Simple and practical 12V to 220Vcrt car power supply

In the wild, when repairing a car breakdown, having a picnic or recreational activity in the countryside, or when there is a power outage in some areas, people will feel very inconvenienced without the mains supply. In some cases, the only way is to extend the cable to bring the mains to a distant place, but this is dangerous, impossible, or impractical. In these situations, a power inverter is needed to convert the 12V car battery into 230V AC mains.

The simple idea of ​​a portable mains power supply was first proposed by a student at Aixcom, which usually includes a high-tech power inverter and a special high-current power supply. After a long time of experimentation, the student, Dirk, made a power inverter for the model aircraft club. During his enthusiastic trial production, he encountered the problem of needing a special integrated circuit, which was the core of the project. When he finally got this elusive chip, the price was shocking, and the whole circuit produced a strong popping noise after being connected, which damaged many components.

After the improved design, the results are as follows: the power inverter can provide sufficient power for a long time in the field, and also emits a strong musical momentum. Dirk also developed and produced a 1 kW enhanced inverter in his model aviation club, which worked well for more than a year despite adverse conditions.

Simplified considerations

It seems doubtful that this circuit method is the simplest on the road to making an inverter. In terms of design, any circuit that can be removed is omitted, and the remaining circuit is 100% of the backbone circuit. For example, there is no voltage stabilization part, and the reduction of battery voltage also causes the AC output voltage to decrease. However, because most mains-powered equipment can continuously and well operate within the range of AC voltage changes of ±10% to 15%, the portable inverter should also be the same. Aixcom abandoned the perfect design of performance and focused on simplifying the equipment, reducing component costs and ensuring reliability in use. Nevertheless, there is still short-circuit and low-voltage protection for the 230V AC output, which cuts off the inverter before the battery voltage drops to a level that no longer starts the car. This is a simple enough circuit for beginners to successfully copy and also provides a practical 230VAC power supply.

Pulse Width Modulation

The heart of this circuit is the SG3526 low-cost switching regulator, which is available from several manufacturers under the part number XX3526, where XX is a combination of letters assigned by the manufacturer. The 3526 supports all well-known switching PSU (Power Supply Unit) power supply devices. The basic working principle of the power inverter is shown in Figure 1. The SG3526 alternately switches the direction of the current through the 12V winding of the power transformer, with the center terminals of the two windings connected together to the positive battery terminal (+12). With each switching action, the current direction is changed once, thereby changing the direction of the magnetic field in the transformer core. As a result, a square wave (approximately) AC voltage is generated on the 230V side of the transformer. In practice, the switch is formed by a complementary push-pull circuit of two FET tubes. The source of the FET tube is grounded through a very small resistor.

The input voltage +Vin of SG3526 can be between 7 and 35V, which is used to establish the reference voltage VREF of 5V. When the input voltage is lower than 7V, the voltage protection cuts off the driver. The driver is powered separately through the +VC connection. The frequency is determined by the resistor RT and the capacitor CT, which is 50Hz in this case. The resistance on RD causes a fixed static time between the driver output A and output B. This is done to eliminate the danger of both drivers (because they are two power FETs) being turned on at the same time when the switch is switched.

The capacitor on the CSOFTSTART pin (CSS, ④) allows the output pulse duty cycle (on/off) to slowly rise to 48% after the power supply voltage switch is turned on or reset. The "Amp" voltage regulator is not used in this production. In another case, when the reference voltage is used as the control quantity, it plays an impedance conversion role. This ensures that the output supplies full duty cycle after startup.

If the voltage between +CS and -CS (in other words, the voltage drop across R8) exceeds 100mV, the current limiter formed by the shunt resistor R8 triggers the circuit breaker procedure. However, by connecting it to ground, an external circuit breaker control can also be used. Because the circuit breaker and reset pins (pins ⑧ and ⑤, respectively) are connected together in this circuit, the modulator is restarted with a soft start after an overload or external disconnection.

Several design suggestions

The transformer is a toroidal transformer with a 230V primary and two 12V secondary windings. Readers in countries with 110V, 117V or 127V mains voltage will of course need a 200W transformer. If you are lucky and find an old toroidal transformer in a junk box, it is not difficult to convert it into two 12V windings. Simply wind 10 turns of litz wire around the core and connect the primary to the mains. Measure the voltage across the new winding and calculate how many turns are needed to get 12V. The output power is 200W and the average current is about 10A, so the cross-section (csa) of the litz wire should be 1.5mm2 or larger.

It is important that the two 12V windings have exactly the same number of turns. If they differ by one turn, the transformer core will saturate, causing the regulator to "hover" in the off-circuit state when a 12V battery is connected. The direction of the windings is also important. Before installing the transformer, connect the two 12V winding terminals in series and apply 230VAC to the primary. The voltage measured across the free ends of the secondary should be 24V.

