Literature Review
ECE 495P
Team #44: Buck Converter
October 14th, 2010
Written by:
Sunghun Cho
Samantha Simpson
Michael Howe
Mallory McGuire
Abstract
Buck converters are an essential part of various power supply systems for many of today’s
electronic devices. Covidien, a biomedical company, is currently using a buck converter in the
power supply of their medical device that applies RF energy to seal vessels. Our goal is to
optimize the buck converter to increase their device’s overall efficiency. To increase the buck
converter’s overall efficiency, we will look into adding an input control device and optimizing
each of the buck converter’s subsystems individually.
Introduction
Step-down or buck converters are a vital part to electronics today. They are capable of
converting a voltage source into a lower regulated voltage source, through the use of a
switching mechanism, a diode, an inductor, and multiple capacitors. Buck converters have
many applications, including instances where the use of a low voltage power supply can reduce
power consumption, as seen in transmitters, microprocessors, and flash memory [1].
A linear regulator can serve the same function as the buck converter, but the buck converter
has higher efficiency and less power loss through thermal dissipation. In exchange for the
higher efficiency, the buck converter has higher noise and is a larger device.
Figure 1 shows the basic components of a buck converter. When the controller sends a high
signal to the PMOSFET, the gate driver of the MOSFET turns on. As the voltage across the
inductor L increases, the current also
increases. In turn, the voltage across Cout
begins to increase with time. When the
controller ceases to send a high signal to
MOSFET, the MOSFET turns off and
Figure 1: Buck Converter Basic Components
current through L and voltage across Cout
is removed. When the MOSFET turns off, the current cannot travel through it and the diode D,
protects the MOSFET by routing the current away from the MOSFET and from ground.
Now each of the buck converter subsystems will be discussed in greater detail.
Optional Input Signal Boost Device
A buck converter in its simplest
form does not require an external
controller. The design Covidien is
currently using for their buck
converter utilizes a boost device as
a controller for the buck converter.
Figure 2: Boost Device Circuit Diagram
The boost device used in the buck converter we plan to optimize is a product Linear Technology
product with part number LTC4444. Other manufacturers make similar parts with comparable
price points. LTC4444 is intended to be used with a buck converter and is capable of handling a
high input voltage up to 100V. Linear Technology describes the chip as a “high voltage gate
driver that drives two N-channel MOSFETs in a synchronous DC/DC converter,” [2]. The boost
device accepts two input signals. The LTC4444 has protection built in to prevent both MOSFETs
from conducting at the same time. The circuit diagram in Figure 2 shows a typical application of
this boost device, which is used in a buck converter.
Another company that produces a similar boost device is Maxim Integrated Products. Maxim
provides more extensive information on the use of the boost converter within the buck
converter circuit. The boost device is used as a control technique. There are other techniques
also being used for similar buck converter circuit designs. The steady state output is controlled
by changing the duty cycle, which can be done using pulse width modulation or with the
addition of a boost converter. The pulse width modulation method establishes an error signal,
and then compares this signal to an oscillating ramp signal. The comparator used to compare
the error signal and ramp signal, outputs another PWM used as a switching signal. This signal
controls the switch as seen in Figure 3.
The previous example was a voltage
mode controller. Another example of
a voltage mode controller is given by
Maxim utilizing a boost device model
Figure 3: Voltage Mode Controller
MAX1932. This device is more
advanced than the Linear Technology boost chip. This chip is an integrated controller with a
digital to analog converter on board. This slightly more advanced chip could allow us to adjust
parameters through electronic means. Many of the same concepts that are used in the
comparator model are used in this Boost converter chip. An error signal is produced when a
divider circuit is created between two pins and is subtracted from the internal reference
voltage of the device. Two differential input signals along with the error signal create a
transconductance amplifier, changing our voltage inputs into a current source. This
sophisticated circuit allows us to adjust the circuit’s gain as well as trim the loop gain. The
output signal that was created is then brought to an internal comparator with the error signal
to control the switch as in the simplified model [3].
