12V->5V @ 5A Buck Converter Board

Owned by Daniel Puratich
Last updated: 2024-08-10 by Meghan Dang
8 min read

Introduction

Table of Contents

Protection Requirements

Protection Justification

We want these boards to be very safe to use so they can be handled by inexperienced members. Our system is 50V maximum so 60V is a bit of margin on that.

This is mostly a learning project for the start of the co-op to get you familiar with our Altium standards, but is also a fun design challenge and will be useful for the team! Whatever circuit is decided upon can be leveraged for other projects as well!

The circuit designed here should be transferable to other projects with similar voltage and current requirements. This allows us to copy paste this circuit around!

 

Questions

  • when you say pad 33 should i make another pin in the schematic or what

  • i just copied the 10k resistor on input side and put it on output side is that fine or should i find a separate resistor

  • i found a capacitor that’s 10uf and 50V but

    • +-20%

    • X5R

    • 0805

  • is the footprint top down as usual ? ?

  • for decoupling caps do the

Design

 

understand project objectives and existing design

  • What are buck converters?

    • voltage regulator that steps down a high input voltage to a lower output voltage

    • consist of:

      • inductor

      • capacitor

        • filter capacitor

        • reduces ripple voltage at load (resistor)

      • resistor

      • active switch

        • typically a transistor (MOSFET)

        • connects and disconnects the inductor from primary source

      • passive switch

        • diode

        • open/close based in terminal voltages - reverse/forward biased

      • Asynchronous

        • active and passive switch

      • Synchronous

        • two active switches (diode is replaced by MOSFET)

    • CCM

      • continuous conduction mode

      • iL > 0 for all t during CCM

  • Analysis of Buck Converters

    • Assume average values are constant

      • Active Switch is Closed/On

        • diode acts as open circuit

        • VL (across inductor) = Vin - Vo (across resistor)

        • iL increasing

      • Active Switch is Open/Off

        • current flow interrupted

        • negative voltage spike develops at inductor

        • diode conducts current → short circuit

        • VL = -Vo

        • iL decreasing

Designing Buck Converters

  • Design Requirements

    • Specs:

      • Nominal Input Voltage, Vin

      • Nominal output Voltage, Vout

      • Nominal Output Current, Iout

      • Efficiency, u

      • Switching Frequency, fs

    • Component Sizing

      • Inductor

        • Inductor Current Ripple Ratio, r = iL/iout

        • Inductor Ripple Current, iL

        • Inductance, L = Vout(1-D)(fsiL)

        • Component should comfortable support desired Iout (Irms > Iout)

        • Consider DCR of inductor (power losses)

      • Capacitor

        • Capacitor Ripple Voltage, Vo

        • Capacitance, C = io/(8fsVo)

      • Diode

        • power losses arise from forward voltage drop

        • Schottky diodes

          • low fwd voltage drops

          • reverse recovery not a problem

        • max current greater than Io

        • max repetitive voltage of diode should be greater than Vin

        • Mount temp

          • 85 C

      • MOSFET

        • metal-oxide semiconductor field-effect transistor

        • drain to source resistance, Rds

          • Vds = IdsRds

          • AVG current rating - IoD

          • Vds > Vin

        • Junction temp Tj

        • Rise & Fall time

  • Efficiency & Power Losses (ASYNC)

    • Efficiency

      • u = Pin/Pout

      • Ptotal loss < Po(i/u - 1)

    • Power Losses

      • inductor

        • PL = Io^2*RESR

      • diode

        • Pd=IoVf(1-D)

      • mosfet

        • conduction loss

          • Pcond - I^@RdsD

        • switching loss

          • image-20240510-151606.png

Protecting Power Paths

  • Power Switch

    • provides electrical connection from voltage sourve or ground to load

    • saves power

    • protects subsystems

    • component protection

    • inrush current protection

    • minimizes PCB size

    • Load Switch

      • safe and reliable distribution of power

        • power distribution

        • power sequencing

        • inrush current control

        • reduced current leakage

    • Integrated Power MUX (Multiplexer)

      • similar to load switch

        • multiple input sources

    • eFuses & Hot Swap

      • additional input power path protection functions

        • current sense monitoring

        • current limiting

        • undervoltage/overvoltage protection

        • thermal shutdown

      • good for hot-plug & transient events that would typically damage system components

        • reduce maintenance costs

        • maximize equipment uptime

      • how they work

        • short-circuit transient event

          • current through efuse increases very rapidly

        • overvoltage event on VIN

          • efuse monitors voltage across internal FET

          • clamps output voltage until VIN falss below threshold

        • overtemperature protection

          • shuts down FET if junction temperature exceeds 150 C (typ)

    • Ideal diode, ORing

      • protect against reverse-polarity conditions

        • monitors external fet

        • reduces power loss

        • blocks reverse current

      • in transient event

        • monitors and adjusts external FET to prevent damage

    • Smart high-side

      • off board load protection

        • monitors outload current

        • detects short-circuit and open-load events

      • adjustable current limits

        • reliable integration to applications with large inrush current or low peak currents

      • robust solution for driving cap, ind, and LED loads

    • Low-side

      • connects load to ground

        • integrated flyback diode

        • eliminate inductive load transients

        • dissipating current in a circular loop

        • solenoids, relays, motors

  • Efuses

    • uses current mirror circuit to measure current without external sense resistor

 

image-20240510-170835.png

  •  

Reverse Current Blocking

  • what is reverse current

    • when Vo > Vin

      • causes current to flow backwards through the system

    • when does it happen?

