In this post I have explained s few simple DC to DC buck-boost converter circuits which can be used for either stepping up the input DC voltage or stepping down the input DC voltage to specific output DC voltage levels.
Since a buck-boost converter allows the input voltage to be either stepped up or stepped down, hence it is named as buck and boost converter or buck-boost converter.
Basic Working principle
The basic working principle of a buck-boost converter can be understood with the help of the following explanation.
Referring to the basic buck-boost topology diagram above, when the switch S is closed, the input voltage VIN is blocked by the diode D, since D is reverse biased. Therefore the voltage has no other option but to flow through the inductor L.
The inductor L due to its inherent reluctance property initially tries to block the current. Due to this, initially the voltage drop across L is high. However, slowly as the reluctance of the coil decreases and the current through the inductor increases.
During this phase the inductor L stores the energy (voltage and current) inside its winding.
Ideally this the point when the switch S needs to be opened.
When switch S is now opened, the voltage VL stored inside the inductor reverses direction, and it passes through the diode D which is now forward biased. In the process VL also passes through the output capacitor and charges it fully.
During this phase the output capacitor stores energy inside it and gets fully charged, simultaneously the load across VOUT also gets the energy from the inductor for ts operation.
Next, at this point if the switch is closed again, the process repeats as explained above, however now the capacitor provides its stored energy to the load.
Depending how how fast or how slow the switch S is closed and opened, the output DC can be a stepped up or boosted output, or it can be a stepped down or buck output.
In a real life electronic buck-boost circuit, the switch S is replaced with a BJT or a MOSFET and this device is switched ON/OFF through an external PWM for generating a buck or a boost output.
Simple Two Transistor Buck-Boost Converter Circuit
The following figure shows how a very simple buck-boost converter could be built using just a couple of transistors and a few other passive parts.
The working of the circuit can be understood from the following points:
Here the circuit is configured to provide a negative boosted output from a 5V input to a -12V output.
As soon as the circuit is powered, the T2 switches ON via R3. This allows T1 to also switch ON through the negative base bias received via R1 and T2.
This causes the full +5V to be applied across the inductor L1 via the collector of T1.
The inductor, due to it reluctance opposes the instant rise in current across it and in the process begins storing energy inside its winding.
Now, slowly the inductor reluctance drops and it starts passing more and more current through. Due to this the base current of T2 is pulled to ground through the inductor and via R2 and C2.
This causes T2 to shut off momentarily.
Now, the stored energy inside the inductor is released back to the output across C1.
C1 now charges fully and simultaneously the output is also supplied with a negatively boosted voltage.
The 12 V diode makes sure that the inverted output boosted voltage does not rise above -12V.
Subsequently C2 begins charging until it is fully charged, which allows T2 to switch ON again, and the process repeats. This repetition process continues at a high frequency.
The frequency of the converter is determined basically by the values of R2 and C2.
By appropriately tweaking the values of R2 and C2, this two transistor circuit could be optimized either to work like a buck converter or a boost converter.
Using IC 555 for the Buck-Boost
This highly efficient and effective buck-boost circuit using the work horse IC 555 would allow you to convert an input source voltage to any required degree, either bucked or boosted, as desired.
We have already learned the concept comprehensively through one of my previous articles where I have explained the versatility of this buck-boost type of topology.
As shown in the circuit diagram below (click to enlarge) the configuration is basically a combination of two distinct stages, viz the upper buck-boost converter stage and the lower IC 555 PWM controller stage.
The buck-boost stage consists of a mosfet which acts like a switch, the inductor which is the main power converting component, the diode which just like the mosfet forms a complementary switch, and the capacitor quite like the inductor forms a complementary power converter device.
The mosfet needs to operate through pulsed triggering so that it alternately switches the input voltage ON and OFF across the inductor in response to its gate voltage.
Therefore the gate voltage should also be in a pulsed form which is accomplished through a IC555 PWM generator stage.
Circuit Operation
The associated IC555 PWM generator is integrated to the mosfet for accomplishing the above discussed operation.
During the ON time of the mosfet, the input voltage is allowed to pass through the mosfet and is applied right across the inductor.
The inductor owing to its inherent property tries to counter this sudden infliction of current by absorbing and storing the power in it.
During the subsequent OFF period of the mosfet, the input voltage is shut off by the mosfet, the inductor now experiences a sudden change in current from peak to zero. In response, the inductor counters this by reversing its stored power across the output terminals via the diode which now acts in the forward biased condition.
The above power from the inductor appears with opposite polarity across the output where the intended load is connected.
The capacitor is positioned to store a portion of the power in it, so that it can be used by the load during the ON time of the mosfet when the diode is reverse biased and power cut off across the load.
This heps to maintain a steady and stable voltage across the load during both ON and OFF cycles of the mosfet.
Using PWM as the Controller
The level of voltage, whether it's a boosted voltage or bucked voltage depends upon how the mosfet is controlled by the PWM generator.
If the mosfet is optimized with higher ON time than the OFF time then the output would generate boosted voltage and vice versa.
However there may be a limit to this, care must be taken not to exceed the ON time beyond the full saturation time of the inductor, and the OFF time must not be below the minimum saturation time of the inductor.
