# Inverting opamp for Buck-Boost Converter

#### kalemaxon89

Joined Oct 12, 2022
190
Moderator’s note: translated from Italian, please only post in English.

Nota del moderatore: tradotto dall'italiano, si prega di postare solo in inglese.

Vorrei
I would like to make a small power supply at home that would allow me to supply a DC voltage over a certain range... as if it were a bench power supply. Of course, I should also think of a way to limit the current and disconnect the circuit in case of overcurrent as in the case of bench power supplies (better not to risk it!).

I would like to try to use a Buck-Boost DC-DC converter (you can suggest other solutions, but I want to try this one) but I have some doubts:

1) Simulation of the Buck-Boost DC-DC converter on LTSpice ... with 12V input, Rload=10 ohm and variable duty cycle from 10% to 80% I get an output range = [-2V .. -50V]. However, if I change Rload I get a completely different range! For example with Rload=1000 I get Vo = [-16V .. -158V].
How is this possible if the formula of Vo of a is -Vi*D/(1-D)??
And above all... if I want to use it to power something... every time I change the load I have a different Rload and I wouldn't know what voltage I'm getting at the output!

2) the Vo output is inverted Vo=-Vi*D/(1-D) ... so I think I simply need an inverting buffer, but I'm not clear on the specifics it should meet.
In my opinion, the parameters that I have to consider when choosing an opamp (in this application .. not in general) are the following:
- Vin(max) and Iin(max)
- Vout(max) and Iout(max)
Whatever?

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#### LowQCab

Joined Nov 6, 2012
3,451
The first question is,
why are You attempting to build a Buck-Boost Converter from scratch ?

Switching-Power-Supplies,
that already have all the bugs worked-out,
are plentiful, and much cheaper than You can build one.

What are You trying to accomplish ?
What will this SMPS be Powering ?
.
.
.

#### Papabravo

Joined Feb 24, 2006
20,378
1. Opamps are not useful for "inverting" power. They apply to signals and require supply voltages that are larger than the output range.
2. You should be careful with extreme boost ratios. The nature of the function D/(1-D) suggests that at large values of D the output will be very hard to control. IMHO the useful range is from 2:1 to 5:1. At a boost ratio greater than 5 the output will be essentially uncontrollable.
3. You should post your simulation so we can offer suggestions.

#### MrAl

Joined Jun 17, 2014
10,588
I would like to make a small power supply at home that would allow me to deliver a DC voltage in a certain range ... as if it were a bench power supply. Of course, I would also have to think of a way to limit the current and disconnect the circuit in case of overcurrent as is the case with bench power supplies (better not to risk it!).

I would like to try using a Buck-Boost DC-DC converter (you can suggest other solutions, but I want to try this one) but I have some doubts:

1) Simulating the DC-DC Buck-Boost converter on LTSpice ... with 12V input, Rload=10 ohms and varying duty cycle from 10% to 80% I get an output range = [-2V .. -50V]. However, if I change Rload I get a completely different range! For example with Rload=1000 I get Vo = [-16V .. -158V].
How is this possible if the formula of Vo of a is -Vi*D/(1-D)??
And most importantly ... if I want to use it to power something ... every time I change load I have a different Rload and I wouldn't know what voltage I'm getting at the output!

2) the Vo output is inverted Vo=-Vi*D/(1-D) ... so I think I simply need an inverting buffer, but I am not clear on the specifications it should meet.
In my opinion, the parameters I need to consider when choosing an opamp (in this application .. not in general) are as follows:
- Vin(max) and Iin(max)
- Vout(max) and Iout(max)
Anything else?
Hi,

Do you mean use an op amp as an error amplifier to regulate the output voltage?
All converters have to have a means to regulate the output as there are certain things that can change the output. The formulas you probably found on the web are purely theoretical and no commercial converter will apply that theory alone, they will also employ control circuit theory, which requires an error amplifier with a reference and feedback. Without feedback it is very unlikely you will get a constant output with a varying load resistance.

#### kalemaxon89

Joined Oct 12, 2022
190
Vorrei fare un piccolo alimentatore a casa che mi permetterebbe di fornire una tensione continua in un certo intervallo ... come se fosse un alimentatore da banco. Certo, dovrei anche pensare ad un modo per limitare la corrente e scollegare il circuito in caso di sovracorrente come nel caso degli alimentatori da banco (meglio non rischiare!).

Vorrei provare ad utilizzare un convertitore DC-DC Buck-Boost (puoi suggerire altre soluzioni, ma voglio provare questa) ma ho qualche dubbio:

1) Simulazione del convertitore DC-DC Buck-Boost su LTSpice ... con ingresso 12V, Rload=10 ohm e duty cycle variabile dal 10% all'80% ottengo un range di uscita = [-2V .. -50V]. Tuttavia, se cambio Rload ottengo una gamma completamente diversa! Ad esempio con Rload=1000 ottengo Vo = [-16V .. -158V].
Come è possibile se la formula di Vo di a è -Vi*D/(1-D)??
E soprattutto... se voglio usarlo per alimentare qualcosa ... ogni volta che cambio carico ho un Rload diverso e non saprei che tensione sto ricevendo in uscita!

2) l'uscita Vo è invertita Vo=-Vi*D/(1-D) ... quindi penso di aver semplicemente bisogno di un buffer invertente, ma non sono chiaro sulle specifiche che dovrebbe soddisfare.
A mio parere, i parametri che devo considerare quando scelgo un opamp (in questa applicazione .. non in generale) sono i seguenti:
- Vin(max) e Iin(max)
- Vout(max) e Iout(max)
Qualsiasi altra cosa?

View attachment 293564
Just updated with schematic @MrAl @Papabravo @LowQCab
True, I had not thought about the power supply that the opamp needs.
I think I only need to control the duty cycle in a certain range (without getting below 10% or above 90% for example) and reverse the output .. but it seems too easy to be true (as you suggest).
I don't need high voltage ranges but I just need to go from a few V to 30-40V

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#### DickCappels

Joined Aug 21, 2008
10,067
If this power supply might power audio or video I suggest simply using an LM317.

It is well regulated, it has protection against excessive currents and excessive temperature, particularly important when used with analog circuits noise is pretty low.

As Larry David might say, “…pretty, pretty, pretty low.”

And best of all, it comes in a 3 lead package, does not need many other parts around it, and is inexpensive.

#### MrAl

Joined Jun 17, 2014
10,588
Just updated with schematic @MrAl @Papabravo @LowQCab
True, I had not thought about the power supply that the opamp needs.
I think I only need to control the duty cycle in a certain range (without getting below 10% or above 90% for example) and reverse the output .. but it seems too easy to be true (as you suggest).
I don't need high voltage ranges but I just need to go from a few V to 30-40V
I don't see any feedback yet for regulation.