These have been models of Voltage Mode Controllers which are what we plan to use in our
design, however, Current Mode Controllers are also available. We plan to input voltage signals
encouraging us to use a voltage mode controller. Depending on the size of the application and
voltage requirement there are a number of boost chips that can be utilized as controllers with
our buck converter.
MOSFETS
Important to the operation of the buck converter is the use of multiple Metal-Oxide
Semiconductor Field Effect Transistors (MOSFETs). First of all, different structures have been
developed over the years to accommodate different applications. The structures have been
improved upon from the standard MOSFET, which has a gate separated from the body by an
insulating layer of silicon dioxide. The body is a heavily doped silicon structure. For an NMOS,
the body is heavy in holes (P+) and has a drain and source area that is heavily doped with
donors (N+). For a PMOS, the opposite is true, such that the body is N+ and the source and
drain are P+. As a voltage is applied to the gate, a depletion area forms until a critical voltage is
reached. Once this threshold voltage (VT) is reached, an inversion layer is created and forms a
channel for current to pass from the source to the
drain. From this basic structure, other models
have been made to be suitable for higher voltage
applications where a low resistance is needed.
The first structure to talk about is the Lateral
Diffusion MOSFET (LDMOS). In addition to the
Figure 4: Structure of an LDMOS
basic MOSFET structure, there is an additional lightly doped N+ region between the oxide and
the drain. This helps the LDMOS withstand higher voltages with thinner gate oxide and a
shorter channel length [5]. This is illustrated in Figure 4 above.
The downside is that current can still flow on the surface of the silicon and that the specific
resistance is still relatively high. The next structure
is the power VMOSFET, which comes from the Vshape in the structure where current flows. The
drawback of this structure, shown in Figure 5 is
that the high electrical field at the tip of the Vgroove [5]. Another structure was created to fix
this problem: the Double Diffusion MOSFET
Figure 5: Structure of a VMOSFET
(DMOS), shown in Figure 6 [5]. In this design, the current travels horizontally through the
channel and then vertically to the drain. The final structure to mention is the UMOSFET. This
structure gets its name from the U-shaped
groove in the gate region. This design, shown in
Figure 7, has a higher channel density to reduce
on resistance [5]. In addition, there is no sharp
oxide tip like in the VMOSFET.
Figure 6: Structure of a DMOSFET
Now that the general structures have
been covered, it is important to
understand how a MOSFET works. For
steady state operation, the MOSFET can
Figure 7: Structure of UMOSFET
be treated as a switch or diode. The case
of an off switch or diode can be seen when the voltage threshold has not been met. The current
cannot flow from the drain to source. However, the current can travel from the source to drain
through the inherent body diode of the MOSFET. There are two situations that can happen
once the threshold voltage has been met. First is when the gate to drain voltage is less than the
threshold voltage. In this case, the current channel becomes pinched, is considered to operate
in the linear region, and can be modeled as a voltage controlled resistor. The other case is when
the gate to drain voltage is greater than the threshold voltage. In this case, the MOSFET
operates in the saturation region and can be modeled as a voltage controlled current source.
More specific to the buck converter are how the high (Control) and low (Sync) side
MOSFETs function in steady state and during transitions in the duty cycle. For steady state
function, there are two times to consider. The first time is when high voltage is allowed to pass
to the output. In this case, the input voltage is high at the Control FET gate is turned on,
allowing current to pass through to the output line. The second time to consider is when the
low voltage is allowed to pass to the output. At this time, the Sync FET is on and the Control FET
is off.
The steady state functionality is rather simple compared to the switching times. The first
time to consider in switching is when the Control FET turns off and the Sync FET turns on.
During this time, as Figure 8 shows, the voltage at the node between the two FETs gets
clamped to below ground because of the inductor current that must keep flowing. Without the
use of a Schottky diode across the Sync FET, there would be a reverse mode of operation on
that FET and current would run from the drain through the body of the FET. The Schottky diode
ensures that this does not happen as it takes the excess current during this time. The second
time in switching is when the Sync FET shuts off and the Control FET turns back on. At this time,
the Control FET must “hard-switch”, meaning it must fight reverse recovery and the voltage
drop across that FET.
