Need Help for fixing/improving boost converter design

Thread Starter

ATOOO

Joined Sep 9, 2022
3
I'm pretty newbie. i designed a boost converter as hobby and need some help about that. not sure if it will work. how can i fix or improve the circuit. i aimed 5 Vin and 15V 10A max out and use 555 for generate pwm signal for mosfets. But i know 555 can generate max %50 duty cycle and it means i can only double input voltage. So i thought and if i connect two booster as serial, i can reach 4x the output voltage(is it possible?). Then i looked for inductors. and found max 5A coils therefore i add 1 more serial connected booster in parallel. In this way i tried the divide output current as 5A+5A. Then for stabilize output voltage, add voltage divider at output and use this voltage at an opamp which used for voltage comparator. Then compared voltage used for decrased duty cycle at 555's 5th pin. (555 runs 60kHz).
Sorry about language i used translate.
 

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DickCappels

Joined Aug 21, 2008
9,353
Just a couple of comments:

Since the boost converters are in parallel their voltages will not add, but the ability to deliver current to a load will be greater than it would be with one converter.

This looks like a learning project. You can save a lot of money and burned fingers (and burned other things) if you start out with a smaller power supply, perhaps just a few watts. Small power supplies are easier to bring up than higher power power supplies. Not only small transistors are cheaper but so is almost everything else.

Rather than resort to an unorthodox trick like combining the outputs of multiple power stages, from which you might learn a few things but never need in real life, why not just move to a different switch mode power supply chip like the SG3524 or MC34063 <=Easy to use)? If you feel you must use an NE555 you can use two - one as an astable multivibrator followed by one as a monostable multivibrator. Using those methods you can concentrate on making you single output stage work well.
 

Ian0

Joined Aug 7, 2020
6,330
You can design any switched-mode supply around a 555 - but it takes some practice and quite a deep understanding of the subject.
You would be far better served by designing your first boost converter around something such as a UC3842, which is designed for the job.
When you have built it and understood how it works, THEN try your hand at 555-based circuits!

A 741 is a really bad choice for the error amplifier on a 5V supply, because if its limited input and output voltage range. (It's a really bad choice for any purpose these days) You might just stand a chance with a 5V rail-to-rail op-amp such as MCP6071, but it needs to be wired as an integrator, and it needs a voltage reference that is lower than the supply voltage. Also, the phase of your feedback is wrong. As the output voltage increases the output of the error amplifier also increases which increases the mark-space ratio, giving higher output.
 

DickCappels

Joined Aug 21, 2008
9,353
In a moment of extreme weakness and laziness I made a 5V to 130V boost converter with an NE555 for experiments with avalanching transistors.
1662749803571.png

But then I grew tired of it being unregulated and switched to a MC43063, and the parts count dropped!
1662749899136.png
This supply is voltage regulated and also current limited. I switched the NPN to a 2N5551 later.

Another version with the MC34063 for a commercial application supported much higher output power.
1662750128428.png
Hones,t I don't get paid to advertise these chips. They are just easy to use. Bode analysis - why bother?
 

crutschow

Joined Mar 14, 2008
30,840
Do you have a real use for 10A @ 15V or did that just sound like a good value.

Note the 5V input will have to supply more than 30A to get that output, since the input power has to be a least a much as the output power (ignoring the circuit inefficiency).
Is you 5V source capable of that?
 

Thread Starter

ATOOO

Joined Sep 9, 2022
3
Do you have a real use for 10A @ 15V or did that just sound like a good value.

Note the 5V input will have to supply more than 30A to get that output, since the input power has to be a least a much as the output power (ignoring the circuit inefficiency).
Is you 5V source capable of that?
i am going to use it for an 775 electric motor yes i have real use for that. and i think i can handle source. i know i cant create energy in this way :D .
 

Papabravo

Joined Feb 24, 2006
19,346
i am going to use it for an 775 electric motor yes i have real use for that. and i think i can handle source. i know i cant create energy in this way :D .
Generally speaking, boost converters are a bad idea for motors, especially with boost ratios above 4-5. Is there a reason why you think this is a good idea? I'd like to hear it if you have one.
 
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Ian0

Joined Aug 7, 2020
6,330
If you have 5V @30A then it probably comes from a power supply (because there aren't 5V batteries) so if you really want 15V @10 A then you simply have the wrong power supply.
The answer is to get the right power supply, not to mess about trying to convert it. True, you won't learn anything about boost converters in the process, but this probably isn't the best opportunity to learn.
 

Thread Starter

ATOOO

Joined Sep 9, 2022
3
i said im newbie. i was not know that boosters is not good option. but i have already tried to make a boost converter design. and im curious about how to improve or fix the design. electric motor is not that important. I just thought if i make a boost converter, i can test with the motor.
im curious so i have couple of questions. for example:

Warn me if im wrong. if this part makes double voltage (bc i have %50 duty cycle)
boost designa.PNG
can i make it 4x with this or is it a big mistake or something

boost designb.PNG
 

Ian0

Joined Aug 7, 2020
6,330
i said im newbie. i was not know that boosters is not good option. but i have already tried to make a boost converter design. and im curious about how to improve or fix the design. electric motor is not that important. I just thought if i make a boost converter, i can test with the motor.
im curious so i have couple of questions. for example:

Warn me if im wrong. if this part makes double voltage (bc i have %50 duty cycle)
View attachment 275922
can i make it 4x with this or is it a big mistake or something

View attachment 275923
Yes, it does make (almost) twice the voltage provided that it is continuous current mode.
In discontinuous current mode, it can make more than that.
Two cascaded converters will make (almost) four times the current, but the calculations for the inductor for the second one will be different, because the voltage and current are different.
But you can get four times the voltage from a single converter with a 75:25 mark-space ratio.
Start with a single converter, when you get that working you can progress on to something more complicated.
A pair of cascaded converters has a d^2 relationship for output voltage (d=duty cycle) - that doesn't make it simpler!
If you want to watch how it works, just use a 555 with a manually adjustable mark-space ratio. Adding feedback and getting it stable is not for the faint-hearted.
 
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