The other is a centralized control type of power supply. There is no independent power supply in the emergency light. When the normal lighting power supply fails, it is powered by the centralized power supply system. In this form of emergency lighting system, the complex electronic circuits inside all the lamps are omitted, and the emergency lighting fixtures are no different from ordinary lamps, and the centralized power supply system equipment is in a dedicated room.
At present, the control power supply of most LED lamps and ordinary lamps is the same in complexity, and the power of each control power source is fixed and cannot be adjusted according to actual needs. The LED emergency control circuit designed in this paper is a complete and independent system, which can be used with LED power supply of different powers, which is simple and convenient, and flexible in use.
First, the new LED emergency lighting principle and advantages and disadvantages
The block diagram of this new LED emergency lighting is as follows:
Figure 1 Block diagram of the new LED emergency lighting fixture
When the utility power is normal, the LED driving power supply supplies power to the control board, and the driving light source emits light. The mains charges the battery through the charger. When the battery is fully charged, the power is cut off by the protection circuit to stop charging. When the utility power is cut off or pulled, the battery supplies power to the emergency part of the light source, and the emergency light source is supplied to the emergency light source through the emergency control circuit.
In this design, the emergency light source may be a whole or a part of the normal illumination source; the normal illumination source and the emergency illumination source may be separated separately or as a whole. Just adjust the peripheral circuit slightly.
The emergency control circuit is a relatively new and unique place in this design. It is mainly composed of MT7201 chip, peripheral circuit and relay. The external detection part can be designed into various forms such as microwave radar detection emergency illumination, infrared detection emergency illumination, sound control, and light control.
This emergency control circuit determines that the electrical layout of the new LED emergency lighting fixtures of this design is very different from the emergency lights currently used. Nowadays, the internal emergency lighting fixtures are composed of transformers, voltage regulators, charging, inverters, batteries, etc., and the rear end is connected to the LED driving power supply to supply power to the LED light source. The advantage is that both ordinary lamps and LED lamps can be used, and the application range is wide. The disadvantage is that there are many electronic components, difficult to overhaul, and high failure rate; and the fixed power level is inconvenient to adjust after design, the circuit is working at normal power supply, and the power consumption is high.
Second, the emergency control circuit principle and test results
The emergency circuit of this design is dedicated to LED lamps. It only needs to adjust few components, and can be used together with peripheral LED power supply to apply to LED lamps of different powers. In normal lighting, only the LED driver power supply part of the circuit works. Low power consumption and corresponding failure rate are also low. The circuit principle is shown in Figure 2.
Figure 2 Emergency Control Circuit Schematic
The MT7201 is a continuous current conduction mode buck constant current source for driving one or more series LEDs. Its input voltage range is from 7V to 30V, and the output current is adjustable up to 1A. LEDs up to 24W can be driven depending on the input voltage and peripherals. The MT7201 is available in a SOT89-5 package. Its pin is shown in Figure 3.
Figure 3 MT7201 pin diagram
The average current of the LED in the circuit is determined by the resistor Rs connected at both ends of Vin and Isense: The relay adopts a small relay with normally open and normally closed electric shock, and the detection part adopts a microwave radar remote control switch. When the human body or object enters the detection range, the sensor immediately Work and start the controlled device. When the human body leaves or the object stops moving, the delay time until the sensor automatically stops working, and the sensor enters the detection state. If the object continues to move within the detection range, the sensor is always active. Of course, according to the needs of the detection part can be replaced by infrared, voice control, light control and so on.
     The average current of the LEDs in the circuit is determined by the resistor Rs connected across Vin and Isense:
The following table gives some typical examples of the relationship between output current and Rs:
The above description is based on the premise that the ADJ pin is floating or equal to the internal reference voltage Vref.
If the LED is open, the inductor will lose connection with the LX, and no current will flow through the entire loop, causing no damage to the device.
Capacitor selection
A low equivalent series resistance (ESR) bypass capacitor must be placed close to the power supply input. The larger the ESR, the greater the efficiency loss. The bypass capacitor is capable of withstanding large peak currents and averaging the input current of the power supply to reduce the impact on the input power supply.
The minimum value of this bypass capacitor is 4.7uF for DC input. For AC input or low voltage input, the bypass capacitor requires 100uF capacitor or similar capacitor. This bypass capacitor is as close as possible to the input pins of the chip.
2. Inductance selection
The recommended inductance parameters for the MT7201 range from 27uH to 100uH. The saturation current of the inductor must be 30% to 50% higher than the output current. The selection of the inductance value needs to take into account the duty cycle of the operation and the opening and closing times of the power switch to ensure that the requirements in the electrical characteristics table are met.
The following formula can be used as a reference:
Power switch on time
Power switch disconnection time
3. Parameter description:
L is the inductance value (H); rL is the inductance parasitic impedance (Ω)
Iavg LED average current (A);
For the inductor current peak-to-peak value (A) {internal setting is 0.3 × Iavg};
VIN supply voltage (V); VLED total LED positive bias drop (V)
RLX power switch on-resistance (Ω); VD Schottky diode positive bias drop (V).
4. Experiments and results
When the mains power fails, the emergency circuit supplies power, the battery voltage is 26.4V; the output current is adjustable, here is set to 300mA; the relay input voltage is DC12V, the emergency light source power is 5.5W; the emergency light source voltage is 18V. The physical map is as follows:
Figure 4 Emergency lighting circuit connection diagram
Figure 5 Circuit diagram of normal power supply
Figure 6 Circuit diagram of emergency lighting
The design circuit is simple and reliable, and the power is adjustable. According to the actual arrangement, the layout of the emergency light source and the whole light source is widely used in factories, mines, tunnels and the like where emergency lighting is required. It is an emergency control circuit specially developed for LED lighting. According to the design, Xi'an Heavy Equipment Yinan Optoelectronics Technology Co., Ltd. has designed a series of emergency lighting fixtures for different occasions.
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