excessive temperature is an important reason for damaging or shortening product life, especially for high-voltage capacitors working in high-voltage, high-current discharge state, the operating temperature is very high. The actual circuit uses 105℃ electrolytic capacitors, but it is required not to exceed 80℃ to prevent the electrolyte from drying up and shortening the service life.

Thermistor RT1 is installed on the circuit board near the high-voltage capacitor to detect the highest temperature inside the signal light. Once it exceeds 80°C, the synchronous pulse output stops, the circuit stops working, and it will automatically work after the temperature drops. If the flashing light is not allowed to stop working, the set high voltage value can be reduced to work, and it will automatically resume after the temperature drops.

2 Conclusion

The wide voltage intelligent flashing light designed by MK6A11P single chip microcomputer has the following conclusions after experiments: various protection functions work normally, adapt to various situations, and the charging rate is ideal when working in a wide voltage range. The flyback boost part uses a synchronous pulse from the single chip microcomputer to turn on and is quickly turned off by NE555, so the operating frequency is stable, and the switch tube can be well protected even if a crash occurs. The overall circuit is simple, the operation is stable, and the product consistency is good, which greatly improves the production capacity, maintenance ability and product competitiveness.