Boost Circuit drops voltage when adding load resistor

Thread Starter

Ritz123

Joined Jul 27, 2022
4
Hi There,

I using LTspice to simulate a boost circuit to bring a 3.7 volts lipo battery up to 5 volts in order to power raspberry pi zero. When I simulate the circuit it works just fine (shown in the first picture) however when I add a load resistor representing the raspberry pi zero the voltage drops back down to 3.7 volts. (shown in the second picture). I have been trying to troubleshoot this circuit problem. Maybe there is a glaring mistake that someone sees that I don’t.

This is the datasheet for the MC34063 IC chip: https://www.ti.com/lit/ds/symlink/mc34063a.pdf

The math for the load resistor:
V = IR
~0.2 volts = (~0.1 amps) R
~2 ohm = R

I got ~0.1 amp from this site: https://www.cnx-software.com/2021/12/09/raspberry-pi-zero-2-w-power-consumption/
I got ~0.365 volts from measuring (the voltage without the load at ~3.94 volts and the voltage with the load at ~3.74 volts) and then took the difference of ~0.2

Please tell me what you think I would greatly appreciate feedback! Thank you!


 

Thread Starter

Ritz123

Joined Jul 27, 2022
4
Thank you for pointing that out ElectricSpidey! I messed up with the load resistor. It should be ~37.4 ohms.
(voltage with the load at ~3.74 volts) / (~0.1 amps)

These are my results after changing the load resistor (shown below). I still have the same problem but at a more progressive pace.
1659555693258.png
 

ronsimpson

Joined Oct 7, 2019
2,250
Post your files! It is almost imposable to read the small schematic.
R3&R4 is wrong. The error amplifier is trying to make a very small volage like 2V.
 

dl324

Joined Mar 30, 2015
14,902
Please tell me what you think I would greatly appreciate feedback!
We only need the schematic at a scale that we can read. Ground always points down.
1659564933934.png
You're simulating this and you can can't figure out why the output voltage drops when you add a load? The divider ratio should give you 1.66V.
EDIT: correct typo

R5 seems a bit small for a circuit that can only provide up to 1.5A.
\( \large I_{SC} = 0.3V/0.08\Omega = 3.75A\)
 
Last edited:

ronsimpson

Joined Oct 7, 2019
2,250
The divider ratio should give you 1.66V.
In a boost circuit, where the error amplifier is trying for 1.6V and the supply is 3.7 the IC will switch once (or less) and never again.

You should fix this design for practice. Then use a IC made for low voltage not this one.
 

dl324

Joined Mar 30, 2015
14,902
In a boost circuit, where the error amplifier is trying for 1.6V and the supply is 3.7 the IC will switch once (or less) and never again.
I think it'll still work. I breadboarded a variation of that circuit a few weeks ago to boost 10V to 12V. While I was testing it, I'm fairly certain I lowered the input voltage to less than 3.7V. The load current was only 50mA and the draw from the power supply was around half an amp. I didn't pay close attention because the input voltage range I was designing for was 10-12V.
 

ronsimpson

Joined Oct 7, 2019
2,250
I think it'll still work.
"It" won't work, but a variation will. The error amplifier is wrong.

At 12V your circuit probably works just fine. At 3.7V the IC is struggling to work.

Pi-0, "needs 2.5A supply" but it really pulls 1A max. By what is in post #1 the current is very dependent on what the Pi is doing. 0.1 to 0.5A is common and some peaks reach 1A.

Can the battery hold up 2A peaks?
 

eetech00

Joined Jun 8, 2013
3,311
Hi There,

I using LTspice to simulate a boost circuit to bring a 3.7 volts lipo battery up to 5 volts in order to power raspberry pi zero. When I simulate the circuit it works just fine (shown in the first picture) however when I add a load resistor representing the raspberry pi zero the voltage drops back down to 3.7 volts. (shown in the second picture). I have been trying to troubleshoot this circuit problem. Maybe there is a glaring mistake that someone sees that I don’t.
Please tell me what you think I would greatly appreciate feedback! Thank you!
I re-calculated the values using the equations on the datasheet.
This is a boost converter for 3.7v in to 5.0v@100ma out.
I used minimum values for the inductor and output capacitor.
You may need to re-calculate the values for an input lower than 3.7v

1659578975223.png
 
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