Car inverter circuit and maintenance

Circuit diagram and troubleshooting experience of vehicle inverter
1. The main indicators of common vehicle inverter products on the market:
input voltage: DC 10V~14.5V; output voltage: AC 200V~220V±10%; output frequency: 50Hz±5%; output power: 70W~150W; conversion efficiency: greater than 85%; inverter working frequency: 30kHz~50kHz.
2. Circuit diagram and working principle of common vehicle inverter products.
The output power of the vehicle inverter with the largest sales volume and the most common one on the market is 70W-150W. The inverter circuit mainly uses pulse width modulation circuits based on TL494 or KA7500 chips. The schematic diagram of the most common vehicle inverter circuit is shown in Figure 1.

                                               
The whole circuit of the vehicle inverter can be roughly divided into two parts. Each part uses a TL494 or KA7500 chip to form a control circuit. The function of the first part of the circuit is to convert the 12V DC provided by the car battery into 30kHz-50kHz, 220V AC through high-frequency PWM (pulse width modulation) switching power supply technology; the function of the second part of the circuit is to use bridge rectification, filtering, pulse width modulation and switching power output technologies to convert 30kHz~50kHz, 220V AC into 50Hz, 220V AC.
 
1. Working principle of vehicle inverter circuit
In the circuit of Figure 1, the chip IC1 and its peripheral circuits, transistors VT1, VT3, MOS power tubes VT2, VT4 and transformer T1 form an inverter circuit that converts 12V DC into 220V/50kHz AC. The chip IC2 and its peripheral circuits, transistors VT5, VT8, MOS power tubes VT6, VT7, VT9, VT10, and 220V/50kHz rectifier and filter circuits VD5-VD8, C12 together form a conversion circuit that converts 220V/50kHz high-frequency AC into 220V/50Hz industrial frequency AC, and finally outputs 220V/50Hz AC through the XAC socket for use in various portable appliances.
In Figure 1, IC1 and IC2 use TL494CN (or KA7500C) chips to form the core control circuit of the vehicle inverter. TL494CN is a dedicated double-ended switching power supply control chip. Its suffix CN indicates that the chip's package shape is a dual-in-line plastic package structure, the operating temperature range is 0℃-70℃, the maximum operating power supply voltage is 7V~40V, and the maximum operating frequency is 300kHz.
The TL494 chip has a built-in 5V reference source with a voltage regulation accuracy of 5 V ± 5% and a load capacity of 10mA. It can be output through its 14th pin for external circuit use. The TL494 chip also has two built-in NPN power output tubes that can provide a driving capacity of 500mA. The internal circuit of the TL494 chip is shown in Figure 2.
                                               