      • 1. power disconnected and Vin drops

      • 2. MOSFET used for load-switching and body diode becomes bwd biased

    • reverse current / voltage is NOT negative voltage (reverse polarity)

    • why it should be blocked

      • damages internal circuitry and pow sups

      • can damage cables and connectors

      • can cause combustion of MOSFETS

    • when should it be blocked

      • power multiplexing

        • switching between multiple pow sups

        • if differences in pow sups high, can cause reverse current

      • ORing

        • each ORing switch sees reverse current when other switch is closed

      • sudden loss of input power

        • voltage on output of switch falls slower than input

          • reverse current will flow across switch

        • to avoid:

          • have a switch with reverse current blocking or larger input than output capacitance

    • how to block reverse current

      • diodes

        • highV, low I apps

        • downsides

          • fwd voltage drop

          • increases pow disspiation

          • drops pow sup by 0.6V-0.8V

          • causes decrease in efficiency and shortened battery life

        • solution

          • schottky diode

          • has lower fwd voltage drop

          • more expensive

          • high reverse current leakage

      • back to back MOSFETs

        • conclusion: not good

      • backwards MOSFET

        • no reverse current when MOSFET is off

        • downside

          • can not switch supply offf

      • switching MOSFET budy terminal

        • typ not possible

    • power switches that block reverse current

      • load switches

        • swithces body terminal of mosfet

        • always-on reverse current blocking

        • downside

          • some reverse current still flows

      • eFuses

        • back to back MOSFET implementation

        • some are always-on

        • some only block when off

    • ti power switch portfolio

Reverse Polarity Protection

  • what is reverse polarity

    • negative voltage

    • reverse connected input power supply

    • reversed battery connection

    • miswiring of field power supply lines

    • typical solution

      • schottky diodes

        • downside

          • power loss from fwd conduction requires thermal management

          • higher power density needs for efficiency

    • other solutions

      • discrete MOSFETs

        • p-channel MOSFET

          • when polarity reversed

          • gate source voltage swings positive, MOSFET turns off, protecting downstream circuits

        • n-channel MOSFET on low side

        • downsides

          • lack of reverse current blocking

          • power dissipation at low VIN

          • size and cost of p-channel

      • ideal diode controller

        • drives external n-channel MOSFET

        • low fwd voltage drop

        • no reverse current

      • eFuses

        • withstand -60V reverse voltage without damage

        • integrated MOSFET replaces external blocking diode

        • benefits

          • improved efficiency

          • reduced system cost and space

          • thermal management is simpler

Overvoltage Protection

  • why is it needed?

    • all components are made for a particular V

    • higher than that V can cause damage to component

    • protects downstream components or clamps output

    • transient overvoltages

      • caused by electrostatics discharge (ESD)

      • voltage ringing from hot-plugs

      • inductive switching surges

    • continuous over voltages

      • present for long periods and stress systems indefinitely

      • caused by

        • failure / miswiring of

          • upstream power supplies

          • voltage regulators

          • converters

        • insertion of

          • noncompliant adapters into a system

    • common protection methods

      • transients

        • ESD diodes

        • transient voltage suppressors (TVSs)

          • nonsecond response time

          • used for low-voltage

        • Zener diodes

          • steer overcurrents to ground plane

          • clamp overvoltages

        • metal-oxide varistors (MOVs)

          • clamps overvoltages

          • absorbs more energy than the former

          • slower response time

          • better for AC mains or high-voltage DC stages

      • continuous

        • overvoltage lockout

          • OVLO pin monitors VIN rail through resistor divider

          • internal comparator turns FETs off

          • eFuse is better and faster at this

          • switch is off until VIN falls below threshold

        • overvoltage clamping

  • Efuse block diagram

image-20240513-175144.png

 

  •  

  • fundamentals of power electronics

 

