With an increase in the mosfets ON time, the circuit starts getting transformed into a Boost converter while with the mosfets OFF time exceeding its ON time results in the circuit behaving like a Buck converter.

Thus the input to the mosfet can be made through an optimized PWM circuit for getting the required transitions across the same circuit.

**Exploring Buck/Boost Topology in SMPS Circuits More Technically:**

with switch mode power supplies are the buck, boost, and the buck boosts.

These are basically non-isolated in which the input power stage shares a common base

with the output power section. Of course we could also find isolated versions

although pretty rare.

uniquely depending upon their exclusive properties.

voltage conversion ratios, the nature of the input and output currents and the

character of the output voltage ripple as well.

Additionally the frequency

response of the duty cycle to the output voltage execution can be considered as

one of the important properties.

topology is the most preferred one because it allows the output to work ways

that is to produce voltages less than the input voltage (buck mode) and also to

produce voltages above the input voltage (boost mode).

opposite polarity from the input, which doesn’t create any issues whatsoever.

form of a pulsating current due to the switching of the associated power switch

(Q1).

cycle.

current because of the associated diode which conducts only in one direction,

causing an ON and OFF pulsating situation during the switching cycle.

The

capacitor is responsible for providing the compensating current when the diode

is in the switched OFF or reverse biased state during the switching cycles.

converter in continuous-mode and discontinuous-mode operation with exemplary

waveforms presented.

The duty-cycle-to-output voltage exchange functionality is

presented after an introduction of the PWM switch design.

schematic of the buck-boost power stage with a drive circuit block added. The

power switch, Q1, is an n-channel MOSFET. The output diode is CR1.

The

inductor, L, and capacitor, C, constitute the efficient output filtering. The

capacitor ESR, RC, (equivalent series resistance) and the inductor DC

resistance, RL, are all analyzed in the . The resistor, R, corresponds to the

load identified by the power stage output.

functionality of the buck-boost power stage, Q1 is constantly turned on and off

with the on- and off-times governed by the control circuit.

This switching

behavior permits to a chain of pulses at the junction of Q1, CR1, and L.

Even

though the inductor, L, is linked to the output capacitor, C, if only CR1

conducts, a successful L/C output filter is established. It cleans the

succession of pulses to result in a DC output voltage.

inductor current setting.

continuously in the inductor over the switching sequence in steady-state

process.

inductor current staying zero for a section of the switching cycle. It begins

at zero, extends to a maximum value, and comes back to zero in the course of

every switching pattern.

afterwards and model suggestions for the inductor value to sustain a selected

mode of functionality as the ability of rated load are presented.

format only over its predicted functioning circumstances since the power stage

frequency response alters substantially between the two distinct techniques of operation.

and a positive voltage, VGS(ON), is supplied from the Gate to the Source

terminals of Q1 by the control circuit to switch on the FET.

RDS(on) however the contro circuit tricky because a suspended drive becomes

necessary. For the identical package dimensions, a p-channel FET possesses a

higher RDS(on) nonetheless typically may not necessitate a floating drive

circuit.

dashed-line outline with terminals tagged a, p, and c. It is discussed

thoroughly in the Buck-Boost Power Stage Modeling portion.

continuous conduction method. The primary objective of this segment would be to

present a derivation of the voltage transformation relationship for the

continuous conduction mode buck-boost power stage.

This will be significant

since it indicates the way the output voltage is determined by duty cycle and

input voltage or on the contrary, how the duty cycle could be determined

depending on input voltage and output voltage.

Steady-state means that the

input voltage, output voltage, output load current, and duty-cycle are constant

as opposed to varying. Capital letters are usually provided to variable labels

to suggest a steady-state magnitude. In continuous conduction mode, the

buck-boost converter takes a couple of states per switching cycle.

The ON State

is each time Q1 is ON and CR1 is OFF. The OFF State is every time Q1 is OFF and

CR1 is ON. An easy linear circuit could symbolize each of the two states in

which the switches in the circuit are substituted by their matching circuit in

the course of each state. The circuit diagram for each of the two conditions is

presented in Figure 2.

the duty cycle, fixed by the drive circuit, depicted in form of a ratio of the

switch ON period to the period of a single full switching sequence, Ts.

The

length of the OFF state is known as TOFF. Because one can find just a couple of

conditions per switching cycle for continuous conduction mode, TOFF is equal to

(1−D) × TS. The magnitude (1−D) is occasionally called D’. These periods are

presented together with the waveforms in Figure 3.

offers a reduced resistance, RDS(on), from its drain to source and manifests a

smaller voltage drop of VDS=IL × RDS(on).

Additionally there is a little

voltage drop across the dc resistance of the inductor equal to IL × RL.

Thereby, the input voltage, VI, minus deficits, (VDS + IL × RL), is put on

across the inductor, L. CR1 is OFF within this period as it would be reverse

biased.

