Working principle of 12V to AC 220V inverter

Today we will introduce an inverter (see Figure 1) which is mainly composed of MOS field effect tubes and ordinary power transformers. Its output power depends on the power of MOS field effect tubes and power transformers, eliminating the tedious transformer winding and is suitable for amateur production by electronics enthusiasts. The following introduces the working principle and production process of the transformer.

Circuit Diagram(1)

Working principle:

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

1. Generation of square wave

Here CD4069 is 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 capacitor C1. Its oscillation frequency is f=1/2.2RC. The maximum frequency of the circuit shown in the figure is: fmax=1/2.2x103x2.2x10-6=62.6Hz, and the minimum frequency is fmin=1/2.2x4.3x103x2.2x10-6=48.0Hz. Due to the error of the components, the actual value will be slightly different. For other redundant phase transmitters, the input end is grounded to avoid affecting other circuits.

Figure 2

2. Field effect transistor driving circuit.

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 3.

Figure 3

3. Field effect transistor power switch circuit.

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

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. This article uses enhancement type MOS field effect transistor, and its internal structure is shown in Figure 4. It can be divided into NPN type and PNP type. NPN type is usually called N-channel type, and PNP type is usually called P-channel type. As can be seen from the figure, for N-channel field effect transistor, its source and drain are connected to N-type semiconductor, and for P-channel field effect transistor, its source and drain are connected to P-type semiconductor. We know that the output current of a general triode is controlled by the input current. But for field effect transistor, its output current is controlled by the input voltage (or field voltage), 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 field effect transistor.

Figure 4

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 5, 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 flows 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, thus 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 flows through its PN junction, and the diode is cut off.

Figure 5

For the field effect tube (Figure 6), 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 6a). 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 6b), 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 gate voltage. Figure 8 shows the working process of the P-channel field effect tube. Its working principle is similar and will not be repeated here.

Figure 6

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 8). 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 at a low level, 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 at a high level, 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 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 1V to 2V, the MOS field effect tube is turned off. The turn-off voltage of different field effect tubes is slightly different. It is also thought that this circuit will not cause a power short circuit due to the simultaneous conduction of the two tubes.

Figure 8

Figure 9

Based on the above analysis, we can draw the working process of the MOS field effect tube part in the schematic diagram (see Figure 9). The working principle is the same as mentioned 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 to complete 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.

The circuit board is shown in Figure 11. The components used can be referred to Figure 12. The inverter transformer uses a finished power transformer with a secondary voltage of 12V, a current of 10A, and a primary voltage of 220V. The maximum drain current of the P-channel MOS field effect tube (2SJ471) is 30A. When the field effect tube is turned on, the resistance between the drain and the source is 25 milliohms. At this time, if a current of 10A passes through, there will be a power consumption of 2.5W. The maximum drain current of the N-channel MOS field effect tube (2SK2956) is 50A. When the field effect tube is turned on, the resistance between the drain and the source is 7 milliohms. At this time, if a current of 10A passes through, the power consumed is 0.7W. From this, we can also know that under the same working current, the heat generated by 2SJ471 is about 4 times that of 2SK2956. So this point should be taken into account when considering the heat sink. Figure 13 shows the position distribution and connection method of the inverter field effect tube introduced in this article on the heat sink (100mm×100mm×17mm). Although the heat generated by the field effect tube when working in the switching state is not very large, the heat sink selected here is slightly larger for safety reasons.

Fig.11

Fig.12

Fig.13

Fig.14

4. Inverter performance test

The test circuit is shown in Figure 15. The input power used for the test here uses a 12V car battery with low internal resistance and large discharge current (generally greater than 100A), which can provide sufficient input power for the circuit. The test load is an ordinary light bulb. The test method is to change the load size and measure the input current, voltage and output voltage at this time. The test results are shown in the voltage and current curve relationship diagram (Figure 15a). It can be seen that the output voltage decreases with the increase of load, and the power consumption of the bulb changes with the voltage. We can also find out the relationship between output voltage and power through calculation. But in fact, since the resistance of the light bulb will change with the voltage applied at both ends, and the output voltage and current are not sinusoidal, this kind of calculation can only be regarded as an estimate. Take a light bulb with a load of 60W as an example:

Fig.15

Figures 16 and 17

Assume that the resistance of the bulb does not change with the voltage. Because R = V2/W = 2102/60 = 735Ω, when the voltage is 208V, W = V2/R = 2082/735 = 58.9W. From this, the relationship between voltage and power can be calculated. Through testing, we found that when the output power is about 100W, the input current is 10A. At this time, the output voltage is 200V. The inverter power efficiency characteristics are shown in Figure 15b. Figure 16 is the temperature rise curve of the field effect tube when the inverter is continuously loaded with 100W. Figure 17 is the output waveform at different loads for your reference.