Application of self-made vehicle-mounted power supply DC converter The DC converter circuit introduced in this article is simple and practical, easy to manufacture, low cost, and the efficiency can reach 96%. It is very easy to use in the car or where there is no mains electricity. The cost of DC converters is much lower than that of inverters. In many places, DC converters can be used to replace inverters, such as mobile phone chargers, satellite receivers, and notebook computers in automobiles. If the output of the DC converter is designed to be 300V DC, the voltage after 220V AC is rectified and filtered is consistent.
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The circuit is shown in Figure 1. The core device of the circuit is the pulse width modulation control chip TL494CN, which is an integrated circuit for switching oscillation and voltage regulation control. Pin 9 and pin 10 output switching pulses with opposite phases. Control VT1 ～ VT4 respectively. When pin 10 is a positive pulse, VT1 is turned off, VT3 is turned on, VT2 is turned on, and VT4 is turned off. When pin 9 is a positive pulse, VT2 is turned off and VT4 is turned on. A boosted high-frequency voltage is generated on the secondary of the transformer, which is then bridge-rectified and filtered to become a DC high-voltage output of approximately 300V. Pin 1, pin 16, pin 2, pin 15 of the integrated circuit TL494CN are the non-inverting input terminal and inverting input terminal of the two comparators in the integrated circuit, respectively. Use these two comparators to achieve over-temperature protection and over-voltage protection of the circuit.
1. Over-temperature protection circuit:
Pin 14 is the 5V reference voltage in the integrated circuit. Pin 16 is connected to this voltage, which is also 5V. The condition for circuit reversal is whether the potential of pin 15 is lower than the 5V voltage of pin 16. If it is lower than this voltage, the circuit will flip and the output will stop. This circuit incorporates a voltage stabilizing diode VD8 to protect the over-temperature protection circuit from the 12V voltage drop. At room temperature, the voltage of 15 feet is U × R1 ÷ (R1 + R5 + Rt), U is the stable voltage of the voltage regulator VD8, which can be 9V. The calculation result is that the voltage of 15 feet is about 5.97V, which is slightly higher than the potential of 16V 5V . When the temperature of the power tube is too high, the resistance value of the thermistor with a positive temperature coefficient becomes larger, and the voltage at pin 15 drops until the comparator turns over after 5V, and the output stops.
2. Overvoltage protection circuit.
The overvoltage protection circuit is composed of VD9, R8, VD1, R4, and C2. It is completed by another comparator inside TL494CN. When the input voltage exceeds 15V, VD8 is turned on. The potential of pin 2 of TL494CN is higher than the potential of pin 1 Invert the comparator and stop the output. Both of these comparator flips are displayed on pin 3 of TL494CN. After the comparator flips, the potential of pin 3 rises. When pin 3 becomes high, it charges C3. When the factor that causes the comparator flips disappears, C3 passes R8 discharge, because the discharge takes a certain time, so the protection circuit can also maintain a period of time, which is conducive to the safety of the output tube. 
The key to the production process is the high-frequency transformer. The high-frequency ferrite core selected by the author is of type EI33, because the operating frequency of this circuit is about 54kHz. This frequency is caused by the capacitors C4 and 6 on the 5 pin of TL494CN. Determined by the resistor R7. Because the frequency is higher, the diode should use fast recovery, especially the four bridge rectifier diodes VD4 ~ VD7 and VD2, VD3 at the output end must use fast recovery diodes. The winding method of the transformer coil is: first wrap 140 turns with Φ0.49mm enameled wire as a secondary, and then use Φ0.96mm enameled wire double wire and wind 6 + 6 turns with a middle tap.
The thermistor adopts positive temperature coefficient, and it must be closely attached to the power tube when used. If there is no problem with the circuit installation, it can be successfully powered on at one time without debugging. The power of this product is 150W, the power tube must add heat sink. The output end of the circuit must be connected to a 1MΩ resistor R12. Some beginners may think that it is useless. In fact, without it, the output voltage will rise to more than 400 volts, which will exceed the withstand voltage value of the output filter capacitor. It is better to choose a larger output filter capacitor, which can be selected between 10uF ～ 100uF, so that the current is large when the output is short-circuited, which is a protection for the output tube. After the circuit is completed, it is safer to string a fuse on the lead line . If it is unsuccessful, use a multimeter to measure the 3-pin voltage. Normally, no voltage above 1V can appear.
Note: The output of this power converter is DC 300V, which is only suitable for small power switching power supply.