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Automotive 12V to USB 5V 2A output power adapter.

1 year ago 613
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Automotive 12V to 5V USB charger assembled PCB

This project presents a nominally 12V to 5V step down power adapter for use in a motor vehicle to supply 5V at up to 2A from a USB socket.

It is intended to be plugged into a cigar lighter socket in a car or other vehicle and can be used with any device that is normally charged from a 5V USB power outlet at up to 1.8A.

Although most recent cars already have built in USB sockets suitable for such use, some may not be capable of supplying enough current to charge devices such as tablets and smartphones. Many cars designed before about 2010 may not have USB outlets at all.

However, most vehicles have cigar lighter outlets allowing a high current connection directly into the vehicle 12V supply. This is clearly unsuitable for use to directly charge 5V input devices so some form of voltage reduction and - due to the noisy and very roughly regulated battery voltage behaviour - voltage regulation is required.

Many of the designs for 12V to 5V adapters already freely available on the web use linear regulation, which will dissipate somewhere in the region of 15W to 20W at a 2A load at maximum alternator output and so require significant heat sinking.

Most do not provide any form of input over-voltage protection against alternator load dump (See: or input reverse polarity connection.

Most do not provide any form of output over-voltage protection in the event of a short circuit failure of the series voltage regulation element, which could result in the connection of the 12V input supply directly to the 5V output.

It is not clear what level of protection is provided by commercially available devices offered for sale.

A review of many of the designs and products offered on the web did not inspire confidence in the safety of connecting an expensive, high-end tablet or smartphone to any of them.

This project presents a design offering clearly defined protection of both input and output connections.

1) Input voltage

8V to 55V DC

2) Load Dump and input reverse polarity protection:

The input side is designed to operate from the nominal 12V vehicle supply but is able to withstand transients, including - in the event of accidental or intermittent vehicle battery disconnection - alternator load dump, up to 55V. It is also protected against accidental reverse polarity connection of the input supply.

3) EMI filtering:

The input side also has low pass filtering to reduce noise being imported into the regulator from the vehicle supply.

The same filtering reduces switching noise being exported from the regulator into the vehicle supply to minimise the possibility of interference with other vehicular equipment.

The filtering is also designed to ensure the stability of the input to the regulator at all input voltages.

4) Output short circuit and over-voltage protection:

The 5V output is protected against output short circuit faults.

The 5V output is also protected against device failure that could result in the 12V input being imposed directly onto the 5V output, such as a series switch transistor failure in the step down regulator chip. In the event of such a failure, the instantaneous voltage imposed on the 5V output is clamped to just below 5.7V. A few milliseconds after the output over-voltage is clamped to 5.7V, a series 2A fuse blows, providing a permanent disconnection from the vehicle supply.

The presence of input and output voltages are indicated by separate green (input) and red (output) LEDs.

5) Output voltage and current:

5V+/-0.25V at up to 1.8A (limited by the rating of the output side USB connector, the regulator chip can supply up to 2A with an up-rated connector).

  • Please note however, that this is not a cheap project to build for two reasons.

a) the L4978 switch mode regulator is one of the few through hole parts available that meet the 8V to 55V input voltage range requirement. Whilst there are cheaper devices available in surface mount packaging, in the interests of making this project accessible to the widest range of users, it was decided to avoid the need for any specialist soldering skills or equipment;

b) The design uses high quality components that are specified and sourced to meet strict design requirements. As is the rule for any switch mode power supply design, it is strongly recommended to build this project using only the specified components or alternatives of known equivalent specifications.

Buying cheap parts of unknown specifications can result in damage to the adapter, the load on charge and possibly to the user through unexpected overheating and consequential risk of fire and even explosion.

  • L4978 Datasheet:

  • Applications Note:

AN1061: Designing with L4978, 2A high efficiency DC-DC convertery:

  • The SMPS stage is designed according to Applications Note: AN1061


based on the specification given in:

4.1 Electrical Specification

Input Voltage range: 8V-55V

Output Voltage: 5.1V ±3% (Line, Load and Temperature)

