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Inverter circuit board and maintenance

Published on Mar 17 2011 // Electrical Circuits

A common style car on the market the main indicators of inverter products
Input voltage: DC 10V ~ 14.5V; output voltage: AC 200V ~ 220V ± 10%; output frequency: 50Hz ± 5%; output power: 70W ~ 150W; conversion efficiency: more than 85%; inverter operating frequency: 30kHz ~ 50kHz.
Second, common car inverter circuit diagram and working principle of the product
Sales of the largest on the market today, the most common car inverter output power of 70W-150W, the inverter circuit TL494 or KA7500 mainly based pulse width modulation circuit chip. The most common car an inverter circuit shown in Figure 1.
Of the circuit board inverter can be divided into two parts, each part of the Caiyong a TL494 or KA7500 control circuit chips, where the first part of the circuit’s role is to provide a 12V car battery, etc. DC, through the high-frequency PWM (pulse width modulation) switching power supply technology into 30kHz-50kHz, 220V AC or so; the role of the second part of the circuit is to use the bridge rectifier, filter, pulse-width modulation and switching power output and other technology, 30kHz ~ 50kHz, 220V convert alternating current of about 50Hz, 220V AC.


 Inverter circuit board and maintenance 1

1. Car inverter circuit works
Figure 1 circuit, the chip IC1 and its peripheral circuits, transistors VT1, VT3, MOS power transistor VT2, VT4 and the composition of the transformer T1 12V AC DC inverter transforms 220V/50kHz circuit. IC2 from the chip and its peripheral circuits, transistors VT5, VT8, MOS power transistor VT6, VT7, VT9, VT10, and 220V/50kHz rectifier, filter circuit VD5-VD8, C12, etc. together constitute the high-frequency AC transform 220V/50kHz 220V/50Hz AC frequency conversion circuit, the final output by XAC 220V/50Hz AC outlet for a variety of portable electrical appliances.
Figure 1, IC1, IC2 used TL494CN (or KA7500C) chip to form the core of car inverter control circuit. TL494CN is a special double-ended switching power supply control chip, the suffix letter of CN, said chip package outline dual in-line for the plastic structure, operating temperature range is 0 ℃ -70 ℃, limit the power supply voltage of 7V ~ 40V, Maximum operating frequency is 300kHz.
TL494 chip with 5V reference, voltage regulators, precision 5 V ± 5%, the load capacity of 10mA, and carried out through its 14-pin for external circuit. TL494 chip also built two NPN power output control, can provide 500mA drive capability. TL494 chip’s internal circuitry shown in Figure 2.

