Design of power supply system for portable aviation inspection equipment

Given that airborne electronic equipment may have a variety of wiring methods and may encounter a set of systems using multiple power supplies, it is necessary to take some measures on the circuit to ensure the normal work of the power supply and the post-circuit. A portable aviation detection equipment power supply system is designed in this paper. The system supports simultaneous access of two power supplies to identify and judge the input power supply and switch between the battery and the external power supply, effectively avoiding the security risks caused by battery backflow current. The battery power is monitored in real-time through the battery power detection circuit and will be reminded when the battery power is too low. Since the circuit uses a MOS tube to control the switching of the power supply, there is almost no voltage drop loss. In addition, the EMC protection circuit is set up at the power supply side to further improve the safety and reliability of the system.


Introduction
In the actual design, for the multi-power supply system, it is generally required that each power supply is independent of the other and does not interfere with each other to supply power to the product, but there will inevitably be multiple power supplies accessing the product at the same time.If certain measures are not taken in the circuit, it may lead to the post-stage circuit not working normally.And the battery side may also lead to certain security risks, such as backflow current, battery heating, and other conditions [1] .Based on this problem, an automatic power switch system is designed in this paper for airborne electronic equipment, which can identify and judge the type of access power supply, and automatically disconnect the battery connection when the external power supply is connected.This system aims to ensure that only one power supply supplies the equipment at the same time, avoids the safety risks caused by current backflow, and at the same time, ensures that the rear circuit will not lose power, which may cause serious consequences such as reset and restart, data loss, and even component damage.In addition, the voltage is smooth when switching.The system has the advantages of low power consumption, fast response, and good electromagnetic compatibility [2] .

Principle of power switching system scheme
This scheme makes full use of the switching characteristics of the MOS tube to achieve the purpose of switching between the battery and external power supply.That is, for PMOS tubes, the gate-source voltage Vgs is less than a certain value, which is more suitable for high-end drive scenarios [3] .At the same time, the MOS tube has almost no voltage loss and is safer and more reliable.A classic circuit is shown below, as shown in Figure 1.Its circuit flow is as follows: when two power supplies are connected to the system at the same time since the external power supply has power, the Vgs of the PMOS is > 0, so the PMOS parasitic diode is cut off, and the rear load is powered by the external power supply; when the external power supply is disconnected, the PMOS is switched on, and the rear load is powered by the battery [4] .Based on this circuit, appropriate improvement and optimization are made according to the actual situation, and a power switching system suitable for aviation airborne electronic equipment is designed.As shown in Figure 2, automatic and seamless power switching can be achieved after testing, and the voltage is stable and reliable when the battery and external power supply are powered separately [5] .When the two power supplies are connected at the same time, the external power supply is preferred for the power supply.At the same time, the battery power supply is cut off, forming a disconnect.
In aviation and airborne electronic equipment, a 28 V voltage power supply is commonly used to power various load equipment, and many chips only need 3 V or even lower operating voltage.In this system, a 3.6 V battery is designed to share the post-load circuit with a 28 V external power supply.When the battery is connected separately, Vg < Vs, the PMOS tube is on, and the battery supplies power to the rear circuit.When the 28 V external power supply is connected, the voltage is first regulated from 18 V to 5 V through the linear voltage regulator chip, and then from 5 V to 3.6 V.The 5 V voltage signal is connected to the gate of the MOS tube as a control signal to control the on-off of the MOS tube, to achieve the purpose of automatic switching between the battery and the external power supply.The 5 V voltage signal acts on the gate, while 3.6 V acts on the source, Vg > Vs, Vgs is about 1.4 V, the PMOS tube is off, and the connection between the battery and the circuit is cut off, to avoid the adverse effects such as battery damage caused by the external power supply for the battery to reverse charge [6] .
The circuit also added EMC electromagnetic compatibility secondary protection circuit, to eliminate the power supply terminal voltage fluctuations and electrostatic impact on the circuit.A filter capacitor is added to the linear voltage regulator chip input and output, to further ensure the stability of the voltage.In addition, the system also has power supply identification, battery remaining power detection, low power reminder, and other functions, as shown in Figure 3.When the I/O pin is high power, the system is powered by the external power supply.You can choose to enable some high-power functions or perform other processing.At the same time, the ADC voltage detection function can be used to detect the remaining battery power with a simple circuit.The diode and capacitor are used to achieve the purpose of decoupling filtering and protecting components, and the remaining battery power can be calculated according to the voltage change detected by the ADC pin of the MCU.At the same time, different methods can be used to warn when the battery power is low according to actual needs.

Reliability test
To ensure that the whole power supply switching system meets the requirements, the oscilloscope is used to capture the voltage change in various cases and analyze whether it meets the expected requirements [7] .In Figures 4 -7 and 10 -11, the voltage unit of the oscilloscope is set to 5 V per cell, and the time unit is set to 4 ns per cell.The dashed line is used to draw the cell, and the solid line is used to represent the voltage reference line at 0 V.In Figure 8 and Figure 9, the oscilloscope voltage unit is set to 500 mV per cell, and the time unit is set to 400 ms per cell.The cells are also drawn with dotted lines, and the solid line is used to represent the voltage reference line at 0 V.This is because the voltage change time at the instant of power switching is about 200 ms, and the voltage amplitude change is about 200 mV.When observed in 5 V and 4 ns units, the voltage change cannot be clearly seen.