In the circuit of Figure 1, R1 and C1 of the 15-pin peripheral circuit of IC1 form a power-on soft-start circuit. When power is on, the voltage across capacitor C1 gradually increases from 0V. Only when the voltage across C1 reaches 5V or above, the pulse width modulation circuit inside IC1 is allowed to start working. When the power is off, C1 discharges through resistor R2 to ensure the normal operation of the soft-start circuit when the power is turned on next time.
R1, Rt, and R2 of the 15-pin peripheral circuit of IC1 form an overheat protection circuit. Rt is a positive temperature coefficient thermistor. The resistance value at normal temperature can be selected in the range of 150 Ω to 300 Ω. Choosing a larger resistance value can improve the sensitivity of the overheat protection circuit.
When installing the thermistor Rt, it should be close to the metal heat sink of the MOS power switch tube VT2 or VT4 to ensure the effectiveness of the overheat protection function of the circuit. The
voltage value U of the 15-pin of IC1 to ground is a relatively important parameter. In the circuit of Figure 1, U≈Vcc×R2÷(R1+Rt+R2)V, and the calculated value at normal temperature is U≈6.2V. Combining Figures 1 and 2, it can be seen that under normal working conditions, the voltage of pin 15 of IC1 should be slightly higher than the voltage of pin 16 (connected to pin 14 of the chip is 5V). The voltage value of 6.2V at room temperature just meets the requirement and leaves a certain margin.
When the circuit works abnormally, the temperature rise of the MOS power tube VT2 or VT4 increases significantly, and the resistance of the thermistor Rt exceeds about 4kΩ, the output of the internal comparator 1 of IC1 will flip from low level to high level, and the pin 3 of IC1 will also flip to high level state, causing the output of the PWM comparator, "OR" gate and "NOR" gate inside the chip to flip, and the output stage transistors VT1 and VT2 will turn to the cut-off state. When the two power output tubes in IC1 are cut off, VT1 and VT3 in the circuit of Figure 1 will be saturated and turned on due to the low level of the base. After VT1 and VT3 are turned on, the power tubes VT2 and VT4 will be in the cut-off state due to the lack of positive bias at the gate, and the inverter power supply circuit stops working.
VDZ1, R5, VD1, C2, and R6 of the peripheral circuit of pin 1 of IC1 constitute a 12V input power supply overvoltage protection circuit. The voltage value of the voltage regulator VDZ1 determines the startup threshold voltage value of the protection circuit. VD1, C2, and R6 also constitute a protection state maintenance circuit. As long as there is a momentary input power supply overvoltage phenomenon, the protection circuit will start and maintain for a period of time to ensure the safety of the subsequent power output tube. Considering the normal change amplitude of the battery voltage during the driving of the car, it is usually more appropriate to select the voltage value of the voltage regulator VDZ1 as 15V or 16V.
C3 and R5 of the peripheral circuit of pin 3 of IC1 are key circuits for maintaining the soft start time on power-up and the circuit protection state. In fact, whether it is the control of the circuit soft start or the startup control of the protection circuit, the final result is reflected in the level state of pin 3 of IC1. When the circuit is powered on or the protection circuit is started, pin 3 of IC1 is high. When pin 3 of IC1 is high, capacitor C3 will be charged. After the cause of the protection circuit starts disappears, C3 discharges through R5. Since the discharge takes a long time, the protection state of the circuit can be maintained for a period of time.
When the 3rd pin of IC1 is at a high level, the capacitor C7 will be charged along R8 and VD4, and the voltage across the capacitor C7 will be provided to the 4th pin of IC2, so that the 4th pin of IC2 remains at a high level. From the internal circuit of the chip in Figure 2, it can be seen that when the 4th pin is at a high level, the potential of the in-phase input terminal of the dead time comparator in the chip will be raised, so that the output of the comparator remains at a constant high level, and the built-in transistors VT1 and VT2 are cut off after the "OR" gate and the "NOR" gate. VT5 and VT8 in the circuit of Figure 1 are in a saturated conduction state, and the subsequent MOS tubes VT6 and VT9 will be in a cut-off state due to the lack of positive gate bias, and the inverter power supply circuit stops working.
The external capacitor C4 (472) at the 5th pin of IC1 and the external resistor R7 (4k3) at the 6th pin are the timing components of the pulse width modulator, and the pulse width modulation frequency determined by them is fosc=1.1÷ (0.0047×4.3)kHz≈50kHz. That is, the operating frequencies of the transistors VT1, VT2, VT3, VT4 and the transformer T1 in the circuit are all around 50kHz, so T1 should use a high-frequency ferrite core transformer. The function of transformer T1 is to boost the 12V pulse to a 220V pulse, with a primary turn of 20×2 and a secondary turn of 380.
The external capacitor C8 (104) at the 5th pin of IC2 and the external resistor R14 (220k) at the 6th pin are the timing components of the pulse width modulator, and the pulse width modulation frequency determined by them is fosc=1.1÷ (C8×R14)=1.1÷(0.1×220)kHz≈50Hz.
R29, R30, R27, C11, and VDZ2 form the overvoltage protection circuit of the 220V output end of the XAC socket. When the output voltage is too high, the voltage regulator tube VDZ2 will break down, causing the voltage of the 4th pin of IC2 to rise to the ground. The protection circuit in the chip IC2 will be activated and the output will be cut off. The MOS tubes VT2 and VT4 in the
car inverter circuit have a certain power consumption and must be equipped with a heat sink. Other devices do not need to install a heat sink. When the car inverter product is continuously used in high-power applications, a small 12V fan must be installed inside it to help dissipate heat.
 