It is important to note
that timing plays a major role
in switching. During
switching, extra caution is
needed to ensure that both
the Control and Sync FET are
never on at the same time. If
this were to happen, the
circuit would effectively be
Figure 8: Relationship Between the Currents, Voltages, and Gate Drive Timing
shorted to ground and lead to excessive power loss and heat-dissipation. For this reason, a
“blanking time” is needed where both the FETs are off to avoid overlapping. During this time is
when power is lost through the FETs.
The two main sources of power loss from FETs are through switching losses and
conduction losses. In order to optimize efficiency in the FETs, each can be looked at separately.
For the Control FET, duty cycle is usually low, so conduction loss is not very important. On the
other hand, the Control FET incurs switching losses at both the turn on and turn off since it
connects the high voltage to the output filter. This means that the switching loss is far more
important to minimize for this transistor. For the Sync FET, just the opposite is seen. There is no
switching loss since it takes advantage of a zero voltage turn on and off. On the other hand, the
duty cycle is usually high, so conduction loss can be very important. Conduction loss is very
straight forward to minimize since it is directly proportional to on resistance from drain to
source of the FET. Less straight-forward is minimizing switching performance. A good indicator
of easier and faster switching is a low gate charge required to form a channel for current to
flow. Another consideration to look at for switching performance is the ratio of charge of gatedrain to gate-source (Qgd/Qgs) [5]. When the Control FET turns on, a dV/dt is impressed upon
the drain of the Sync FET, so a lower Qgd/Qgs reduces the probability of simultaneous
conduction of the Control and Sync FET. One final condition to look at for switching
performance is how well the Sync FET deals with reverse recovery. The term Qrr is used to
measure reverse recovery charge, so a lower Qrr is better for higher frequencies (above 500
kHz) [5].
Inductor
An inductor is a passive electrical component that can store energy in a magnetic field and its
inductance is measured in Henries (H). Inductance is resulted from the magnetic field which
tends to resist changes in current. The amount of energy stored in an inductor is expressed as E
1
=2 𝐿𝐼 2 . Inductor has a characteristic that when there is more current regardless of the value of
inductors, more energy is stored, but, it only grows linearly with respect to the inductance
value (see Figure 1) [1].
Figure 9: Energy stored Vs. Current
Inductors are broadly used in analog circuit and signal process as well as the energy storage
devices in some switched-mode power supplies. Inductors are also used in electrical
transmission systems, where they are used to limit switching currents and fault current and to
depress voltages from lightning strikes.
The operation of an inductor within the buck converter is of high importance, as it is used to
store energy when the MOSFET switch disconnects the load from the source voltage. The
MOSFET alternates between connecting the inductor to source voltage and discharging the
inductor into the load. When the semiconductor switch is on, the current in the inductor ramps
up and energy is stored. When the switch is turned off, this energy is released into the load. In
other words, the inductor is used as an energy storage device.
Selecting appropriate inductors is one of the most
important parts to optimizing the buck converter.
Figure 10: Buck Converter Circuit
Designers must know how to predict performance of
Diagram
certain applications accurately and decide the most efficient inductors, as it is directly related
to cost and performance [1]. When selecting an inductor for buck convertor, following
parameters are needed to be calculated or defined [7].





Output Voltage
Switching frequency
Maximum input voltage
Duty cycle
Maximum ripple current
To find the required inductor values there are two methods, the basic method and volt-seconds
method. Using intuition is not recommended and it can be rather misleading [8].