Output ripple: 34mV

Output Current range: 1mA-2A

Max Output Ripple current: 20% Iomax

Current limit: 3A

Switching frequency: 100kHz

Target Efficiency: 85%@2A Vin = 55V, 92%@0.5A Vin = 12V


SMPS copy

Notes copy



ID Name Designator Footprint Quantity Manufacturer Part Manufacturer Supplier Supplier Part Price LCSC Assembly Mounted Description Supplier order code Alternate supplier Alternate supplier order code Notes
1 L4978 U1 DIP8 1 L4978 8V - 60 Buck converter in DIP8 package from 6868234 Farnell 1077138
2 220n C5,C7,C8,C1,C13 C_MLCC_RES_DIP_5.5x3mm_P5mm 5 C330C224K1R5TA Farnell KEMET C330C224K1R5TA Multilayer Ceramic Capacitor, Gold Max, C330 Series, 0.22 µF, ± 10%, X7R, 100 V, Radial Leaded 1457691
3 120u L2 PANASONIC_ELC18B121L_120uH_3A 1 ELC18B121L Farnell PANASONIC ELECTRONIC COMPONENTS ELC18B121L CHOKE, 120UH, 3.A, 0R065 1749226
4 SB360 D3 SB360_DO-201AD_H 1 SB360-E3/54 Farnell VISHAY SEMICONDUCTOR SB360-E3/54 Rectifier Diode, Single, 60 V, 3 A, 680 mV, 100 A, 150 °C 9550380
5 2N3906 Q1 2N3906_TO92_TRIPOD 1 2N3906 Farnell MULTICOMP 2N3906 Bipolar (BJT) Single Transistor, High Speed Switching, PNP, 40 V, 250 MHz, 625 mW, 200 mA, 100 1574372
6 220u C2,C9 C_ALU_D10mm_P5mm 2 EEUFR1J221L Farnell PANASONIC ELECTRONIC COMPONENTS EEUFR1J221L Electrolytic Capacitor, FR Series, 220 µF, ± 20%, 63 V, 10 mm, Radial Leaded 2079302
7 470u C3,C4,C12 C_ALU_D12.5mm_P5mm 3 EEUFR1J471 Farnell PANASONIC ELECTRONIC COMPONENTS EEUFR1J471 Electrolytic Capacitor, FR Series, 470 µF, ± 20%, 63 V, 12.5 mm, Radial Leaded 2079308
8 100m R2,R3 RC07 2 LR1LJR10 Farnell TE CONNECTIVITY LR1LJR10 Through Hole Resistor, LR Series, 0.1 ohm, 500 mW, ± 5%, 350 V, Axial Leaded 2330244
9 SC-03-06G T1 Kemet_SC-03-06G 1 SC-03-06G Farnell Kemet 600uH 3A Common Mode Choke 2364241
10 LED_RED D4 LED3MM 1 703-0090 (Any approximately 2V forward drop, red, 3mm lead spacing LED. Almost any sized LED can be fitted if the leads are bent or wired to the PCB.) Farnell MULTICOMP 703-0090 LED, 3MM, RED, 100MCD, 643NM 2112100
11 STP36NF06L M1 STP36NF06L_H 1 STP36NF06L Farnell STMICROELECTRONICS STP36NF06L MOSFET Transistor, N Channel, 30 A, 60 V, 0.032 ohm, 10 V, 2.5 V 9935614
12 1N5401G D1 1N5401G_DO-201AD_H 1 1N5401G Farnell TAIWAN SEMICONDUCTOR 1N5401G Rectifier Diode, Single, 100 V, 3 A, 1.1 V, 125 A, 150 °C 1863154
13 20k R4 RC07 1 Various Farnell 20k 250mW Metal Film 2% or better. 9341498
14 2.7n C6 C_MLCC_RES_DIP_5.5x3mm_P5mm 1 D272K25Y5PH63J5R Farnell VISHAY BC COMPONENTS D272K25Y5PH63J5R Ceramic Disc & Plate Capacitor, D Series, 2700 pF, ± 10%, Y5P, 100 V, Radial Leaded 1835023
15 22n C10 C_MLCC_RES_DIP_5x3mm_P2.5mm 1 K223K15X7RF53L2 Farnell VISHAY K223K15X7RF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 0.022 µF, ± 10%, X7R, 50 V, Radial Leaded 1141773
16 9.1k R5,R1 RC07 2 Various Farnell 9.1k 250mW Metal Film 2% or better. 9342320
17 1.2k R6,R9 RC07 2 Various Farnell 1.2k 250mW Metal Film 2% or better. 9341226
18 2.4k R7,R11,R12 RC07 3 Various Farnell 2.4k 250mW Metal Film 2% or better. 9341595
19 FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM F1 FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM 1 64600001003 Farnell LITTELFUSE 64600001003 Fuse Holder, 250 V, 250 V, 6.3 A, Cartridge Fuse Holder, Solder Pin, 1 1271691 Fuse is: ANTISURGE, 2A, 5X20MM (Farnell order code: 1271686)
20 LED_GREEN D2 LED3MM 1 703-0087 (Any approximately 2V forward drop, green, 3mm lead spacing LED. Almost any sized LED can be fitted if the leads are bent or wired to the PCB.) Farnell MULTICOMP 703-0087 LED, GREEN, 3MM, 572NM, STANDARD 2112096
21 1u L1 MURATA_11R102C_1uH_3A 1 11R102C Farnell MURATA POWER SOLUTIONS 11R102C INDUCTOR, 1UH, 20%, 3A, TH RADIAL 2062664
22 unplated_M3_clearance_hole 3M5_H1,3M5_H2,3M5_H3,3M5_H4 hole_3mm5 4 n/a You don't have to buy these: they come free with the PCB.
24 1.5k R8 RC07 1 Various Farnell 1.5k 250mW Metal Film 2% or better. 9341323
25 100 R10 RC07 1 Various Farnell 100R 250mW Metal Film 2% or better. 9341099
27 220p C11,C14 C_MLCC_RES_DIP_5x3mm_P2.5mm 2 K221J15C0GF53L2 Farnell VISHAY K221J15C0GF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 220 pF, ± 5%, C0G / NP0, 50 V, Radial Leaded 1141767
28 USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-0003 SKT1 USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-0003 1 Farnell MOLEX 48405-0003 USB, 3.0 TYPE A, RECEPTACLE, TH 2401585


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