The inductor current, IL, passes from the input supply, VI, by way of

Q1 and to ground. In the course of the ON state, the voltage put on across the

inductor is constant and the same as VI − VDS − IL × RL.

Following the polarity

norm for the current IL presented in Figure 2, the inductor current boosts due

to the executed voltage. Furthermore, because the applied voltage is

fundamentally consistent, the inductor current rises linearly. This boost in

inductor current in the course of TON is drawn out in Figure 3.

determined by utilizing a form of the well-known formula:

presented as:

current. Furthermore observe that through this interval, every bit of the

output load current comes in by the output capacitor, C.

With reference to

Figure 2, while Q1 is OFF, it offers an increased impedance from its drain to

source.

Consequently, because the current running in the inductor L is unable

to adjust instantly, the current switches from Q1 to CR1. As a result of the

reducing inductor current, the voltage across the inductor reverses polarity

until rectifier CR1 turns into forward biased and flips ON.

The voltage

connected across L turns into (VO − Vd − IL × RL) in which the magnitude, Vd,

is the forward voltage drop of CR1. The inductor current, IL, at this point

passes from the output capacitor and load resistor arrangement via CR1 and to

the negative line.

Observe that the alignment of CR1 and the path of current

circulation in the inductor signifies that the current running in the output

capacitor and load resistor grouping leads to VO to be a minus voltage. In the

course of the OFF state, the voltage connected across the inductor is stable

and the same as (VO − Vd − IL × RL).

Preserving our likewise polarity

convention, this connected voltage is minus (or reverse in polarity from the

connected voltage in the course of the ON time), due to the fact that the

output voltage VO is negative.

Therefore, the inductor current lowers

throughout the OFF time. Furthermore, because the connected voltage is

basically steady, the inductor current reduces linearly. This reduction in

inductor current in the course of TOFF is outlined in Figure 3.

provided by:

current. In stable state situations, the current rise, ΔIL(+), in the course of

the ON time and the current reduction through the OFF time, ΔIL(−), has to be

identical.

Or else, the inductor current could offer an overall boost or

reduction from cycle to cycle that would not be a stable condition

circumstance.

Thus, both of these equations may be equated and worked out for

VO to acquire the continuous conduction form buck-boost voltage change-over

affiliation:

= TON/TS and (1−D) = TOFF/TS, the steady-state equation for VO is:

supposed to be similar to TS. This can be genuine only for continuous

conduction mode as we are going to discover in the discontinuous conduction

mode evaluation. An essential scrutiny ought to be made at this point:

Fixing

the two values of ΔIL on par with each other is exactly equal to leveling out

the volt-seconds on the inductor. The volt-seconds employed on the inductor is

the product of the voltage employed and the period that the voltage is applied

for.

This can be the most effective way to estimate unidentified magnitudes

for example VO or D with regards to common circuit parameters, and this

approach is going to be used frequently within this article. Volt-second

stabilize on the inductor is a natural requirement and ought to be perceived at

least additionally as Ohms Law.

voltage was implicitly supposed to be consistent without any AC ripple voltage

throughout the ON time and the OFF period.

This is an accepted simplification

and entails a couple of individual outcomes. First, the output capacitor is

believed to be sizable adequately that its voltage conversion is minimal.

Second, the voltage the capacitor ESR is in addition deemed to be minimal. Such

assumptions are legitimate since the AC ripple voltage will definitely be significantly

lower than the DC portion of the output voltage.

that VO could be tweaked by fine-tuning the duty cycle, D.

This connection

draws near zero as D arrives near zero and rises without destined as D draws

near 1. A typical simplification consider VDS, Vd, and RL are tiny enough to

neglect. Establishing VDS, Vd, and RL to zero, the above formula simplifies

noticeably to:

circuit operation would be to contemplate the inductor as a power storage part.

Each time Q1 is on, energy is poured over the inductor.

While Q1 is off, the

inductor supplies back part of its energy to the output capacitor and load. The

output voltage is regulated by establishing the on-time of Q1. For instance, by

raising the on-time of Q1, the quantity of power sent to the inductor is

amplified.

Additional energy is subsequently sent to the output in the course

of the off-time of Q1 causing an increase in the output voltage. In contrast to

the buck power stage, the typical magnitude of the inductor current is not the

same as the output current.

To associate the inductor current to the output

current, looking at Figures 2 and 3, observe that the inductor current to the output

solely while in the off state of the power stage.

This current averaged over a

whole switching sequence is the same as the output current since the

approximate current in the output capacitor ought to be equivalent to zero. The

connection between the average inductor current and the output current for the

continuous mode buck-boost power stage is provided by:

inductor current is proportional to the output current, and because the

inductor ripple current, ΔIL, is unrelated of output load current, the minimal

and the highest values of the inductor current follow the average inductor

current precisely.

As an example, if the average inductor current declines by 2A owing to a load current reduction, in that case the lowest and highest values

of the inductor current reduce by 2A (considering continuous conduction mode

is preserved).