  • select topology options for solving the challenge

    • eFuse between input and load

    • external nMOS

    • potential external Rilim

  • component selection for each option

    • eFuse options

      • TPS26621

        • typ. for 24V system

        • price

          • $3.73 per piece for 10

        •  

      • TPS2663x

        • price

          • $5.3 per piece for 10

        • features

          •  

      • TPS2663x

        • intended for 24V supply

        • price

          • $3.8 per piece for 10

        • features

          • operating voltage

            • 4.5V-60V

          • adjustable current limit

            • 0.6A-6A

          • reverse polarity protection

            • external N-channel FET

          • reverse current blocking

            • 0.17us

            • can be supported with the nMOS FET

          • over voltage clamp

            • 35V-39V

          • overvoltage protection at 34V

          • PGOOD look this up

        • operating temp

          • -40°C ~ 125°C

        • system example

 

  • overload protection equation

 

 

 

high side switch

 

  • Pros and Cons

    • eFuse

      • pros

        • less to implement

        • has all necessary features

      • cons

        • overvoltage clamping fixed at ~38V

          • needs to be clamped much lower (12V-5V buck with 17V abs max)

        • expensive

    • high-side driver

      • pros

        • wider input range

        • several additions can be made

        • clamping is NOT fixed, can be adjusted using TVS diode and cutoff can also be adjusted with resistor divider

      • cons

        • requires more external components

        • less functionality and already implemented features

        • also expensive

  • BJT current sensing

    • lowkey unnecessary

Resistor Calculations

  • Resistor OVLO ladder

    • R3

      • 31.6kΩ

    • R2

      • 88.7kΩ

    • R1

      • 330kΩ

 

 

  • RSNS

    • 10mΩ

  • RSET

    • 100Ω

  • RISCP

    • 1.3kΩ

  • RIWRN - NOT NEEDED

    • 39.2kΩ

  • RIMON

    • 9.76kΩ

FETs

  • back 2 back

capacitors

  • 10nF 25V

    • CTMR

      • 68nF 10%

    • CBST

      • 88nF 10%

  • LED circuit

    • resistor

      • 1K ohms

    • LED

      • green LED from stock

    • when choosing a resistor for LED, set it to around 2mA-3mA and calculated based off of that fwd current, indicator LEDs should be easy to look at.

  •  

 

12s servo module

  • capacitor

    • 1uF 25V

      • put in altium ggs

  • feedback resistor

    • RT0603BRD0759K7L

 

  • inductor selection:

    • low dcr

      • ~20mOhm

    • look at graphs

    •  

 

  • Optimizing final design

    • IWRN to GND because

      • overcurrent protection not needed

    •  

Component creation in Altium library

In general, followed all specs from

  • Resistors

    • all 0603, used existing symbols and footprints

    • Capacitors

      • 0603 & 0805, used existing symbols and footprints

    • Inductor

      • used existing symbol and created custom footprint

    • ICs

      • made custom symbol and footprint

      • edited symbol as needed to best suit the application

Schematic Styling

  • place smaller caps closer to IC

  • make power path clear

  • all gnds on same sides should be at equal heights

  • make pullup/pulldown resistors obvious

  •  

 

  • output voltage for pixhawk and RPi

    • pixhawk

      • operating voltage range is 4.75-5.25V

      • maximum operating voltage is 5.7V

      • make voltage on higher end because max operating voltage is higher

      • around 5.15 volts?

    • RPi https://datasheets.raspberrypi.com/rpi4/raspberry-pi-4-datasheet.pdf

      • ideal voltage is 5.1V

      • absolute max is 6V

      • aim for 5.1/slightly higher

    • decided on 5.3 voltage due to voltage drop across wires

  • placement

    • minimize feedback loop

    • big puzzle

    • use common sense

    • place caps in order of least to greatest from IC

    • make them easy to connect to one another, either in line, or around a single straight corner

    • make the power flow easy to read

    • all components should be relatively easy to spot/ you should know what you’re looking at

  • board dimensions

    • 37mmx37mm

  • layout

    • make traces as small as possible/fewest corners

    • polygons should have all or mostly 45 degree corners

    • thermal reliefs on gnd pads of high current pads

      • LEDs

      • connectors

      • big inputs

    • lock in placement before starting layout, makes everything easier

      • can also actually lock placement by turning off primitives

    •  

  • what meme should i put?

    • spongebob and patrick: besties 4evah

 

Validation

  • Ordering

    • we might wait for other boards to order a bunch at a time

    • board

    • components

  • Testing

    • writeup test plans first

    • do protections work

    • does board work

  • Further validation gets pretty grindy imo and and we can skip

 

Simulations

 

simulated input caps on LTSpice to prevent problems like

Test Plan

Protections:

  • test if RPP works

    • plug in battery opposite way

  • test if overvoltage works

    • supply with higher voltage than its operating range

      • 80V

Board:

  • does it step down input to ~5V

    • test with supply that is within operating range (6~17V)

    • test with power supply at all these ranges

  • can it power RPi and Pixhawk

    • plug them into their respective connectors and find out

  • does indicator LED turn on

    • look at it

  • measure efficiency @ 12V input & 1-3A

  • plu

 

Efficiency

 

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