Introduction

WARG-Altium365 Link: https://warg.365.altium.com/designs/74D65A2E-05AC-4B8C-AE55-D8D307F760C7#design

• Who

  • Reserved for @Kevin Li co-op student

  • @Robert Tang doing the engineering

  • high level architecture defined by @Daniel Puratich

• What

  • Buck Converter

    • Buck Converters

  • 6S LiHv input

    • 27V absolute max on LiHv

  • 5V output voltage

  • rpp

    • Reverse Polarity Protection Circuits

    • Kind of nice because autonomy software people like to use this hardware

  • 5 A output current

  • 30.5x30.5 mount pattern

    • Mounting Hole & Pattern Specifications

  • usb-c (for rpi) and jst (for pixhawk) output conns

    • Pixhawk Electrical Connector Standards

    • USB-C connects to the raspberry pi easily, can copy circuit from 12V->5V @ 5A Buck Converter Board

    • Pixhawk standard connector for POWERx style port so this can be used as a redundant supply

    • To be clear, both of these output connectors should be present. Only one will be used in any given system though (so both should be sized on the PCB to handle the full 5 A of current).

  • xt30 input connector or just solderpads for input

    • both in library already (2.54mm pitch solderpads)

    • if solderpads are used, give space for epoxy so we can strain relieve

    • clear polarity label would be nice if solderpads are used

• Why

  • Powering the Raspberry Pi 5 for Fixed Wing 2025 if we decide we want to mount the RPi into this system and dont want to use RPi Interface Rev C . Currently there is no plan to do this but it is nice. Ground testing power supply for the raspberry pi is also a use case, but 12V->5V @ 5A Buck Converter Board kind of accomplishes this already.

  • Redundant Pixhawk power supply for Fixed Wing 2025 . See https://docs.px4.io/main/en/flight_controller/pixhawk6x.html for details.

Engineering

@Robert Tang to fill out when the time comes

Background Knowledge

Buck Converter

DC-DC Step-down voltage regulator.

Open

Diagram of a buck converter

Components:

• Source (Vin)

• Switch

  • Usually a MOSFET, because manually switching on/off the switch is tooo slow

• Diode

  • In synchronous buck converter, a second MOSFET is added

• Inductor

  • Stores and release energy (as current)

• Capacitor

  • Smooth the voltage output

Calculating the conversion factor in CCM

*In this application we are only interested in Continuous Conduction Mode (CCM), where essentially i_L is larger than 0 for all time during CCM.

Some assumptions to be made before the calculation:

1. Assume CCM

2. Average steady state (Over period, the average values will be constant)

3. Vin and Vout constant with respect to time

4. Diodes and FETs are ideal

The calculation:

When the switch is on, during a time T_on the circuit looks like the following, where the diode acts like a open circuit (because of the reverse polarity).

Open

Switch on

The Inductor voltage, and the rate of change of current through the inductor can be tested using the following equation, their graphs are also shown.

When the switch is off, (after T_on) for a time T_offthe diode acts like a wire, and the voltage source (Vin) is replaced by an open circuit, the inductor is supplying current to the rest of the circuit.

Here the inductor voltage and the current will change

Combine the two we can get the graphs for the change in inductor voltage and current over a period T, and we also know that T_on+T_off=T

Since we have assumed 'average steady state', which implies that the change in current are the same, so we can equate the two changes in current

Finally we get the transfer function Vout/Vin=D, the duty cycle

As for how to design a buck converter, I referred to Kevin’s documentation on bucker converter IC Buck Converters

Synchronous Buck Converter

For maximum efficiency the buck converter we selected will be synchronous, with a little bit of research, I discovered that the main difference is that the diode is replaced/connected in parallel with another FET. The turning on and turning off of the switch is controlled by the on/off of the two FETs.

The benefit for that is the increased efficiency and reduced diode loss (R_dson*I_d<Vf*I)

Power losses

Inductors: has DC Resistance (DCR) loss, P_L=I_0^2*R_ESR

Diode: forward voltage drop (note that it is only conducting for (1-D)*T seconds) P_D=I_0*V_f*(1-D)

MOSFET: 1. Conduction loss, P_Cond=I_0^2*R_ds*D

2. Switching loss, P_sw=[V_ds*I_0*(t_on+t_off)]/2T_s

Component selection

Buck Converter IC

This is the most important part, once we have selected the IC we can select other components (inductors, capacitors, resistors) based off the specs of the IC.

Since we are converting from 24V to 5V, we need to consider a buck converter IC with: maximum input voltage larger than 24V; output voltage adjustable/fixed at 5V; rated output voltage should be larger than 5A; if possible the switching frequency should be a wide range; step-down, buck converter only (no buck-boost).

After applying all of the above constraints to DigiKey search query, my options came down to around 30 ICs, at first I was only considering TI’s IC, and the main IC are the LMR and TPS series. After I enlarge my options to other manufacturers, I found other good chips. Applying more close constraints and looking at the Datasheets more carefully, it comes down to 2 options:

With this chip, the footprint configuration looks really complicated, other than that there is no critical weakness of this IC

The only problem with this chip, is that the switching frequency is fixed, that may cause some problems when selecting Inductors, luckily, after calculation I can source pretty good inductors based on the fixed frequency, so in the end I chose the MP2491CGQB-Z chip

EN Current Calculation:

R_Pullup +3kOhm (internal pullup) = (24V-7V)/I_EN (7Volt is the internal Zener Diode)

In this case, R_Pullup=220kR, so I_EN=62.7uA, smaller than the 100uA, which is good.