2. Parameters of components in the circuit

The parameters of each component in the circuit are listed in the attached table.
                                         

3. Maintenance points of vehicle inverter products
Since vehicle inverter circuits generally have a power-on soft start function, it takes 5s-30s after the power is turned on before there is an AC 220V output and the LED indicator lights up. When the LED indicator is not on, it indicates that the inverter circuit is not working. When the
power is turned on for more than 30s and the LED indicator is not on, it is necessary to measure the AC voltage value at the XAC output socket. If the voltage value is about the normal 220V, it means that only the circuit of the LED indicator part has a fault; if the AC voltage value at the XAC output socket is 0, it means that the cause of the fault is that the inverter circuit of the front stage of the inverter is not working, and it may be that the protection circuit inside the chip IC1 has been started.
The method to determine whether the internal protection circuit of the chip IC1 is started is: use the DC voltage block of the multimeter to measure the DC voltage value of the 3rd pin of the chip IC1 to the ground. If the voltage is above 1V, it means that the protection circuit inside the chip has been started, otherwise it means that the cause of the fault is caused by the action of the non-protection circuit.
If the voltage value of the chip IC1's 3-pin to ground is above 1V, indicating that the protection circuit inside the chip has been activated, it is necessary to further use the DC voltage block of the multimeter to test the DC voltage between the 15-pin and 16-pin of the chip IC1, as well as the DC voltage between the 1-pin and 2-pin of the chip IC1. Under normal circumstances, the DC voltage of the chip IC1's 15-pin to ground in the circuit of Figure 1 should be higher than the DC voltage of the 16-pin to ground, and the DC voltage of the 2-pin to ground should be higher than the DC voltage of the 1-pin to ground. Only when these two conditions are met at the same time, the DC voltage of the chip IC1's 3-pin to ground can be normal 0V, and the inverter circuit can work normally. If it is found that a certain test voltage does not meet the above relationship, just find the cause of the fault according to the corresponding branch to solve the problem.
IV. Parameters and replacement of main components of vehicle inverter products
The main components in the circuit of Figure 1 are driver tubes SS8550 and KSP44, MOS power switch tubes IRFZ48N and IRF740A, fast recovery rectifier diodes HER306 and PWM control chip TL494CN (or KA7500C).
SS8550 is a PNP type transistor in TO-92 package. The method of identifying its pin electrodes is that when facing the printed identification surface of the transistor, pin 1 is the emitter E, pin 2 is the base B, and pin 3 is the collector C.
The main parameters of SS8550 are: BVCBO=-40V, BVCEO=-25V, VCE(S)=-0.28V, VBE(ON)=-0.66V, fT=200MHz, ICM=1.5A, PCM=1W, TJ= 150℃, hFE=85～160(B), 120～200(C), 160～300(D). The
surface mount device model corresponding to SS8550 in TO-92 package is S8550LT1, and its package is SOT-23.
SS8550 is a common and easy-to-buy transistor in the market, and the price is relatively cheap, with a single price of only about 0.3 yuan.
KSP44 is an NPN transistor in TO-92 package. The identification method of its pin electrode is that when facing the printed identification surface of the transistor, its pin 1 is the emitter E, 2 is the base B, and 3 is the collector C. The
main parameter indicators of KSP44 are: BVCBO=500V, BVCEO=400V, VCE(S)=0.5V, VBE(ON)=0.75V, ICM=300mA, PCM=0.625W, TJ=150℃, hFE=40～200.
