Detailed explanation of the working principles of products commonly used by engineers

Inverter Principle

An inverter must be something that is composed of an inverter device. It is directly different from a transformer, that is, it can realize DC input and then output AC. The working principle is the same as that of a switching power supply, but the oscillation frequency is within a certain range. For example, if the frequency is 50HZ, the output is AC 50HZ. An inverter is a device that can change its frequency.

Transformers generally refer to devices in a specific frequency band, such as industrial frequency transformers, which are the transformers we usually see. Their input and output must be within a certain range, such as 40-60HZ, to work.

Answer your questions below

1. It is an inverter device, and its working principle is similar to that of a switching power supply. Of course, you can also imagine it as a transformer. Scientifically speaking, its working principle is to control the output of the oscillation signal through an oscillation chip or a specific circuit, such as outputting a 50HZ signal. This signal is then amplified to drive the MOS tube [field effect tube or thyristor] to switch continuously. After the DC power is input, it forms a certain AC characteristic through the switching action of the MOS tube. After being corrected by the correction circuit, a sinusoidal AC similar to that on the power grid can be obtained, and then it is sent to a transformer. This transformer is an industrial frequency transformer. It is a 220V to 24V transformer, that is, if the input is 220V, the output is 24V, and if the input is 24V, the output is 220V. In fact, it is a general 24V transformer.

Then the transformer outputs, and then sends it to the voltage stabilizing circuit, protection circuit, and load for use. Another point to explain is that we regard this inverter as a transformer. The transformer does not refer to who has a larger current. The transformer looks at the capacity, that is, volt-ampere [the product of volts and amperes, the product of voltage and current]. For example, a transformer with a 220V 5A input can output 24V xA if we do not consider the loss:

220*5=24*x, so the product of the left and right sides is the same, but in practical applications, losses should be taken into account, so the input needs to be slightly larger than the output.

Therefore, the power [watts] or capacity [volt-amperes] values ​​on both sides of the transformer should be close to the same. It is not what you said.

2. Usually the 220V electricity obtained by the inverter in the car is 220V 50HZ. The higher-end ones are sine waves, and the cheaper ones are generally square waves.

The sine wave is the same as the electricity used in the socket, and the square wave can also be used, but if you use a device with a motor such as a fan, there will be some noise. The reason why square waves are used is because this modulation method is relatively low cost.

Generally, the maximum power of the inverter in the car is no more than 500 watts, and the air conditioner is usually more than 700 watts. Moreover, do you really want to install a home air conditioner in your car? The air conditioners in cars, including those in buses, use the engine to directly drive the compressor, not electricity. If there is an additional electricity conversion process in the middle, the loss will be greater. It is also difficult to install, so it is better to use a car air conditioner.

It can be connected to laptops, TVs, DVD players, etc., as long as it is used within its rated power, but it should be noted that it is connected to the car battery. Although it is generally 11V, it automatically protects the power off to avoid the car from failing to start due to low voltage, but it is still not suitable for use when the engine is not running. So if the load is relatively large, it is recommended to start the engine. If it is used to charge a mobile phone, there is no problem.

3. There is a module called DC-DC on the electric car, which is also called a DC converter. This module inputs 48V and outputs 12V. So you only need to buy a 12V input car inverter to use it. Of course, if you can buy an inverter with 48V input, it will be better, but it is probably difficult to buy. Moreover, this module can generally only provide 5A current, no more than 10A, and it is also used for headlights, so it is easy to overload. It is recommended that, if possible, buy an extra DC converter, which is specifically for your inverter. Then, if the DC converter can only provide 5A, then the inverter output The input should be less than 5A, otherwise the module may be damaged. Of course, some DC converters have very large currents. If the repair place doesn't have it, you can go to some electrical appliance stores or ask them to repair it and give you a high-current one, or connect multiple DC converters in parallel. In short, don't let it overload. If you can buy an inverter with 48V input, then connect it directly in parallel to the battery. If you don't want to use a DC converter and it's not 48V, then you can consider, for example, if yours is 24V, then choose 2 adjacent batteries and connect them to the top to get 24V. You can install a switch to control it.

But there is a disadvantage to this installation. The voltage of these two batteries will be lower than that of the other two. This may cause unbalanced discharge, resulting in the car not being able to travel far, and may damage the battery.

Sine wave inverter schematic diagram

Here we will introduce the working principle of this inverter in detail.

Square wave signal generator (see Figure 3)

Figure 3

Here, six inverters CD4069 are used to form a square wave signal generator. R1 in the circuit is a compensation resistor, which is used to improve the unstable oscillation frequency caused by the change of power supply voltage. The oscillation of the circuit is completed by charging and discharging the capacitor C1. Its oscillation frequency is f=1/2.2RC. The maximum frequency of the circuit shown in the figure is: fmax=1/2.2×3.3×103×2.2×10-6=62.6Hz; the minimum frequency fmin=1/2.2×4.3×103×2.2×10-6=48.0Hz. Due to the error of the components, the actual value will be slightly different. For other redundant inverters, the input end is grounded to avoid affecting other circuits.

