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

2 years ago  

License: CC-BY-NC-SA 3.0

Status: Complete

SMPS12V to 5V adapterUSB chargerL497812V to 5V converter5V USB car adapter
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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: https://en.wikipedia.org/wiki/Load_dump) 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 ST.com 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


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


IDValueQty.PackageComponentsManufacturer PartDescriptionSupplierSupplier order codeAlternate supplierAlternate supplier order codeNotes
1L49781DIP8U1L49788V - 60 Buck converter in DIP8 package from st.comuk.rs-online.com6868234Farnell1077138
2220n5C_MLCC_RES_DIP_5.5x3mm_P5mmC5,C7,C8,C1,C13C330C224K1R5TAKEMET C330C224K1R5TA Multilayer Ceramic Capacitor, Gold Max, C330 Series, 0.22 µF, ± 10%, X7R, 100 V, Radial LeadedFarnell1457691
4SB3601SB360_DO-201AD_HD3SB360-E3/54VISHAY SEMICONDUCTOR SB360-E3/54 Rectifier Diode, Single, 60 V, 3 A, 680 mV, 100 A, 150 °CFarnell9550380
52N390612N3906_TO92_TRIPODQ12N3906MULTICOMP 2N3906 Bipolar (BJT) Single Transistor, High Speed Switching, PNP, 40 V, 250 MHz, 625 mW, 200 mA, 100Farnell1574372
6220u2C_ALU_D10mm_P5mmC2,C9EEUFR1J221LPANASONIC ELECTRONIC COMPONENTS EEUFR1J221L Electrolytic Capacitor, FR Series, 220 µF, ± 20%, 63 V, 10 mm, Radial LeadedFarnell2079302
7470u3C_ALU_D12.5mm_P5mmC3,C4,C12EEUFR1J471PANASONIC ELECTRONIC COMPONENTS EEUFR1J471 Electrolytic Capacitor, FR Series, 470 µF, ± 20%, 63 V, 12.5 mm, Radial LeadedFarnell2079308
8100m2RC07R2,R3LR1LJR10TE CONNECTIVITY LR1LJR10 Through Hole Resistor, LR Series, 0.1 ohm, 500 mW, ± 5%, 350 V, Axial LeadedFarnell2330244
9SC-03-06G1Kemet_SC-03-06GT1SC-03-06GKemet 600uH 3A Common Mode ChokeFarnell2364241
10LED_RED1LED3MMD4703-0090 (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.)MULTICOMP 703-0090 LED, 3MM, RED, 100MCD, 643NMFarnell2112100
11STP36NF06L1STP36NF06L_HM1STP36NF06LSTMICROELECTRONICS STP36NF06L MOSFET Transistor, N Channel, 30 A, 60 V, 0.032 ohm, 10 V, 2.5 VFarnell9935614
121N5401G11N5401G_DO-201AD_HD11N5401GTAIWAN SEMICONDUCTOR 1N5401G Rectifier Diode, Single, 100 V, 3 A, 1.1 V, 125 A, 150 °CFarnell1863154
1320k1RC07R4Various20k 250mW Metal Film 2% or better.Farnell9341498
142.7n1C_MLCC_RES_DIP_5.5x3mm_P5mmC6D272K25Y5PH63J5RVISHAY BC COMPONENTS D272K25Y5PH63J5R Ceramic Disc & Plate Capacitor, D Series, 2700 pF, ± 10%, Y5P, 100 V, Radial LeadedFarnell1835023
1522n1C_MLCC_RES_DIP_5x3mm_P2.5mmC10K223K15X7RF53L2VISHAY K223K15X7RF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 0.022 µF, ± 10%, X7R, 50 V, Radial LeadedFarnell1141773
169.1k2RC07R5,R1Various9.1k 250mW Metal Film 2% or better.Farnell9342320
171.2k2RC07R6,R9Various1.2k 250mW Metal Film 2% or better.Farnell9341226
182.4k3RC07R7,R11,R12Various2.4k 250mW Metal Film 2% or better.Farnell9341595
19FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM1FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MMF164600001003LITTELFUSE 64600001003 Fuse Holder, 250 V, 250 V, 6.3 A, Cartridge Fuse Holder, Solder Pin, 1Farnell1271691Fuse is: ANTISURGE, 2A, 5X20MM (Farnell order code: 1271686)
20LED_GREEN1LED3MMD2703-0087 (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.)MULTICOMP 703-0087 LED, GREEN, 3MM, 572NM, STANDARDFarnell2112096
211u1MURATA_11R102C_1uH_3AL111R102CMURATA POWER SOLUTIONS 11R102C INDUCTOR, 1UH, 20%, 3A, TH RADIALFarnell2062664
22unplated_M3_clearance_hole4hole_3mm53M5_H1,3M5_H2,3M5_H3,3M5_H4n/aYou don't have to buy these: they come free with the PCB.
241.5k1RC07R8Various1.5k 250mW Metal Film 2% or better.Farnell9341323
251001RC07R10Various100R 250mW Metal Film 2% or better.Farnell9341099
27220p2C_MLCC_RES_DIP_5x3mm_P2.5mmC11,C14K221J15C0GF53L2VISHAY K221J15C0GF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 220 pF, ± 5%, C0G / NP0, 50 V, Radial LeadedFarnell1141767
28USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-00031USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-0003SKT1MOLEX 48405-0003 USB, 3.0 TYPE A, RECEPTACLE, THFarnell2401585



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Very nicely done. To adapt for other output voltages, would you simply have to adjust the R6/R7 voltage divider ratio? Or would that cause problems for the overvoltage protection portion downstream? I would like to build something similar with multiple outputs at common voltages (3.3V, 5V, 6V, 7.4V).




If you want to individual overvoltage protection limits for each output then you need to change the overvoltage protection Rtop and Rbot values using the formula that is given in the schematic. It depends on how far above the required output voltage the respective overvoltage limit needs to be set. This design is set to ensure no damage can occur to a USB charger input circuit. The absolute maxiumum voltage of these are not well specified compared to say the 7V limit typical of many 5V logic devices.

You need to carefully check the requirements of whatever is connected as your load.

Changing R6 and R7 will change the output voltage of the SMPS according to the formula for Rtop1 and Rbot1 in the schematic but this is not like a linear regulator. If you change the output voltage then, strictly, you need to redesign the rest of the SMPS stage: the loop compensation R5, C10, C11 and L2, C12.

That said the L4978 design is reasonably tolerant of changes in output voltage for a given set of R5, C10, C11, C12 and L2. There is a table of R6 and R7 (Rtop1 and Rbot1) values for a range of common output voltages given in Figure 21 of the ST apps note AN1061 at:


If you do change the output voltage, I would recommend testing the SMPS stage into a dummy load with line and load transients to ensure that the circuit is stable: it doesn't have any unexpected overshoots, undershoots or instability before connecting it to a real load!

Overshoots in particular need to be checked an excessive overshoot may turn the overvoltage clamp on and so could possibly blow the fuse.




@example Thanks for the thorough reply! Good point about Rtop and Rbot. My initial thought was that leaving these as-is would simply cause Vclamp to scale with voltage coming out of the SMPS. But, yeah, it probably warrants looking more closely at expected loads on each of the supplies.


It's very useful, thank you!

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