24V-48V->12V @ 3A Buck Converter Board
- Meghan Dang
- Kevin Tu
Introduction
Owners
@Kevin Tu Engineer
@Meghan Dang @Andrew Chai Reviewer
Project Goals
Protected buck board that takes input from 6S-12S and steps it down to 12V
Reverse Polarity
Overvoltage
Undervoltage
Overcurrent
Project Requirements
Input voltage: ~24V-48V
Output voltage: 12V @ 3A
Documentation:
IC selection:
Buck Converter Options:
$5.75
4.3V - 60V Input
0.8V - 50V, 3A Output
4.89 mm x 3.90 mm
Protection features include overtemperature shutdown, VOUT overvoltage protection (OVP), VIN undervoltage lockout (UVLO), cycle-by-cycle current limit, and short-circuit protection with frequency foldback.
$6.16
6V - 75V Input
1.225V - 70V, 3A Output
6.50 mm x 4.40 mm
Fault protection features include, current limiting, thermal shutdown and remote shutdown capability.
$7.40
3.6V - 48V Input
0.9V- 18V, 3A Output
6.50 mm × 4.40 mm
Switch Current-Limit Protection, Short-Circuit and Overcurrent Protection of FET
$10.44
4V - 60V Input
1.23V - 57V, 3A Output
10.16 mm × 8.51 mm
The output switch includes cycle-by-cycle current limiting, as well as thermal
shutdown for full protection under fault condition.
Selected Buck Converter: LMR16030SDDAR
4.3-V to 60-V input range
3-A continuous output current, 40-µA operating quiescent current, 1-µA shutdown current
155-mΩ high-side MOSFET
Current mode control
Adjustable switching frequency from 200 kHz to 2.5 MHz
Thermal, overvoltage and short protection
8-pin HSOIC with PowerPAD™ package
External Component Selection:
Frequency Selection (subject to change): 500kHz
By extension, Rt will be a resistor with a value of 49.9kΩ.
RC0603FR-07499KL (0603 Footprint)
$0.16
Voltage Divider Resistance Selection:
To calculate the values of the resistors of the voltage dividers, the ref voltage value is to be considered first. Based on the datasheet to minimize the effects of noise RFBB has been selected to have a value of 10kΩ. Calculating the ratio of RFBT to have a value of 150kΩ.
RFBT : RC0603FR-07150KL (0603 Footprint)
$0.16
RFBT : RC0603FR-0710KL (0603 Footprint)
$0.16
Feed forward Capacitor:
This capacitor ensures high frequency feedback can bypass the resistor divider network, allowing for faster transient response time, at low frequencies this capacitor has high impedance and is negligible.
$0.22
0603 size, 100V, 390pF, +-5% tol, C0GNP0
Inductor Selection:
Using the formula given in the datasheet, and selecting a K value of 0.3 (Medium Ripple, Medium Space) the minimum inductance is found to be 20 Micro Henrys. We will select an inductor with an inductance of 22 Micro Henrys as they are more common and cheaper.
$1.58
22 µH Shielded Drum Core, Wirewound Inductor 4 A 43mOhm Max Nonstandard
Output Capacitor Selection
For a target ripple of 1%, the minimum equivalent capacitance of the output Capacitor is found to be 3.4091µF. The Equivalent Series Resistance of the capacitors must also be less than 73.3 mΩ.
Capacitor Selection:
$0.32
@ 12V 25 °C, 500kHz
Effective Capacitance: 1.47 micro Farads
ESR: 3.89 mΩ
$0.22
@ 12V 25 °C, 500kHz
Effective Capacitance: 0.473 micro Farads
ESR: 14.9 mΩ
Need to consider Phase Margin as well. /// Add LT Spice sims later
Based on Texas Instrument’s WEBENCH tool this board will be selecting
2x 35SVPF39M
Electrolytic Capacitor rated 35V 39uF with 30 mΩ ESR
@12V 25 °C, 500kHz
Effective Capacitance: 3.92
ESR: 2.36 mΩ
When the 3 capacitors are in parallel we achieve a equivalent capacitance of 3.413 micro Farads.
Diode Selection:
The diode selected will need a max reverse voltage rating of 25% greater than our maximum input voltage. In this case the diode needs a rating of 60V. When selecting the diode the reverse current leakage and forward voltage must also be considered to reduce power loss.
$0.62
Diode 60 V 3A Surface Mount SOD-123H
Input Capacitor Selection:
Based on the datasheet guidelines we will select a decoupling capacitor with a capacitance of 0.1μF, and a voltage rating of >= 96V to account for the derating of ceramic capacitors. Similarly we will choose decoupling capacitor with a capacitance of 4.7μF and a rating of >= 96V.
Medium Decoupling Capacitor GRM31CZ72A475KE11L
$1.10
4.7 µF ±10% 100V Ceramic Capacitor X7R 1206 (3216 Metric)
1.32uF effective
ESR 4.62mOhm
For Sims
0.005 ohm
0.0005 uH
1.32uF
High Decoupling Capacitor GRM188R72A104KA35J
$0.22
0.1 µF ±10% 100V Ceramic Capacitor X7R 0603 (1608 Metric)
0.054 uF effective
ESR 59.7mOhm
For Sims:
0.02 Ohm
0.0003 uH
0.054uF
Bulk Capacitor GRM32EC72A106KE05L
$1.74
10 µF ±10% 100V Ceramic Capacitor X7S 1210 (3225 Metric)
2.7uF effective
2.03mOhm
For Sims
0.004 Ohms
0.0003 uH
2.7 uF
Boot strap Capacitor Selection:
Based on the datasheet the recommended capacitor is 0.1 μF and rated 16 V or higher.
$0.26
0.1 µF ±10% 16V Ceramic Capacitor X7R 0603 (1608 Metric)
Reverse Polarity Protection
Considered using a high side drive, however decided to use a simple discrete gate with a PMOSFET instead as other than reverse polarity protection the other necessary fault protections are built into the buck converter IC. Furthermore most high side drive ICs (eg. TPS1H100) require external circuitry to achieve reverse polarity protection.
Reference Video: How to protect circuits from reversed voltage polarity!
P MOSFET Selection:
The P MOSFET needs a drain source voltage rating significantly higher than the maximum input of 48V. The continuous drain current must also be greater than 3A.
$1.16
P-Channel 60 V 14A (Tc) 1.7W (Ta) Surface Mount TO-252-3
Gate Source Voltage of +- 20
$1.40
P-Channel 60 V 7.7A (Ta) 1W (Ta) Surface Mount POWERDI3333-8
Zener Diode Selection
Since the gate source voltage of our P MOSFET is +- 20, and the Rds is best at -10V. We need to choose a Zener diode with a voltage rating between 10V and 20V.
$0.22
Zener Diode 15 V 200 mW ±6% Surface Mount SOD-323
Resistor
For the pull down resistor we will be using a 100kOhm resistor
$0.16
100 kOhms ±1% 0.1W, 1/10W Chip Resistor 0603 (1608 Metric) Moisture Resistant Thick Film
Overall Expected Cost
5.75 + 0.16 + 0.16 + 0.16 + 1.58 + 0.32 * 2 + 0.22 + 0.62 + 1.10 + 0.22 + 0.26 + 1.16 + 0.22 + 0.16
= $12.41