KSP44 is a high-voltage transistor commonly used in telephones. When KSP44 is damaged and cannot be purchased, it can be replaced with the transistor KSE13001 commonly used in fluorescent lamp circuits. KSE13001 is a product of FAIRCHILD, and its main parameters are BVCBO=400V, BVCEO=400V, ICM=100mA, PCM=0.6W, hFE=40～80. Although the package form of KSE13001 is also TO-92, the order of its pin electrodes is different from that of KSP44, which should be paid special attention to when replacing. The method of identifying the pin electrodes of KSE13001 is that when facing the printed identification surface of the transistor, its pin electrode 1 is the base B, 2 is the collector C, and 3 is the emitter E.
IRFZ48N is an N-channel enhancement MOS fast power switch tube packaged in TO-220 form. Its pin electrode order 1 is the gate G, 2 is the drain D, and 3 is the source S. The main parameter indicators of IRFZ48N are: VDss=55V, ID=66A, Ptot=140W, TJ=175℃, RDS(ON)≤16mΩ.
When IRFZ48N is damaged and cannot be purchased, it can be replaced with the N-channel enhancement MOS switch tube IRF3205 with the same package form and pin electrode order. The main parameters of IRF3205 are VDss=55V, ID=110A, RDS(ON)≤8mΩ. Its market price is only about 3 yuan each.
IRF740A is an N-channel enhancement MOS fast power switch tube in TO-220 package. Its pin electrode sequence 1 is gate G, 2 is drain D, and 3 is source S. The
main parameter indicators of IRF740A are: VDSS=400V, ID=10A, Ptot=120W, RDS(ON)≤550mΩ.
When IRF740A is damaged and cannot be purchased, it can be replaced with N-channel enhancement MOS switch tube IRF740B, IRF740 or IRF730 with exactly the same package and pin electrode sequence. The main parameters of IRF740 and IRF740B are exactly the same as IRF740A. The main parameters of IRF730 are VDSS=400V, ID=5.5A, and RDS(ON)≤1Ω. Although the parameters of IRF730 are slightly worse than those of the IRF740 series, for inverters with a power below 150W, its parameter indicators are more than enough.
HER306 is a 3A, 600V fast recovery rectifier diode with a reverse recovery time of Trr=100ns. It can be replaced by HER307 (3A, 800V) or HER308 (3A, 1000V). For vehicle inverters with a power below 150W, the fast recovery diode HER306 can be replaced with BYV26C or the most easily purchased FR107. BYV26C is a 1A, 600V fast recovery rectifier diode, and its reverse recovery time Trr = 30ns; FR107 is a 1A, 1000V fast recovery rectifier diode, and its reverse recovery time = 100ns. Considering the reverse recovery time parameter of the device, BYV26C is more suitable for replacement.
TL494CN and KA7500C are PWM control chips. By analyzing various car inverter products on the market, it can be found that some car inverter products use two TL494CN chips, some use two KA7500C chips, and some use one of each of the two chips. What is even more bizarre is that some products are deliberately mysterious, and the logo of one of the TL494CN or KA7500C chips is polished, and then various weird chip models are marked, which makes maintenance personnel feel confused. In fact, as long as you look at the peripheral circuit of the chip, you will know that the chip used must be TL494CN or KA7500C.
After carefully reviewing and comparing the original PDF documents of the TL494CN and KA7500C chips, it was found that the external pin arrangement of the two chips is exactly the same, and even their internal circuits are almost exactly the same. The only difference is that the reference source size of the internal op amp input of the two chips is slightly different, which has no effect on the function and performance of the circuit. Therefore, the two chips can be completely replaced with each other, and the parameters of the peripheral circuit of the chip do not need to be modified when replacing. The successful replacement experience in actual use has also confirmed the feasibility of this replacement and the reliability of the circuit performance after replacement.
Since it is difficult to find the KA7500C chip on the market, and even if it can be bought, its price is at least twice that of the TL494CN chip, the successful experience and method of using TL494CN to directly replace the KA7500C chip introduced here is indeed good news for manufacturers and maintenance personnel of vehicle inverter products.