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).
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.
AN1061: Designing with L4978, 2A high efficiency DC-DC convertery:
The SMPS stage is designed according to ST.com Applications Note: AN1061
DESIGNING WITH L4978, 2A HIGH EFFICIENCY DC-DC CONVERTERY
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, firstname.lastname@example.orgA Vin = 12V
|1||L4978||U1||DIP8||1||8V - 60 Buck converter in DIP8 package from st.com|
|2||220n||C5,C7,C8,C1,C13||C_MLCC_RES_DIP_5.5x3mm_P5mm||5||KEMET C330C224K1R5TA Multilayer Ceramic Capacitor, Gold Max, C330 Series, 0.22 µF, ± 10%, X7R, 100 V, Radial Leaded|
|3||120u||L2||PANASONIC_ELC18B121L_120uH_3A||1||PANASONIC ELECTRONIC COMPONENTS ELC18B121L CHOKE, 120UH, 3.A, 0R065|
|4||SB360||D3||SB360_DO-201AD_H||1||VISHAY SEMICONDUCTOR SB360-E3/54 Rectifier Diode, Single, 60 V, 3 A, 680 mV, 100 A, 150 °C|
|5||2N3906||Q1||2N3906_TO92_TRIPOD||1||MULTICOMP 2N3906 Bipolar (BJT) Single Transistor, High Speed Switching, PNP, 40 V, 250 MHz, 625 mW, 200 mA, 100|
|6||220u||C2,C9||C_ALU_D10mm_P5mm||2||PANASONIC ELECTRONIC COMPONENTS EEUFR1J221L Electrolytic Capacitor, FR Series, 220 µF, ± 20%, 63 V, 10 mm, Radial Leaded|
|7||470u||C3,C4,C12||C_ALU_D12.5mm_P5mm||3||PANASONIC ELECTRONIC COMPONENTS EEUFR1J471 Electrolytic Capacitor, FR Series, 470 µF, ± 20%, 63 V, 12.5 mm, Radial Leaded|
|8||100m||R2,R3||RC07||2||TE CONNECTIVITY LR1LJR10 Through Hole Resistor, LR Series, 0.1 ohm, 500 mW, ± 5%, 350 V, Axial Leaded|
|9||SC-03-06G||T1||Kemet_SC-03-06G||1||Kemet 600uH 3A Common Mode Choke|
|10||LED_RED||D4||LED3MM||1||MULTICOMP 703-0090 LED, 3MM, RED, 100MCD, 643NM|
|11||STP36NF06L||M1||STP36NF06L_H||1||STMICROELECTRONICS STP36NF06L MOSFET Transistor, N Channel, 30 A, 60 V, 0.032 ohm, 10 V, 2.5 V|
|12||1N5401G||D1||1N5401G_DO-201AD_H||1||TAIWAN SEMICONDUCTOR 1N5401G Rectifier Diode, Single, 100 V, 3 A, 1.1 V, 125 A, 150 °C|
|13||20k||R4||RC07||1||20k 250mW Metal Film 2% or better.|
|14||2.7n||C6||C_MLCC_RES_DIP_5.5x3mm_P5mm||1||VISHAY BC COMPONENTS D272K25Y5PH63J5R Ceramic Disc & Plate Capacitor, D Series, 2700 pF, ± 10%, Y5P, 100 V, Radial Leaded|
|15||22n||C10||C_MLCC_RES_DIP_5x3mm_P2.5mm||1||VISHAY K223K15X7RF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 0.022 µF, ± 10%, X7R, 50 V, Radial Leaded|
|16||9.1k||R5,R1||RC07||2||9.1k 250mW Metal Film 2% or better.|
|17||1.2k||R6,R9||RC07||2||1.2k 250mW Metal Film 2% or better.|
|18||2.4k||R7,R11,R12||RC07||3||2.4k 250mW Metal Film 2% or better.|
|19||FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM||F1||FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM||1||LITTELFUSE 64600001003 Fuse Holder, 250 V, 250 V, 6.3 A, Cartridge Fuse Holder, Solder Pin, 1|
|20||LED_GREEN||D2||LED3MM||1||MULTICOMP 703-0087 LED, GREEN, 3MM, 572NM, STANDARD|
|21||1u||L1||MURATA_11R102C_1uH_3A||1||MURATA POWER SOLUTIONS 11R102C INDUCTOR, 1UH, 20%, 3A, TH RADIAL|
|22||unplated_M3_clearance_hole||3M5_H1,3M5_H2,3M5_H3,3M5_H4||hole_3mm5||4||You don't have to buy these: they come free with the PCB.|
|23||TL431AILP||U2||TL431AILP_TO92_TRIPOD||1||TEXAS INSTRUMENTS TL431AILP. VOLTAGE REF, SHUNT, 2.495V-36V, TO-226AA|
|24||1.5k||R8||RC07||1||1.5k 250mW Metal Film 2% or better.|
|25||100||R10||RC07||1||100R 250mW Metal Film 2% or better.|
|26||M023.5MM||JP1||SCREWTERMINAL-3.5MM-2||1||CAMDENBOSS CTB3051/2BK TERMINAL BLOCK, WIRE TO BRD, 2POS|
|27||220p||C11,C14||C_MLCC_RES_DIP_5x3mm_P2.5mm||2||VISHAY K221J15C0GF53L2 Multilayer Ceramic Capacitor, Mono-Kap Series, 220 pF, ± 5%, C0G / NP0, 50 V, Radial Leaded|
|28||USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-0003||SKT1||USB_3.0_TYPE_A_RECEPTACLE_PTH_MOLEX _48405-0003||1||MOLEX 48405-0003 USB, 3.0 TYPE A, RECEPTACLE, TH|