Inductor

From the data sheet the design requirements of the inductors is that:

1. DC current rating 25% larger than output current (5A)

2. DC Resistance should be smaller thatn 15mOhms

3. I_sat (Saturation current) should be larger or equal to high side switch current

The current ripple can be calculated in the following equation

And the value of inductor can be calculated using the following equation, substitute in all the values.

I chose standardize inductor values, and calculate the K backwards (4.7uH, 5.1uH, 5.6uH, 6.2uH)

Based on that, I started looking on Digikey for the different inductors. The constraints are that inductors should be shielded, and with current rating and saturation current around 7-10A.

5.6uH:

https://www.digikey.ca/en/products/detail/sumida-america-inc/177CDMCCDS-470MC/9490436 *if set K=0.35, but still bigggg

https://www.digikey.ca/en/products/detail/vishay-dale/IHLP6767GZER560M11/2139396

https://www.digikey.ca/en/products/detail/pulse-electronics/PA4344-563NLT/5641842

https://www.digikey.ca/en/products/detail/codaca/VSAB1770-560M/16731527

https://www.digikey.ca/en/products/detail/coilcraft/XGL1010-563MED/21381839 *Seems to be the best one but not available on DigiKey

https://www.digikey.ca/en/products/detail/shenzhen-sunlord-electronics-co-ltd/MWSA1206S-470MT/14120328 *set at 0.35, relatively smaller

https://www.digikey.ca/en/products/detail/bourns-inc/SRP1265A-470M/4876628 *0.35, smaller (the above one is the smallest available)

4.7uH:

https://www.digikey.ca/en/products/detail/bourns-inc/SRP6060FA-4R7M/9351055

*The rated current and isat are a little bit high, small size (6mmx6mm) (1.86)

https://www.digikey.ca/en/products/detail/abracon-llc/ASPIAIG-F6060-4R7M-T/20096290

*Rated:10, isat:9A, small size (1.57)

https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/744314490/1638566

*4.9uH actually, small size, 6.5A/6.5A, higher price (3.8)

https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/7443340470/3476751

*8mm*7mm, 7.5A/8A, 2.14

The final selected inductor is https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/7443340470/3476751

Capacitor

Several capacitors are needed for this application, and most of the capacitors can be found in the WARG library.

*Several notes includes: use standardize value (e.g. 22uF=10+10+1+1); ceramic preferred, use X5R or better.

Other capacitors includes those such as feedforward capacitors, I just refer the value of them via the datasheet, the details are not expanded here.

Input Capacitors:

For simplification I just used a 20uF capacitor (connected 2 10uF in parallel),

Output Capacitors:

The effective capacitance can be calculated as follows,

To compensate the DC-bias and looking at the capacitors the WARG library already have, I selected 2 10uF capacitors with equivalent capacitance around 4.24uF at 5V DC and connect them in parallel.

Resistors

For the ILIM pin, a resistor is needed to set the current limit, according to the graph in the data sheet, to get a current limit of 6A we need 60.4kOhm resistor

For the FB pin, we need to build a voltage divider network to set the output voltage from the feedback voltage.

Updated (10/3): to compensate for the DCR losses, the voltage aimed to be regulated is changed to 5.1V

In this case, R1/R2=9.2, R2=10kOhms, R1=92kOhms, the library values I found are 10k and 91k.

System Block Diagram

Reverse Polarity Protection

One feature of the buck converter that is nice to have is RPP. It can be implemented using the following methods:

1. Using a diode in series, this is the most simple method, however for higher current (>3A) application there will be great power losses.

2. MOSFETS

   1. P-MOSFET on the high side

      1. More expensive, due to the P-MOSFET

   2. N-MOSFET on the low side

      1. Less expensive, but requiring the power ground and load ground to be different

3. IC

   1. Small voltage drop across MOSFET

   2. Small current

Overall, choosing a P-MOSFET and an IC are equally favorable, for the sake of learning I have chosen to use IC.

RPP IC selection:

RPP diagram:

PCB Layout

Some guidelines in the datasheet of the Buck IC:

Placement

Initial placement:

Updated Placements (From 9-30 to 10-2)

Routing

PDN Analysis

Goal is to calculate loop impedance for the following:

• VBUS: Input connector -> NMOS, NMOS -> Buck IC

• SW: Buck IC -> Inductor

• 5V: Inductor -> Pixhawk Conn, Inductor -> USB-C Conn, also do Inductor to farthest decoupling cap down (basically before the via punch down to L4).

Used the Power Analyzer by Keysight tool, not able to calculate the loop impedance directly, use the voltage drop and current simulated to calculated loop impedance.

Pixhawk Wiring

The Pixhawk connector that I find is the 5024430670 Molex Clickmate connector, linked below:

https://www.digikey.ca/en/products/detail/molex/5024430670/2380429

The link below is the pixhawk wiring standard

https://github.com/pixhawk/Pixhawk-Standards/blob/v6xupdate/DS-009%20Pixhawk%20Connector%20Standard.pdf

The wiring harness that WARG use is 1:1 harness, as pictured, the red cable indicating pin 1

Comparing the cable with the pin placements on my PCB shows that it is correct