Common indicators and working principles of vehicle inverters

1. Main indicators of common car 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 operating frequency: 30kHz～50kHz.

2. Circuit diagram and working principle of common vehicle inverter products

The output power of the most popular and common vehicle inverter 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 entire 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 power provided by the car battery into 30kHz-50kHz, 220V AC power 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 power into 50Hz, 220V AC power. 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 rectification and filtering circuits VD5-VD8, C12 and other circuits 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 package 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 is 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 powered 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 turned off, C1 discharges through resistor R2 to ensure that the soft-start circuit works normally when powered on next time.

The R1, Rt, and R2 of the peripheral circuit of the 15th pin 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 value can improve the sensitivity of the overheat protection circuit. The thermistor Rt should be installed 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 15th pin of IC1 to the 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 the 15th pin of IC1 should be slightly higher than the voltage of the 16th pin (connected to the 14th pin of the chip is 5V). The voltage value of 6.2V at normal temperature just meets the requirements and leaves a certain margin.

When the circuit works abnormally, the temperature rise of MOS power tube VT2 or VT4 increases significantly, and the resistance of thermistor Rt exceeds about 4kΩ, the output of comparator 1 inside IC1 will flip from low level to high level, and pin 3 of IC1 will also flip to high level state, causing
the output of PWM comparator, "OR" gate and "NOR" gate inside the chip to flip, and the output stage transistor VT1 and transistor VT2 will turn to 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, 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.

The VDZ1, R5, VD1, C2, and R6 of the peripheral circuit of IC1's 1st pin 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 variation 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 the key circuits for maintaining the soft start time of power-on and the protection state of the circuit. In fact, no matter the control of the circuit soft start or the start 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 at a high level. When pin 3 of IC1 is at a high level, capacitor C3 will be charged. This causes C3 to discharge through R5 after the cause of the protection circuit starts disappears. Because the discharge takes a long time, the protection state of the circuit can still 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 will remain 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 will remain at a constant high level, and the built-in transistors VT1 and VT2 will be 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 will stop 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 elements of the pulse width modulator, and the pulse width modulation frequency determined is fosc=1.1÷(0.0047×4.3)kHz≈50kHz. That is, the operating frequencies of transistors VT1, VT2, VT3, VT4 and 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 12V pulses to 220V pulses. Its primary turns are 20×2 and its secondary turns are 380.

The external capacitor C8 (104) at pin 5 and the external resistor R14 (220k) at pin 6 of IC2 are the timing elements of the pulse width modulator. 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 an overvoltage protection circuit for the 220V output end of the XAC socket. When the output voltage is too high, the voltage regulator VDZ2 will break down, causing the voltage between pin 4 of IC2 and the ground to rise. The protection circuit in chip IC2 will be activated and the output will be cut off.

The MOS tubes VT2 and VT4 in the vehicle inverter circuit have a certain power consumption and must be equipped with a heat sink. Other devices do not need to be equipped with a heat sink. When the vehicle inverter product is continuously used in high-power applications, a small 12V fan must be installed inside 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 the vehicle inverter circuit generally has 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 means that the inverter circuit is not working.

When the LED indicator does not light up after the power is turned on for more than 30 seconds, it is necessary to measure the AC voltage value at the XAC output socket. If the voltage value is about 220V, it means that only the circuit of the LED indicator part is faulty; if the AC voltage value at the XAC output socket is 0, it means that the fault is caused by the inverter circuit of the inverter front stage not working, and the protection circuit inside the chip IC1 may have been activated.
The method to determine whether the internal protection circuit of the chip IC1 is activated 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 activated, otherwise it means that the fault is caused by the action of the non-protection circuit.

If the voltage value of the chip IC1's 3rd pin to the 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 15th and 16th pins of the chip IC1, as well as the DC voltage between the 1st and 2nd pins of the chip IC1. Under normal circumstances, the DC voltage of the chip IC1's 15th pin to the ground in the circuit of Figure 1 should be higher than the DC voltage of the 16th pin to the ground, and the DC voltage of the 2nd pin to the ground should be higher than the DC voltage of the 1st pin to the ground. Only when these two conditions are met at the same time, the DC voltage of the chip IC1's 3rd pin to the 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.

4. Main component parameters and replacement 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 transistor in TO-92 package. The identification method of its pin electrodes is that when facing the printed logo 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 relatively common and easily available triode on the market. It is also relatively cheap, with a single piece costing only about 0.3 yuan.

KSP44 is an NPN transistor in TO-92 package. The method of identifying its pin electrodes is that when facing the printed logo surface of the transistor, its pin 1 is the emitter E, 2 is the base B, and 3 is the collector C.

The main parameters 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 KSE13001, a transistor commonly used in fluorescent lamp circuits. KSE13001 is a product of FAIRCHILD. Its main parameters are BVCBO=400V, BVCEO=400V, ICM=100mA, PCM=0.6W, and 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. Pay special attention to this 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 in TO-220 package. Its pin electrode sequence is 1 for gate G, 2 for drain D, and 3 for source S. The main parameters 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 IRF3205, an N-channel enhancement MOS switch tube with exactly the same package and pin electrode sequence. The main parameters of IRF3205 are VDss=55V, ID=110A, RDS(ON)≤8mΩ. Its market price is only about 3 yuan per tube.
IRF740A is an N-channel enhancement MOS fast power switch tube in TO-220 package. Its pin electrode sequence is 1 for gate G, 2 for drain D, and 3 for source S.

The main parameters 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 the same package form and pin electrode arrangement. The main parameters of IRF740 and IRF740B are exactly the same as those of IRF740A. The main parameters of IRF730 are VDSS=400V, ID=5.5A, RDS(ON)≤1Ω. Although the parameters of IRF730 are slightly worse than those of IRF740 series, its parameters are more than enough for inverters with power below 150W.

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 power below 150W, the fast recovery diode HER306 can be replaced by BYV26C or the most easily available FR107. BYV26C is a 1A, 600V fast recovery rectifier diode with a reverse recovery time of Trr = 30ns; FR107 is a 1A, 1000V fast recovery rectifier diode with a reverse recovery time of 100ns. Considering the reverse recovery time parameter of the device, BYV26C is more suitable for replacement.

TL494CN and KA7500C are PWM control chips. An analysis of various car inverter products on the market shows that some car inverter products use two TL494CN chips, some use two KA7500C chips, and some use one of each chip. What is even more bizarre is that some products are deliberately mysterious, polishing the logo of one of the TL494CN or KA7500C chips, and then marking various weird chip models, which makes maintenance personnel feel confused. In fact, as long as you compare 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 two chips, TL494CN and KA7500C, 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 by each other, and the parameters of the peripheral circuit of the chip do not need to be modified. 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. Therefore, the successful experience and method of using TL494CN to directly replace the KA7500C chip introduced here is indeed good news for the manufacturers of vehicle inverter products and the majority of maintenance personnel.