A
boost converter (
step-up converter) is a power converter with an output DC voltage greater than its input DC voltage. It is a class of
switching-mode power supply (SMPS)A switched-mode power supply is an electronic power supply unit that incorporates a switching regulator in order to provide the required output voltage.-Terminology:...
containing at least two
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
switches (a
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
and a
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's...
) and at least one
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
storage element. Filters made of
capacitorA capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force...
s (sometimes in combination with
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s) are normally added to the output of the converter to reduce output voltage ripple.
Power can also come from DC sources such as batteries, solar panels, rectifiers and DC generators.
A
boost converter (
step-up converter) is a power converter with an output DC voltage greater than its input DC voltage. It is a class of
switching-mode power supply (SMPS)A switched-mode power supply is an electronic power supply unit that incorporates a switching regulator in order to provide the required output voltage.-Terminology:...
containing at least two
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
switches (a
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
and a
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's...
) and at least one
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
storage element. Filters made of
capacitorA capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force...
s (sometimes in combination with
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s) are normally added to the output of the converter to reduce output voltage ripple.
Overview
Power can also come from DC sources such as batteries, solar panels, rectifiers and DC generators. A process that changes one DC voltage to a different DC voltage is called DC to DC conversion. A boost converter is a
DC to DC converterIn electronic engineering, a DC to DC converter is an electronic circuit which converts a source of direct current from one voltage level to another. It is a class of power converter.- Usage :...
with an output voltage greater than the source voltage. A boost converter is sometimes called a step-up converter since it “steps up” the source voltage. Since power must be conserved, the output current is lower than the source current.
History
For high efficiency, the
SMPSA switched-mode power supply is an electronic power supply unit that incorporates a switching regulator in order to provide the required output voltage.-Terminology:...
switch must turn on and off quickly and have low losses. The advent of a commercial
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
switch in the 1950’s represented a major milestone that made SMPSs such as the boost converter possible. Semiconductor switches turned on and off more quickly and lasted longer than other switches such as
vacuum tubeIn electronics, a vacuum tube, electron tube , thermionic valve, or valve is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space...
s and electromechanical relays. The major
DC to DC converterIn electronic engineering, a DC to DC converter is an electronic circuit which converts a source of direct current from one voltage level to another. It is a class of power converter.- Usage :...
s were developed in the early-1960s when semiconductor switches had become available. The
aerospaceAerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
industry’s need for small, lightweight, and efficient power converters led to the converter’s rapid development.
Switched systems such as SMPS are a challenge to design since its model depends on whether a switch is opened or closed. R.D. Middlebrook from Caltech in 1977 published the models for DC to DC converters used today. Middlebrook averaged the circuit configurations for each switch state in a technique called state-space averaging. This simplification reduced two systems into one. The new model led to insightful design equations which helped SMPS growth.
Applications
Battery powered systems often stack cells in series to achieve higher voltage. However, sufficient stacking of cells is not possible in many high voltage applications due to lack of space. Boost converters can increase the voltage and reduce the number of cells. Two battery-powered applications that use boost converters are
hybrid electric vehiclesA hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles , which combine an internal combustion engine and one or more electric motors.-Power:...
(HEV) and lighting systems.
The
Toyota PriusThe Toyota Prius is a full hybrid electric mid-size car developed and manufactured by the Toyota Motor Corporation. It first went on sale in Japan in 1997, making it the first mass-produced hybrid vehicle. It was subsequently introduced worldwide in 2001. The Prius is sold in more than 40...
HEV uses a 500 V motor. Without a boost converter, the Prius would need nearly 417 cells to power the motor. However, a Prius actually uses only 168 cells and boosts the battery voltage from 202 V to 500 V. Boost converters also power devices at smaller scale applications, such as portable lighting systems. A white LED typically requires 3.3 V to emit light, and a boost converter can step up the voltage from a single 1.5 V alkaline cell to power the lamp. Boost converters can also produce higher voltages to operate
cold cathodeA cold cathode is an element used within some nixie tubes, gas discharge lamps, discharge tubes, and vacuum tubes. The term cold cathode refers to the fact that the cathode is not independently heated...
fluorescent tubes (CCFL) in devices such as
LCDA liquid crystal display is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures...
backlightA backlight is a form of illumination used in liquid crystal displays . Backlights illuminate the LCD from the side or back of the display panel, unlike frontlights, which are placed in front of the LCD...
s and some
flashlightA flashlight is a portable electric spotlight that emits light from a small incandescent lightbulb or from one or more light-emitting diodes...
s.
