Buck and Boost Converter Together

Thread Starter

reefat

Joined Aug 16, 2020
2
I am designing a power supply using a buck regulator and a boost converter or two buck regulators together, and feeding them from a single (1S 3.7V) battery or a double (2S 7.4V) LiPo battery pack. The goal is power up a 3.3V MCU (i.e. ESP32 or Arduino) and one or two motors via a motor driver.

Here I have 4 different options depending on the single or double cell LiPo batteries and also depending whether to use 6 or 12 volts motor(s). But my design has to work with all 4 options. Assume we are using a BMS (for the safety and power management of the LiPo battery) that could draw sufficient power from the LiPo to run both MCU and Motors.

Questions:

Preface: First notice, in option 1, 2 and 4, we are feeding a buck regulator and a boost converter from a 1S or 2S LiPo cell(s). And in option 3, we are feeding two buck regulators from a 2S LiPo pack.

  1. Is there going to be any issue for feeding a buck and a boost converter from the same source (i.e. the same battery)?
  2. Since the buck/boost converters run on a specific frequency, will it cause any issue if the frequency of one doesn't match with another?
  3. Efficiency is one of the major concerns, because we are using a battery here. Do you have any suggestion? Do you have any better design idea?

Here are the 4 options (see circuits below):

Screen Shot 2020-08-16 at 10.04.27 PM.png
 

Irving

Joined Jan 30, 2016
3,845
Ditch the BMS, you dont need it if you're controlling the usage, BMS is only useful if you have no control of usage. If you're worried use LiFePO4 cells instead. Assume you're usin a off-board charger and not tryng to charge in situ, make life a lot simpler for yourself.

Next, whats your power budget? Before you can make any decisions on power supply configuration, just what are you trying to acheive? How big, how long for, what losses can you tolerate, is it continuous or peaky load, etc, etc. Boost converter driving a motor is potentially a waste of energy, 15-20% loss in converter and 15-20% loss in driver - both are switching circuits so far better to match motor and supply but you need much more info. You dont need step down to motor if motor volts < battery volts, driver can take care of that.

Start with motor - whats it doing, speed, torque, power output, losses, etc. That will inform battery choice/size/chemistry/etc.
 

Thread Starter

reefat

Joined Aug 16, 2020
2
Thank you @Irving for walking me through this. I liked the idea of LiFePO4. Interestingly, even Tesla hasn't rolled it out completely on their Model 3 yet! Anyhow, as you advised, I will start with the motor and calculate how much power I will need based on the desired lifetime of the product.
 
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