Automobile laptop power supply without winding elements. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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To power most laptops, a constant voltage of about 19 V is required. Known automotive power converter circuits for them are built on the principle of a step-up pulse converter using a transformer or a storage choke. Unlike them, the proposed device implements a push-pull inverter with a diode-capacitor voltage doubler.

Stabilization of the output voltage is carried out by the method of pulse-width (SHI) regulation.

Main Specifications

• Input voltage, V ....... 11 ... 15
• Output voltage, V ....... 19
• Maximum output current, A ....... 4,7
• Efficiency, % .......61...83
• Conversion frequency, kHz ....... 25
• Dimensions, mm ....... 130x85x60
• Weight, kg ....... 0,6

Rice. 1 (click to enlarge)

The scheme of the device is shown in fig. 1. The device is based on a specialized chip KR1114EU4 (DA1), which is a push-pull SHI controller. This microcircuit generates rectangular pulses with controlled duration. Elements C4 and R7 set the frequency of the internal oscillator of the microcircuit to about 25 kHz, and the divider R3R4 sets the minimum pause between pulses of about 8 μs (approximately 20% of the pulse repetition period). This pause prevents the flow of through current through the transistors during switching. Stabilizing feedback is made using a voltage divider R1R2R6 and an internal op-amp of the DA1 chip. The inputs of this op amp (pins 1 and 2 of the DA1 chip) receive feedback and reference voltage signals, and the control voltage is formed at the output of the op amp (pin 3 of the DA1 chip).

Capacitor C1 smooths out the voltage ripple coming from the output of the converter. The frequency correction of the op-amp is carried out by means of a circuit on the elements R5 and C3. The outputs of the DA1 chip (pins 8-11) - the collectors and emitters of its output transistors - control the inverter on two complementary pairs of powerful transistors VT1-VT4 connected in a bridge circuit.

A voltage doubler is connected to the output of the inverter, containing a Schottky rectifier VD1 - VD4 and capacitors C5-C7. Resistors R9 and R12 limit the output current of the DA1 chip to 0,17 ... 0,25 A and, accordingly, the base current of transistors VT1-VT4 in order to prevent their overload. The current through the bases of these transistors is chosen so as to limit the current of their collectors at the level of 5 ... 10 A.

The doubler works like this. Suppose an internal transistor is open between pins 8 and 9 of the microcircuit. At this time, transistors VT1 and VT4 are open, and VT2 and VT3 are closed. In this case, the capacitor C5 is charged through the diode VD1, and C6 through VD4 gives charge to the capacitor C7, which supplies the load. This is followed by a pause, during which both internal output transistors of the microcircuit and transistors VT1 and VT4 are closed. After a pause, the internal transistor opens between pins 10 and 11 of the microcircuit and transistors VT2 and VT3 open, while VT1 and VT4 remain closed. In this case, the capacitor C6 is charged through the diode VD2, and the capacitor C5 through VD3 gives a charge to the capacitor C7. Then a pause follows again, during which all transistors VT1-VT4 are closed, after which the process repeats. Capacitor C2 suppresses interference in the input power circuit, and also prevents the penetration of impulse noise generated by the converter into the vehicle's on-board network.

When the input voltage drops below the minimum allowable value (it is 10...11 V and depends on the load current), the converter exits the voltage stabilization mode, its output voltage decreases.

Fig. 2

The appearance of the assembled board of the device is shown in fig. 2. Chip KR1114EU4 (DA1) can be replaced by analogues MB3759R, TL494CN, KA7500V, IR9494. Instead of transistors KT8102B (VT1, VT3) and KT8101B (VT2, VT4), you can use KT8102A and KT8101A, respectively. Transistors VT1 and VT2 should be installed on one heat sink, P "3 and VT4 - on the other. The area of ​​\u200b\u2beach heat sink is about XNUMX cmXNUMX. On a complementary pair transistor with a lower base current transfer coefficient, there is more voltage drop and heat dissipation, so installing transistors in each arm bridge to a common heat sink makes it possible to equalize their thermal conditions without the need for electrical isolation of transistors from the heat sink.

Schottky diodes 2D219A (VD1-VD4) can be replaced by others from the 2D219, 2D2998, KD2998 series. These diodes are installed without heat sinks. You can use MBR1035, MBR1045, KD271-KD273 with indices "A" or "AC", installing each diode on a separate heat sink with an area of ​​10 ... 15 cm2 or through insulating gaskets on a common heat sink with an area of ​​60 cm2. Diodes KD271 - KD273 with other indices are suitable, as well as series KD213, 2D231, 2D251, 2D252, 2D2992, 2D2993, KD2995-KD2997, but the heat sink area per diode must be increased to 25 cm2.

Oxide capacitors C2, C5-C7 must be designed for a permissible pulse current of at least 3 A and have the lowest possible equivalent series resistance (ESR), i.e., belong to the "Low ESR" category. This reduces the output voltage ripple and improves the reliability of the device. For example, Jamicon capacitors of the WL, TL or TZ series are suitable. If necessary, each of them can be replaced by several identical capacitors connected in parallel. In this case, we can tentatively assume that the allowable pulse current grows in proportion to the number of connected capacitors.

Since a significant impulse current flows in the power circuits of the device, when wiring the printed circuit board, it is important that the common wire and the positive power bus used in the power part of the device are connected to the corresponding conductors of the low-current part at the terminals of the capacitor C2. Otherwise, generation failures and other malfunctions of the device are possible.

The engine of the tuned resistor R1 sets the output voltage of the converter, it can be in the range of 18 ... 20 V. The connections of the converter input to the on-board network and the output to the laptop are made in the same way as in the previous design.

Author: K. Gavrilov

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