The forgoing evaluation was for the buck-boost power stage

functionality in continuous inductor current mode. The following segment is a

explanation of steady-state functionality in discontinuous conduction mode. The

primary outcome is a derivation of the voltage conversion relationship for the

discontinuous conduction mode buck-boost power stage.

Evaluation

is reduced and the conduction mode shifts from continuous to discontinuous.

Remember for continuous conduction mode, the average inductor current trails the

output current, i.e. in case the output current reduces, in that case so will

the average inductor current.

Besides, the lowest and highest peaks of the

inductor current pursue the average inductor current accurately. In case the

output load current is decreased below the fundamental current level, the

inductor current would be zero for a part of the switching sequence.

This would

be apparent from the waveforms presented in Figure 3, because the peak to peak

level of the ripple current is unable to alter with output load current.

In a

buck-boost power stage, if the inductor current tries to below zero, it simply

halts at zero (because of the unidirectional current movement in CR1) and

continues there until the outset of the subsequent switching action. This

working mode is known as discontinuous conduction mode.

A power stage working

in discontinuous conduction format possesses three distinctive states through

every switching cycle in contrast to 2 states for continuous conduction format.

The inductor current state in which the power stage is at the periphery between

continuous and discontinuous setting is presented in Figure 4. In this the

inductor current simply collapses to zero while the following switching cycle

commences just after the current attains zero. Observe that the values of IO

and IO(Crit) are laid out in Figure 4 since IO and IL include opposing

polarities.

into discontinuous conduction pattern. This condition is drawn in Figure 5. The

discontinuous mode power stage frequency response is pretty dissimilar from the

continuous mode frequency response which is presented in the Buck-Boost Power

Stage Modeling segment. Additionally, the input to output connection is fairly

diverse as presented down this page derivation:

buck-boost power stage voltage change-over ratio, recollect that you have three

distinctive states that the converter considers through discontinuous

conduction mode functionality.

The ON State is when Q1 is ON and CR1 is OFF.

The OFF State is when Q1 is OFF and CR1 is ON. The IDLE condition is when each

Q1 and CR1 are OFF. The initial two conditions are very much like the

continuous mode situation and the circuits of Figure 2 are relevant apart from

that TOFF ≠ (1−D) × TS. The rest of the switching sequence is the IDLE state.

Additionally, the DC resistance of the output inductor, the output diode

forward voltage drop, as well as the power MOSFET ON-state voltage drop are

usually supposed to be minute enough to overlook.

The time period of the ON

state is TON = D × TS where D is the duty cycle, fixed by the control circuit,

indicated as a ratio of the turn on time to the time of one full switching

sequence, Ts. The length of the OFF state is TOFF = D2 × TS. The IDLE period is

the rest of the switching pattern which is presented as TS − TON − TOFF = D3 ×

TS. These periods are put up with the waveforms in Figure 6.

for the inductor current rise and drop are enumerated below.

issued by:

inductor current, Ipk since in discontinuous mode, the current begins at 0

every cycle.

state is presented by:

current rise, ΔIL(+), in the course of the ON time and the current reduction

while in the OFF time, ΔIL(−), are identical. Thus, both of these equations

could be equated and addressed for VO to acquire the initial of two equations

to be utilized to solve for the voltage conversion ratio:

divided by the output load R). It is the average over one switching sequence of

the inductor current at that time when CR1 becomes conductive (D2 × TS).

equation to acquire:

current (VO divided by R) just derived and the one for the output voltage, both

of them with regards to VI, D, and D2. We at this point unravel each formula

for D2 as well as fix the two equations on par with one another. Utilizing the

resultant equation, an illustration for the output voltage, VO, could be

procured.

transformation affiliation is written by:

dissimilarities between the two conduction modes. For discontinuous conduction

mode, the voltage change relationship is a function of the input voltage, duty

cycle, power stage inductance, the switching frequency, and the output load

resistance. For continuous conduction mode, the voltage change-over connection

is just influenced by the input voltage and duty cycle.

run in a choice between continuous conduction mode or discontinuous conduction

mode. For a specific usage, one conduction mode is chosen while the power stage

was made to sustain the identical mode.

Hi swagatam,plz provide me some help regarding bk-bst converter ckt witk solar panel for pump load with these conditions.1)solar panel (two 12v/1.08A(total 40w) panel in series). 2)a dc pump load(24v/0.4A). The problem is that the ckt should perfm buck for vg>24v and bst opn for vg<24v with i/p vg varing btn 15-34.Arduino is used for duty cycle.help me to choose L&C value.Also any suggestion regarding circuit.

Hi Charan, I can only only suggest you the relevant articles for the info, doing it personally will be be difficult for me due to lack of time, you can refer the following articles for the formulas and calculation help:

http://www.homemade-circuits.com/search/label/Buck%2FBoost