Field effect transistor drive circuit.

Figure 4

Since the maximum amplitude of the oscillation signal voltage output by the square wave signal generator is 0~5V, in order to fully drive the power switch circuit, TR1 and TR2 are used here to amplify the oscillation signal voltage to 0~12V, as shown in Figure 4.

MOS field effect tube power switch circuit.

This is the core of the device. Before introducing the working principle of this part, let's briefly explain the working principle of MOS field effect tube.

Figure 5

MOS field effect transistor is also called MOS FET, which is the abbreviation of Metal Oxide Semiconductor Field Effect Transistor. It generally has two types: depletion type and enhancement type. The enhancement type MOS field effect transistor used in this article has its internal structure shown in Figure 5. It can be divided into NPN type and PNP type. NPN type is usually called N-channel type, and PNP type is also called P-channel type. As can be seen from the figure, for the N-channel field effect transistor, its source and drain are connected to the N-type semiconductor, and for the P-channel field effect transistor, its source and drain are connected to the P-type semiconductor. We know that the output current of a general triode is controlled by the input current. But for a field effect transistor, its output current is controlled by the input voltage (or electric field), and it can be considered that the input current is extremely small or there is no input current, which makes the device have a high input impedance, and this is also the reason why we call it a field effect transistor.

Figure 6

To explain the working principle of MOS field effect tube, let's first understand the working process of a diode with only one P-N junction. As shown in Figure 6, we know that when a forward voltage is applied to the diode (the P terminal is connected to the positive pole and the N terminal is connected to the negative pole), the diode is turned on and current passes through its PN junction. This is because when the P-type semiconductor terminal is at a positive voltage, the negative electrons in the N-type semiconductor are attracted and rush to the P-type semiconductor terminal with a positive voltage, while the positrons in the P-type semiconductor terminal move toward the N-type semiconductor terminal, thereby forming a conduction current. Similarly, when a reverse voltage is applied to the diode (the P terminal is connected to the negative pole and the N terminal is connected to the positive pole), the P-type semiconductor terminal is at a negative voltage, the positrons are gathered at the P-type semiconductor terminal, and the negative electrons are gathered at the N-type semiconductor terminal. The electrons do not move, and no current passes through its PN junction, and the diode is cut off.

Figure 7a Figure 7b

For the field effect tube (see Figure 7), when there is no voltage on the gate, it can be seen from the previous analysis that no current will flow between the source and the drain, and the field effect tube is in the cut-off state (Figure 7a). When a positive voltage is applied to the gate of the N-channel MOS field effect tube, due to the effect of the electric field, the negative electrons of the source and drain of the N-type semiconductor are attracted and rush to the gate, but due to the obstruction of the oxide film, the electrons gather in the P-type semiconductor between the two N-channels (see Figure 7b), thereby forming a current and making the source and drain conductive. We can also imagine that there is a groove between the two N-type semiconductors, and the establishment of the gate voltage is equivalent to building a bridge between them, and the size of the bridge is determined by the size of the gate voltage. Figure 8 shows the working process of the P-channel MOS field effect tube, and its working principle is similar and will not be repeated here.

Figure 8

The following is a brief description of the working process of the application circuit composed of C-MOS field effect tube (enhancement MOS field effect tube) (see Figure 9). The circuit combines an enhancement P-channel MOS field effect tube and an enhancement N-channel MOS field effect tube. When the input end is low, the P-channel MOS field effect tube is turned on, and the output end is connected to the positive pole of the power supply. When the input end is high, the N-channel MOS field effect tube is turned on, and the output end is connected to the power ground. In this circuit, the P-channel MOS field effect tube and the N-channel MOS field effect tube always work in opposite states, and their phase input and output ends are opposite. Through this working mode, we can obtain a larger current output. At the same time, due to the influence of leakage current, the gate voltage has not reached 0V, usually when the gate voltage is less than 1 to 2V, the MOS field effect tube is turned off. Different field effect tubes have slightly different turn-off voltages. Because of this, the circuit will not cause a power short circuit due to the simultaneous conduction of the two tubes.

Fig. 9

Based on the above analysis, we can draw the working process of the MOS field effect tube circuit part in the schematic diagram (see Figure 10). The working principle is the same as described above. When this low voltage, high current, 50Hz frequency alternating signal passes through the low voltage winding of the transformer, it will induce a high voltage AC voltage on the high voltage side of the transformer, completing the conversion from DC to AC. It should be noted here that in some cases, such as when the oscillation part stops working, sometimes a large current will pass through the low voltage side of the transformer, so the fuse of this circuit cannot be omitted or short-circuited.