 Inverter circuit board and maintenance 2

Figure 1 circuit IC1 of 15 feet external circuit R1, C1 soft-start circuit composed of power. When the power is the voltage across the capacitor C1 gradually increased by the 0V, only when the voltage across C1 reaches 5V or more, it allows the internal pulse-width modulation circuit IC1 to work. When the power supply after power failure, C1 discharges through the resistor R2, to ensure that the next power-soft-start circuit to work.
IC1 15-foot peripheral circuits R1, Rt, R2 components overheat protection circuit, Rt is a positive temperature coefficient thermistor, the resistance at room temperature in the range of 150 Ω ~ 300Ω optional, appropriate larger selection can improve the thermal protection circuit start sensitivity.
When thermistor Rt installed close to the MOS power switches or VT2 VT4 metal heat sink, so as to ensure the effective thermal protection circuit.
IC1 pin 15 to ground voltage U is a more important parameter circuit in Figure 1 U ≈ Vcc × R2 ÷ (R1 + Rt + R2) V, room temperature, the calculated value U ≈ 6.2V. With Figure 1, Figure 2 shows that, under normal operating conditions required IC1 pin voltage should be slightly higher than the 15 16-pin voltage (pin 14 connected to the chip as 5V), the voltage value of 6.2V at room temperature just to meet the size requirements, and slightly left a certain margin.
When the circuit is abnormal, MOS power transistor VT2 VT4 temperature rise or a substantial increase in the resistance of thermistor Rt when more than about 4kΩ, IC1 internal comparator 1 output will be low turn high, IC1 3 feet then turning to a high state, causing the PWM comparator chip, "or" gate and "or" gate of the output of flip occurred, the output stage transistors VT1 and VT2 transistors are turned OFF state. When the two power output of IC1 tube end, the Figure 1 circuit VT1, VT3 will be extremely low because of the base while the saturated conduction, VT1, VT3 conduction, the power transistor gate VT2 and VT4 will be no positive result bias and is off, power inverter circuit to stop working.
1 IC1 pin peripheral circuit VDZ1, R5, VD1, C2, R6 constitute a 12V input supply voltage protection circuit, the regulator VDZ1 the regulator protection circuit determines the value of the start threshold voltage value, VD1, C2, R6 is also composed of protection of the state to maintain the circuit, as long as the occurrence of transient phenomena of the input power over-voltage protection circuit will start and remain for some time after class to ensure the safety of the power output tube. Taking into account the vehicle was in the normal battery voltage changes in the size range, typically the value of zener regulator VDZ1 selected 15V or 16V is more appropriate.
3-pin peripheral circuit IC1 of C3, R5 constitute the soft-start time to maintain power and maintain the critical state of circuit protection circuit, in fact, either the control or soft start circuit protection circuit of the start control, the final results are reflected in the IC1 3 feet on the state level. Circuit protection circuit on power-up or start, IC1 3 feet high. When IC1′s 3 feet high, the capacitor C3 will be charged. This results in protection circuit starts the incentive disappears, C3 discharges through R5, the discharge time required for a longer, making the circuit of the protection status is still maintained for some time.
When IC1′s 3 feet high, will be along the R8, VD4 to charge the capacitor C7, while the voltage across the capacitor C7 provide IC2 4 feet, 4 feet to IC2 is held high status. Chip from the circuit of Figure 2 shows, when the 4 pin is high, it will raise the dead-time chip phase comparator input with the potential to make the comparator output remains constant high level, the "or" door , "or" door to built-in transistor transistor VT1 and VT2 are closed. Figure 1 circuit VT5 and VT8 in saturated conduction state, the subsequent level management VT6 and VT9 MOS gate will be no positive bias and for all is off, power inverter circuit to stop working.
5-pin external capacitor IC1 C4 (472) and 6-pin external resistor R7 (4k3) for the pulse width modulator of the timing components, determined by the PWM frequency is fosc = 1.1 ÷ (0.0047 × 4.3) kHz ≈ 50kHz. The circuit transistor VT1, VT2, VT3, VT4, the operating frequency of the transformer T1 are 50kHz or so, so T1 high-frequency ferrite core transformer should be used, the role of the transformer T1 is 12V to 220V step-up pulse pulse , the primary number of turns for the 20 × 2, 380 secondary turns.
5-pin external capacitor IC2 C8 (104) and 6 pin external resistor R14 (220k) for the pulse width modulator of the timing components, determined by the PWM frequency is fosc = 1.1 ÷ (C8 × R14) = 1.1 ÷ (0.1 × 220) kHz ≈ 50Hz.
R29, R30, R27, C11, VDZ2 composition of XAC 220V outlet circuit output overvoltage protection, when the output voltage is too high will cause the regulator VDZ2 breakdown, the IC2 pin 4 to ground voltage rise, the protection of the chip IC2 circuit action, cut off the output.
Car inverter circuit MOS tube VT2, VT4 has some power, heat sink must be installed, no need to install additional heat sink devices. When the car power inverter products continue to be applied where the larger, the need to install in its internal 12V small fan to help cooling.