Operating principle
The key principle that drives the boost converter is the tendency of an inductor to resist changes in current.
When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being
discharged, it acts as an energy source (somewhat like a battery). The voltage it produces during the
discharge phase is related to the rate of change of current, and not to the original charging
voltage, thus allowing different input and output voltages.
The basic principle of a Boost converter consists of 2 distinct states (see figure 2):
- in the On-state, the switch S (see figure 1) is closed, resulting in an increase in the inductor current;
- in the Off-state, the switch is open and the only path offered to inductor current is through the flyback diode
A flyback diode is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.- Induction at the Opening of a Contact:According to Lenz's law, if the current through an inductance changes, this...
D, the capacitor C and the load R. This results in transferring the energy accumulated during the On-state into the capacitor.
- The input current is the same as the inductor current as can be seen in figure 2. So it is not discontinuous as in the buck converter and the requirements on the input filter are relaxed compared to a buck converter.
Continuous mode
When a boost converter operates in continuous mode, the current through the inductor never falls to zero. Figure 3 shows the typical waveforms of currents and voltages in a converter operating in this mode.
The output voltage can be calculated as follows, in the case of an ideal converter (i.e using components with an ideal behaviour) operating in steady conditions:
During the On-state, the switch S is closed, which makes the input voltage appear across the inductor, which causes a change in current flowing through the inductor during a time period (t) by the formula:
At the end of the On-state, the increase of I
L is therefore:
D is the duty cycle. It represents the fraction of the commutation period T during which the switch is On. Therefore D ranges between 0 (S is never on) and 1 (S is always on).
During the Off-state, the switch S is open, so the inductor current flows through the load. If we consider zero voltage drop in the diode, and a capacitor large enough for its voltage to remain constant, the evolution of I
L is:
Therefore, the variation of I
L during the Off-period is:
As we consider that the converter operates in steady-state conditions, the amount of energy stored in each of its components has to be the same at the beginning and at the end of a commutation cycle. In particular, the energy stored in the inductor is given by:
Therefore, the inductor current has to be the same at the beginning and the end of the commutation cycle. This can be written as
Substituting and by their expressions yields:
This can be written as:
Which in turns reveals the duty cycle to be:
(multiply D by 100 if you wish D in percent)
From the above expression it can be seen that the output voltage is always higher than the input voltage (as the duty cycle goes from 0 to 1), and that it increases with D, theoretically to infinity as D approaches 1. This is why this converter is sometimes referred to as a
step-up converter.
Discontinuous mode
In some cases, the amount of energy required by the load is small enough to be transferred in a time smaller than the whole commutation period. In this case, the current through the inductor falls to zero during part of the period. The only difference in the principle described above is that the inductor is completely discharged at the end of the commutation cycle (see waveforms in figure 4). Although slight, the difference has a strong effect on the output voltage equation. It can be calculated as follows:
As the inductor current at the beginning of the cycle is zero, its maximum value (at ) is
During the off-period, I
L falls to zero after :
Using the two previous equations, δ is:
The load current I
o is equal to the average diode current (I
D). As can be seen on figure 4, the diode current is equal to the inductor current during the off-state. Therefore the output current can be written as:
Replacing I
Lmax and δ by their respective expressions yields:
Therefore, the output voltage gain can be written as flow:
Compared to the expression of the output voltage for the continuous mode, this expression is much more complicated. Furthermore, in discontinuous operation, the output voltage gain not only depends on the duty cycle, but also on the inductor value, the input voltage, the switching frequency, and the output current.
See also
- Buck Converter
A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter it is a switched-mode power supply that uses two switches , an inductor and a capacitor....
- Buck–boost Converter
Two different topologies are called buck–boost converter.Both of them can produce an output voltage much larger than the input voltage....
- Split-Pi
Split-Pi is a power converter topology that can theoretically produce an arbitrary output voltage, either higher or lower than the input voltage. In practice the upper voltage output is limited to the voltage rating of components used. It is essentially a Boost converter followed by a Buck ...
(patented boost–buck converter technology)
- The hydraulic ram
A hydraulic ram, or hydram, is a cyclic water pump powered by hydropower. It functions as a hydraulic transformer that takes in water at one "hydraulic head" and flow-rate, and outputs water at a higher hydraulic-head and lower flow-rate...
can be seen as analogous to a boost converter, using the electronic–hydraulic analogyThe electronic–hydraulic analogy is the most widely used analogy for "electron fluid" in a metal conductor. Since electric current is invisible and the processes at play in electronics are often difficult to demonstrate, the various electronic components are represented by hydraulic...
.
External links