The FETs used in the circuit should be able to handle 55V, 72A, and be marked with 12MΩ RD-S(ON). Of course, other types of FETs can be used, making sure they can handle at least 40V, 40A, and have an RD-S(ON) not exceeding 50MΩ. Usually, power FETs can also be connected in parallel, but make sure that each tube has its own gate resistor. If you want the inverter output power to be greater than 200W, then a parallel structure can be used. In this case, a current limiter can be adapted, using a small shunt resistor R8, or modifying the R16-R17 divider value to achieve this.

Using a high-power inverter to power both ordinary light bulbs and halogen bulbs (powerful lamps) can cause trouble. Both bulbs present a very low "cold" resistance, causing the inverter output voltage to drop, or even drive the circuit breaker out of operation. The result is a lock-in voltage that is insufficient to heat the filament to its normal temperature. Fortunately, the 200W inverter presented here should be able to supply bulbs up to 150W. It should be noted that capacitor C6 can be increased in value, but there is no limit to the range, because the circuit's ability to resist short circuits is sufficient. It is also possible to significantly increase the soft start time by using C5, or not to use it at all. That is probably the safest solution.

Comparator IC1 monitors the battery voltage and ambient temperature, comparing the measurement with the 5V reference voltage from the 3526. In the event of an error, the two open collector outputs pull down the disconnect control input (pin ⑧) to ground potential. A PTC resistor is used to determine the shutdown temperature. Depending on the exact type of circuit, the size of R6 needs to be slightly changed. Early Aixcom inverter prototypes used the D901-D60-A40 (disconnection temperature of 60°C). It is also possible to use a temperature switch with a range of 60°C to 80°C, or a temperature fuse with a range of 90°C. Although the latter is cheaper, it needs to be replaced when it blows.

If a sufficiently large heat sink is used, a simple wire connection is sufficient without a PTC resistor. The voltage monitor turns off at about 12V, and can be adapted to other levels by changing R1 and R5. In the comparator, R2 and R4 determine the size of the delay, which prevents the power inverter from automatically turning on in the event of a fault. After the switch is turned on, the reference voltage rises slowly, and its speed depends on the charging time of C2, so the monitor is activated after a few seconds.

Car batteries can supply dangerously high currents. To prevent the inverter from catching fire, it must be protected with a 25A to 35A automotive fuse. The 230VAC output voltage, even if generated by a battery, is dangerous.

structure

Single-sided printed circuit board is used for the design of printed circuit board. Although it has a large grounding area and wide copper foil strip, it must be thickened by tinning to pass the large current of the transformer. It is recommended to start with the installation of AMP (fast-on) plugs (blade terminals) because they require a lot of force to insert into the circuit board. Don't forget the connection of the shunt resistor. R8 should be installed on the surface of the printed circuit board, slightly upward, to help keep it as cool as possible. R8 can also be replaced by a 5W resistor. Ensure that all polar components (transistors, electrolytic capacitors, diodes and integrated circuits) are installed correctly. When the transistor is installed on the heat sink, an insulating gasket must be used.

Power on debugging

Only one multimeter is needed for this project. Start with the inverter without the transformer. Connect it to the power supply on the test bench and debug the two protection circuits: adjust the input voltage to debug the voltage protection circuit, debug the temperature protection circuit with the help of a soldering iron, and use potentiometers and other tools as needed. In any case, the output is transferred to ground and the LED lights up when the voltage at the positive input of the comparator drops below the voltage at the negative input. If the protection circuit appears to be working, measure the signal on the two grids. If there is an error, the two grids read 0V. In the absence of errors, the oscilloscope shows two clear rectangular wave signals with a pulse width of 10ms. Make the same measurement with a multimeter and read half the power supply voltage.

Only after the debugging is correct can the toroidal transformer be connected. At this point, understand what happens when IC1 is removed from its socket, because in this case, only the current limiter can trigger the circuit breaker function. If a normal 100W bulb does not light up within a few seconds, measure the voltage on the circuit breaker control (pin 8 of 3526 or D2 anode). If the measured value is less than 5V, the current limiter and soft start time are not normal.

Once the bulb is lit, carefully check the inverter for shorts. If you can use an oscilloscope, measure the FET current (voltage across R8) and use R16 to increase the current limit point to 20% below the allowed drain current. This is of course done with the 230VAC output shorted.

Normally, without load, the noise generated by the transformer is greater than expected for normal operation. This is because the rectangular wave transforms the magnetic field strongly and quickly. Under no-load conditions, core saturation is indicated by the transformer's chaotic sound. Measured with an oscilloscope, the current does not rise according to the sawtooth waveform, but has spikes (overshoot). In this case, the transformer 12V winding just needs a few turns. If there is a problem, another method is to choose a lower value of R11 and increase the oscillation frequency a little. The final frequency can be taken as 55Hz, which is not a problem for most loads, but this circuit is not suitable for powering an alarm clock.

Actual Results

The voltage regulation link is omitted for simplicity and cost reasons, and the output voltage is determined by the input voltage. The output voltage of the author's prototype is loaded by a 150W halogen lamp, and the output voltage also depends on the transformer winding and the output current. If you want a 230VAC rated output voltage at a 13VDC input, you should use a transformer with two 11V windings. The maximum efficiency measured by the prototype is 94%.