Introduction
Owners
Meghan Dang Engineer
Daniel Puratich Reviewer
Project Goals
Adding protection to this buck converter to ensure we don’t blow any of them up!
Altium Files
Previous Rev: https://warg.365.altium.com/designs/F25BBA87-F3F3-4A89-9BEF-E1A42A8DDA7B
used as a starting place for this design, but ultimately this design is a lot more than a rev 2
This Rev: https://warg.365.altium.com/designs/1283F32D-4074-4D12-B0B3-2243CA1F31A2
Table of Contents
Protection Requirements
Input
Overvoltage protection up to 60V
reverse polarity protection up to -60V
Output
Overcurrent protection: no component damage (operation not required though) if excessive current is drawn. auto retryIn theory the IC already supports this though there is some doubt as per RCA: Fried Boards
Overvoltage protection: no component damage to the board to 60Vso:either put 2 IC’s, one at input one at outputorimplement something else for overcurrent and over voltage
Dropping this design requirement in favor of saving time
Sundry Changes
layout match 30x30mm Mounting Hole Specifications
A single power indicator LED
Objectives
high efficiency
protect for alternative (unintended) ground paths (im forcing high side switching with this requirement, but for good reason, we often short things out to GND accidentally).
minimize cost, component count, complexity, and board space
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
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
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
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
typ. for 24V system
price
$3.73 per piece for 10
price
$5.3 per piece for 10
features
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
TPS48110 https://www.digikey.ca/en/products/detail/texas-instruments/TPS48110AQDGXRQ1/17748316
suitable for
12V-24V-48V system designs
things to note:
Place a TVS diode at the input to clamp the voltage transients during hot-plug and fast turn-off events
this is a overvoltage cut-off system
is it still good ? ? ? ? ?
yes can just add TVS diode with 17V rating to clamp voltage at input - refer to 9.5 Layout
voltage range
3.5V-80V
operating temp
-40 ~ 125 C
output RPP
-30V (look into increasing this)
is increasing possible?
is there input RPP?
back to back nMOS’s
adjustable overcurrent protection
IWRN, ISCP
adjustable undervoltage lockout
UVLO
adjustable overvoltage protection
+- 2% (CHECK IF CLAMP OR NOT)
protection threshold programming using resistor ladder
calculated using equations 19 & 20
component selection for this
resistors
RSNS (eq 13)
RSET (recc. range - 50-100 ohms)
RISCP (eq 15)
RSNS
RIWRN (eq 14)
RIMON based on eq 21
capacitors
based on fault timer period
CTMR
MOSFETs
Q1
read data sheet and base ratings off of the things in first sentence
Bootstrap Capacitor, CBST
based on equation 17 in DS
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
R2
R1
RSNS
RSET
RISCP
RIWRN - NOT NEEDED
RIMON
FETs
capacitors
10nF 25V
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
feedback resistor
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 https://uwarg-docs.atlassian.net/wiki/spaces/EL/pages/2232221707/WARG+Schematic+Symbol+and+Footprint+Guidelines?search_id=b7cb4267-4616-4e90-8a9d-d911a8e7ce35
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 https://docs.px4.io/main/zh/flight_controller/cuav_pixhawk_v6x.html
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
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