Basic Method
From general rule V= L*dl/dt, the following equation is found:
VIN-VSW-V0= 𝐿 ×
∆𝐼
𝐷/𝑓
VIN =applied DC input voltage
D = duty cycle
VSW= voltage across the switch when it is ON
f = switching frequency in Hz
Then, solve for ∆𝐼 and
∆𝐼
(𝑉𝐼𝑁 − 𝑉𝑆𝑊 − 𝑉𝑜 )×𝐷
𝑜
𝐿×𝑓×𝐼𝑜
r= 𝐼 =
r= constant for a given application
Now, in a buck converter, the formula for the duty cycle is
D=𝑉
𝑉𝐷 +𝑉𝑜
𝐼𝑁 −𝑉𝑆𝑊 +𝑉𝐷
Then, r can be rewritten as
(𝑉𝐼𝑁 − 𝑉𝑆𝑊 − 𝑉𝑜 )×(𝑉𝐷 +𝑉𝑜)
R = (𝑉
𝐼𝑁 − 𝑉𝑆𝑊 + 𝑉𝑜 )×𝐿×𝑓×𝑙0
Finally, L is found as
𝐿=
(𝑉𝐼𝑁 − 𝑉𝑆𝑊 − 𝑉𝑜 ) × (𝑉𝐷 + 𝑉𝑜)
× 106 µ𝐻
(𝑉𝐼𝑁 − 𝑉𝑆𝑊 + 𝑉𝑜 ) × 𝑟 × 𝑓 × 𝑙0
Volt-seconds method
The general rule V= L*dl/dt is equal to V*dt=L*dl.
Here, V*dt is the applied voltseconds. So following equation is found.
Et=V∆𝑡 = 𝐿∆𝐼 𝑉µ𝑠𝑒𝑐𝑠
Et=voltage across the winding of the inductor times the duration in µ𝑠𝑒𝑐𝑠.
Now, r can be written as
𝐸𝑡
r=𝐿 ×𝐼
𝑂
Then solving for L,
𝐸𝑡
L = 𝑟 ×𝐼
𝑂
From ∆𝐼 =
𝑉𝐼𝑁 −𝑉𝑆𝑊 −𝑉0
𝐿
𝐷
× 𝑓 , it can be found that inductance is inversely proportional to the
ripple current. In other words, when larger inductor is used, ripple current would be reduced
while smaller inductor increases the ripple current. Therefore, it can be said that ripple current
is determined by size of inductance. Moreover, there is a characteristic that large ripple current
means the peak current is greater and it makes greater saturation of the inductor, and it gives
more stress on the transistor. So, it should be made sure that the saturation current of the
inductor should be greater than peak current to handle peak current.
Since selecting the right inductor is one of the most important parts to optimizing the buck
converter, understanding the relationship among components and using right equations are
very important.
Conclusion
Buck converters are an essential part to power supply systems of various devices, including
Covidien’s medical device. Optimizing the efficiency of a buck converter can be achieved by
including the optional signal boost controller and looking into the individual efficiencies of each
of the converters subsystems: MOSFETS and output inductor. By using the methods described
in this literature review, we hope to increase the efficiency of the buck converter and the
power supply system as a whole.
Bibliography
[1] Schelle, Donald, and Jorge Castorena. Buck Converter Design Demystified. Power Electronics
Technology, June 2006. Web. 10 Oct. 2010.
[2] "Linear Technology." Linear Technology - High Voltage Synchronous N-Channel MOSFET
Driver. Web. 13 Oct. 2010.
<http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1142,C1041,P39339>
[3] "DC-DC Converter Tutorial - Maxim." Analog, Linear, and Mixed-signal Devices from
Maxim/Dallas Semiconductor. Maxim, 29 Nov. 2001. Web. 13 Oct. 2010. <http://www.maximic.com/app-notes/index.mvp/id/2031>
[4] Bai, Yuming. Optimization of Power MOSFET for High-Frequency Synchronous Buck
Converter. Diss. Virginia Polytechnic Institute and State University, 2003. Blacksburg: Yuming
Bai, Virginia. Print.
[5] Persson, Eric. "How FET Selection Can Optimize Synchronous Buck Converter Efficiency."
EETimes. 7 July 2006. Web. 12 Oct. 2010. <http://www.eetimes.com/design/powermanagement-design/4012090/How-FET-selection-can-optimize-synchronous-buck-converterefficiency>
[6] “S4w's Wind Generator Notes” 2007. 1 Oct. 2010. <http://www.s4wsbox.com/?q=node/40>
[7] “Switching Regulator Inductor Design” 2010. Cooper Bussmann. 10 Oct. 2010.
<www.cooperbussmann.com>
[8] “Selecting Inductors for Buck Converters” 2002. National Semiconductor Corporation. 10
Oct. 2010. <www.national.com>