Power switchover instant
The power switch moment includes four situations: connecting and disconnecting the external power supply when the battery is powered, and connecting and disconnecting the battery when the external power supply is powered.According to the circuit principle of this system, since the battery voltage is slightly higher than 3.6 V, connecting and disconnecting the external power supply when the battery is powered should produce smooth and stable voltage changes.When the external power supplies, regardless of whether the battery is connected, the voltage value should be stable, that is, when the external power supply is connected, the MOS tube will be controlled to disconnect the battery.
The oscilloscope is used to capture the waveform of the power switch moment to ensure that the power switch will not cause power failure and restart of the post-stage equipment, affecting the normal operation of the electronic equipment [8] .It can be found that the voltage change of the output to the rear circuit at the moment of power switching is smooth and slow, and the voltage is continuous.There is no power loss, and there is no relatively large voltage fluctuation, which meets the expected requirements of power supply reliability [9] .

Two power supplies are supplied at the same time
To prevent the current from pouring back into the battery, the ammeter is used for testing.The specific method is to connect the ammeter in series to the battery end circuit.Provides 4.7mA of current to the circuit when the battery is connected alone, as shown in Figure 12.When the external power supply is connected, it can be observed that there is neither current output nor reverse current inflow at the battery end, to ensure the safety and reliability of the system, as shown in Figure 13.

System workflow
First of all, the power supply is identified and judged by the detection circuit (Figure 3) and enters different operating modes according to the judgment results.Due to the limited amount of power that the battery can store, the battery is usually used in low-power mode to avoid frequent battery replacement, only the necessary functions are retained.At the same time, DAC is used to detect the remaining power in the battery (Figure 3), and timely alarm reminds the need to replace the battery when the power is insufficient.When the power supply is switched or connected to the system at the same time, since there is no power consumption limit when using an external power supply, it will enter normal mode.The battery and the external power supply switching circuit (Figure 2) will automatically select the power supply to ensure that the current will not flow from the external power supply to the battery, causing safety hazards [10] .Figure 14 is the workflow block diagram of the power supply system of portable aviation inspection equipment.

Conclusion
This paper introduces a portable power supply system for aviation inspection equipment, which supports automatic switching between two power supplies without losing power when switching, and has the advantages of battery power detection, low power reminder, power identification, and EMC circuit protection.It is mainly used in the power supply circuit of airborne electronic equipment.After testing, the system is stable and reliable, and the power supply voltage is stable, which has a certain promotion value and use value.

Figure 1 .
Figure 1.Power switching circuit of MOS tube.Based on this circuit, appropriate improvement and optimization are made according to the actual situation, and a power switching system suitable for aviation airborne electronic equipment is designed.As shown in Figure2, automatic and seamless power switching can be achieved after testing, and the voltage is stable and reliable when the battery and external power supply are powered separately[5] .When the two power supplies are connected at the same time, the external power supply is preferred for the power supply.At the same time, the battery power supply is cut off, forming a disconnect.In aviation and airborne electronic equipment, a 28 V voltage power supply is commonly used to power various load equipment, and many chips only need 3 V or even lower operating voltage.In this system, a 3.6 V battery is designed to share the post-load circuit with a 28 V external power supply.When the battery is connected separately, Vg < Vs, the PMOS tube is on, and the battery supplies power to the rear circuit.When the 28 V external power supply is connected, the voltage is first regulated from 18 V to 5 V through the linear voltage regulator chip, and then from 5 V to 3.6 V.The 5 V voltage signal is connected to the gate of the MOS tube as a control signal to control the on-off of the MOS tube, to achieve the purpose of automatic switching between the battery and the external power supply.The 5 V voltage signal acts on the gate, while 3.6 V acts on the source, Vg > Vs, Vgs is about 1.4 V, the PMOS tube is off, and the connection between the battery and the circuit is cut off, to avoid the adverse effects such as battery damage caused by the external power supply for the battery to reverse charge[6] .The circuit also added EMC electromagnetic compatibility secondary protection circuit, to eliminate the power supply terminal voltage fluctuations and electrostatic impact on the circuit.A filter capacitor is added to the linear voltage regulator chip input and output, to further ensure the stability of the voltage.

Figure 2 .
Figure 2. Battery and external power switching circuit.

3. 1 .
Batteries and external power supplies are supplied separately 3.1.1.The battery is powered separately.The test battery is a lithium battery with a nominal voltage of 3.6 V, and the actual voltage value is 3.68 V.The oscilloscope probe measures the battery voltage at the rear circuit, as shown in Figure 4.

Figure 13 .
Figure 13.Battery current after the external power supply is connected.

Figure 14 .
Figure 14.Block diagram of system workflow.