For instance suppose it takes 3ms for the inductor to become fully saturated, the ON time in this case can be set within 0 - 3ms, and not beyond that, This will result in a boost from minimum to maximum depending upon the value of the chosen inductor.
The associated pot wit the IC555 PWM generator can be effectively tweaked for acquiring any desired buck-boost voltage at the output.
The inductor value is a matter of trial and error, try to incorporate as many winding as possible for acquiring better and efficient results and diverse range.
Circuit Diagram
The above design can be suitably upgraded for implementing an automatic output voltage correction with the help of the following modifications:
The 1K preset may be appropriately set initially for determining the desired control point.
IC 555 Pinouts
You said in this article:
“The 1K preset may be appropriately set initially for determining the desired control point.”
Please how will I set this?
It means you have adjust the preset with some trial and error to find the expected working output voltage.
Thanks, the preset is not respond to any change. The 50k preset was changing the duty cycle, but the feedback preset for the 555 timer does not respond any change in the wiper.
Are you getting the buck boost output from the circuit, and is the output output voltage varying in response the duty cycle changes?
Yes, with the duty cycle but not the feedback, I don’t know how the feedback back will work?
I hope you have connected the output filter capacitor which should keep the output stable (if your input supply is stable).
Anyway, you can try this modification in the feedback section. Connect the wiper arm of the preset with the pin#5, and connect one outer end of the preset to the buck-boost output and the other outer end of the preset with the ground, and check the response.
Nice to look at some of your designs as always. It’s me again and this time, I’m needing to build a boost circuit to work with the LED lights in my RV. So here’s the deal. Sometimes my Bvoltage goes a little low to power the lights I have been building. So what I want to do is construct some really simple boosters to power with around 9.5 to 13.8 v input, I need to output to stay at a constant give or take a volt. I’m looking at the simple 2 transistor setup which shows an inverted output. Going to put this together on a bread board, add a bridge rect. to the output to invert again? Everything would be isolated this way as I see it with the boost circuit running between the single push tactile switches in the lights and the push on push off circuits, this way the nothing is powered on until I do so. So I would according to this circuit need a 7805 in there to regulate the incoming V to run the Q’s and that 12v Z could be replaced with something like a 14v Z. This would keep the Lumens up no matter how low the Bat V goes. Space is limited in the Poly cases I’m using but I think I could squeeze this in there. I have some SMD 555’s on the way also but the rest of the pieces, I have in hand now. Not the FR107 but another FR part that should be fine, was also thinking maybe a 4001 would be fitting as these lights only draw +/- 0.05A? Multiple strings of SMD high intensity white LEDs in series/par. Beyond that, I have a couple of Mains LED fixtures that are supposed to be dimable but not able to track down the controllers for those as yet. Looking for a way to get a 120VAC input for these with a way to dim them, the circuits in these fixtures indicates as far as I can recall, 36 to 57v drive. I’ll have to get back to you on this later if you have any ideas.? For now lets work on the drivers for the DC powered fixtures I’m working on now. Cool beans my friend.
Thank you Bret, for the detailed explanation.
Yes, the first circuit looks good and may be suitable for your application. I have another article on boost converters specifically, which also includes a few good designs. You can find the article under the following link:
https://www.homemade-circuits.com/how-to-make-simple-boost-converter-circuits/
Do let me know if you have any further questions.
On my first build on the proto board the 2 Q circuit failed to deliver. The output was less then the 9v battery I was using to power the circuit. I ohmed out the circuit and scratched my head and pondered what went wrong but found nothing and moved on after an hr or so. I never did figure out why the circuit didn’t work. I’m waiting on some SMD 555’s and when those come in, I’m going to try a different approach. I really need to get a boost circuit up and working to install in those LED arrays that I can switch on with the functioning momentary push switches and want to build a few more fixtures to put outside in water tight poly cases with clear lens for lighting outside, under my hoods of vehicles and where ever.
In the first circuit I think the 100uF polarity must be reversed.
By the way the circuit was designed by the elektor electronics engineers so it cannot be wrong.
What is the motivation in designing dc-dc convertor/please add something that motivates.the study of dc-dc convertor !
Designing DC Dc converters require complex calculations, so unfortunately motivation might not work for this type of projects.
Hello noob here, can i ask what specifications am I going to use for my device. Im trying to create a wind turbine using a three phase permanent magnet ac generator, from the generator, it will be connected to a rectifier to convert it from ac to dc, then connected to a buck booster, but the question is what type of buck booster would i use?
Hello sir. I enjoy following your work. But for some reason you always work with low voltage. For example, I could not find a voltage regulator whose input voltage is max 100v, output voltage can be adjusted 12, 24, 48 v. Do you have any such work?
Thank you Ibrahim, you can try a bench power supply as explained in the following article, you can get a 100V max adjustment feature from this design:
How to Design a Bench Power Supply Circuit
Hi sir
Please what is the formula used in determining the frequency of the PWM generator used in this circuit thanks for your time
Hello Emmanuel, the formula can be found at the bottom section of this article:
https://www.homemade-circuits.com/timer-ic-555-explained/
Bonjour monsieur. C’est aussi possible de contrôler les grandes tensions telles que 400V?
Hello Daoud, yes high voltages upto 400V can be also controlled, provided the inductor is calculated correctly.
D’accord monsieur. Merci
You are welcome!