2. Circuit components parameters
Circuit parameters of the components listed in the Annex.

 Inverter circuit board and maintenance 3

III. Car inverter product maintenance points
The inverter circuit board generally has the power soft-start function, so we have to wait after power-5s-30s will be after the 220V AC output, and LED indicator lights up. When the LED indicator is not lit, then the inverter did not work.
When the power is more than 30s, LED light has not lit, you need to measure the AC outlet XAC output voltage value, if the voltage is around 220V normal, then only part of the circuit LED lights appeared failure; if XAC by measuring the AC voltage at the output socket is 0, then the cause of the malfunction before the inverter stage of the inverter circuit does not work is probably the protection circuit inside the chip IC1 has been launched.
Internal protection circuitry to determine whether the chip IC1 is started: with a multimeter to measure DC voltage blocking chip IC1′s pin 3 to ground DC voltage value, if the voltage is above 1V is illustrated in the on-chip protection circuit has been launched, or that failure cause of action due to non-protection circuit.
If the chip IC1 to ground voltage of 3 feet above the value of 1V, indicating that the chip protection circuit is activated, the need to further block the DC voltage with a multimeter test chip IC1 of the DC voltage between the feet 15,16, and chip IC1′s DC voltage between 1,2 feet. Under normal circumstances, the circuit chip IC1 in Figure 1, 15 feet on the ground DC voltage should be higher than 16 feet on the ground DC voltage, 2 feet on the ground of the DC voltage should be higher than 1 foot on the ground DC voltage, only when these two conditions are met, the chip IC1 3 feet on the ground to the normal DC voltage of about 0V, inverter circuit to work properly. If that does not meet the test voltage of a relationship, just press the corresponding branch to find the cause of the malfunction, you can solve the problem.
IV. Car inverter main components of the product parameters and substitution
The main circuit in Figure 1 device has driven tube SS8550, KSP44, MOS power switch IRFZ48N, IRF740A, fast recovery rectifier diode and the PWM control chip HER306 TL494CN (or KA7500C).
SS8550 form for the TO-92 package PNP-type transistor. The pin electrode identification, when the printing for the logo side transistor, the pin 1 is the emitter E, 2 for the base B, 3 for the collector C.
SS8550 The main parameters for the: 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).
TO-92 package with the SS8550 form the corresponding surface mount device model S8550LT1, the package is SOT-23.
SS8550 is more common on the market today, easy to buy the transistor, the price is relatively cheap, the price alone just 0.3 yuan.
KSP44 TO-92 form for the NPN-type transistor package. The pin electrode identification, when the printing for the logo side transistor, its emitter pin 1 E, 2 for the base B, 3 for the collector C.
KSP44 the main parameters as: BVCBO = 500V, BVCEO = 400V, VCE (S) = 0.5V, VBE (ON) = 0.75V, ICM = 300mA, PCM = 0.625W, TJ = 150 ℃, hFE = 40 ~ 200 .
KSP44 is commonly used in high-voltage transistor telephone, when KSP44 damaged and can not buy, can be used in the common fluorescent lamp circuit transistor KSE13001 for substitution. KSE13001 FAIRCHILD products for the main parameters BVCBO = 400V, BVCEO = 400V, ICM = 100mA, PCM = 0.6W, hFE = 40 ~ 80. KSE13001 the package while the same for the TO-92, but the pin electrode but with KSP44 different sort, which is in substitution to pay attention. KSE13001 pin electrode identification method is to identify when the printing surface facing the transistor, its base pin electrode 1 B, 2 for the collector C, 3 for the emitter E.
IRFZ48N form for the TO-220 package, N-channel MOS-enhanced fast power switch. The pin 1 is the sort gate electrode G, 2 for the drain D, 3 for the source S. IRFZ48N the main parameters as: VDss = 55V, ID = 66A, Ptot = 140W, TJ = 175 ℃, RDS (ON) ≤ 16mΩ.
When IRFZ48N damage can not be bought, the electrodes can be sorted package and pin the same N-channel enhancement mode MOS switch IRF3205 for substitution. The main parameters for IRF3205 VDss = 55V, ID = 110A, RDS (ON) ≤ 8mΩ. Its market price is only about 3 yuan each.
IRF740A form for the TO-220 package, N-channel MOS-enhanced fast power switch. The pin 1 is the sort gate electrode G, 2 for the drain D, 3 for the source S.
IRF740A the main parameters as: VDSS = 400V, ID = 10A, Ptot = 120W, RDS (ON) ≤ 550mΩ.
When IRF740A damage can not be bought, the electrodes can be sorted package and pin the same N-channel enhancement mode MOS switch IRF740B, IRF740 or IRF730 for substitution. IRF740, IRF740B IRF740A the main parameters and the same. IRF730 The main parameters for VDSS = 400V, ID = 5.5A, RDS (ON) ≤ 1Ω. Although the parameters of which IRF730 IRF740 slightly worse than the series, but for 150W power inverter is the following, the parameter index is already more than sufficient.
HER306 for the 3A, 600V fast recovery rectifier diode, the reverse recovery time Trr = 100ns, available HER307 (3A, 800V) or HER308 (3A, 1000V) for substitution. Following the 150W power inverter car, one of the fast recovery diode HER306 can BYV26C or most likely to buy the FR107 for substitution. BYV26C as 1A, 600V fast recovery rectifier diode, the reverse recovery time Trr = 30ns; FR107 is 1A, 1000V fast recovery rectifier diode, the reverse recovery time = 100ns. Reverse recovery time from the device to consider the parameter index, substitution is more appropriate when used BYV26C more.
TL494CN, KA7500C for the PWM control chip. On a variety of car currently on the market to analyze the inverter products can be found, and some car inverter products used in the two TL494CN chip, some use the two KA7500C chip, there are two chips use a different even more strange is that some products actually trick, or will one of them TL494CN chip ID KA7500C were polished and then marked with a variety of odd chip model, so that maintenance personnel feel more confused. In fact as long as the control chip peripheral circuits to look, that must be used TL494CN chip or KA7500C.
After careful inspection, contrast TL494CN, KA7500C two chips original pdf data and found that the two external pins arranged in exactly the same chip, and even its internal circuit is also almost identical, only difference is the internal operation of two chips reference input put a little bit difference in size, functionality and performance of the circuit has no effect, so these two chips can replace the use of each other, and the substitution of the external circuit when the parameters of the chip do not have to make any changes. The actual use experience of the successful substitution, also confirmed the feasibility of such substitution and the substitution after the performance of circuit reliability.
As the market has been difficult to find a KA7500C chips, and even if they can buy, its price is at least more than twice the chip TL494CN, TL494CN therefore presented here direct substitution KA7500C chip using the successful experience and approaches, for the car reverse Variable Product manufacturers and the general maintenance staff is indeed a good news.