voltage drops on this switcher everywhere?

Thread Starter

Miracletech

Joined Nov 15, 2019
160
I'm using this circuit design:
"https://www.eevblog.com/forum/proje...onverter/?action=dlattach;attach=294867;image"
action=dlattach;attach=294867;image"
But I changed some values:
R1:220 ohms
R3:470 ohms
C1:470PF
M1: TIP41C.
When I turn the converter on and connect a 500MA Load,
The output voltage significantly drops from 5.6V to 4.6V, And
there is voltage drop across my inductor(???)
My question is; Is there any fault on my side? I used every
other thing of same value except the components above.
Is my inductor saturating, Because I'm getting 0.07v drop
across it, which is odd.
Any ideas what is going on?
(By the way, I'm stepping up 3.7v to 5V for Led strip. I like
discrete.)
 
Last edited by a moderator:

Papabravo

Joined Feb 24, 2006
15,761
A TIP41C used as M1 is not going to work, because it NOT A MOSFET! It is a BIPOLAR JUNCTION TRANSISTOR! I gotta say: "That is kind of a serious oversight".
BTW there will almost always be a voltage drop across the inductor of boost converter. How did you think it was supposed to work?
 

Thread Starter

Miracletech

Joined Nov 15, 2019
160
I tried, My Device is acting stupid.
I know it uses a Mosfet, But I've only Got Mr. TIP41C, which is older than.( Well, Dunno) but, Is there any reason why the voltage is dropping on load?

Is the Transistor switching causing the unexpected voltage drop? Is the transistor damaged? The circuit is working, With the BJT.
 

Papabravo

Joined Feb 24, 2006
15,761
I tried, My Device is acting stupid.
I know it uses a Mosfet, But I've only Got Mr. TIP41C, which is older than.( Well, Dunno) but, Is there any reason why the voltage is dropping on load?
You cannot substitute a Bipolar Junction Tranistor for a MOSFET period. The part is mentioned in the TI Power Data Book from 1972, that is how old the part is. The ones I had were beasts. I think there is a pretty good chance that it is undamaged, but that is not a 100% guarantee.
 

Thread Starter

Miracletech

Joined Nov 15, 2019
160
Gate- Base
Drain- Collector
Source-Emitter.
...? It works, when I reduced R3 To 470ohms.

What I'm trying to get is; Is it that The TIP41C is the old man causing the voltage to drop because of its high VCEsat, Or I'm driving it improperly, Or, It cannot do the task, Or, Its too big?
 

Papabravo

Joined Feb 24, 2006
15,761
Gate- Base
Drain- Collector
Source-Emitter.
...? It works, when I reduced R3 To 470ohms.
That's because it allows Q2 to turn on, but it sure isn't working as was intended since putting an actual load on it causes the output voltage to drop. Regulators actually regulate; this implementation cannot do that. Abandon this iteration and get the proper parts. Even then it is still a marginal design because Q2 does not have a stable bias source.
 

Thread Starter

Miracletech

Joined Nov 15, 2019
160
I've seen Boost converters use BJT's. For high currents, But I don't know why this one is giving me an undesirable voltage drop.
I even added a pull down resistor of 10K to Q2, Same problem exists.
 

Thread Starter

Miracletech

Joined Nov 15, 2019
160
Any ideas what I'm gonna do?
Also, when the base is shorted to Ground, Does it affect the transistor performance?
By the way, The 2ND Circuit, I'll modify it to make it an actual Boost converter.
 
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