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Transcript
Literature Number: SNVP001
Ease Power Supply Design with Design Tools
by
Jeff Perry, Senior Manager, WEBENCH Design Tools
National Semiconductor Corp.
As most electrical system design engineers have experienced, power supply
design is often left until the last minute. With a deadline looming and the boss
looking over one’s shoulder, an engineer is in a rush to get something fast.
Faced with the complexity of implementing a DC to DC switching power supply
design under these circumstances, designers can be put under stress and
possibly make the wrong decisions.
But it doesn’t have to be this way. The latest power supply design tools provided
by IC manufacturers, such as National Semiconductor’s WEBENCH® Design
Environment, provide an efficient alternative to the traditional trial and error
approach to design. A user can do virtual testing and optimization before
constructing a prototype thereby reducing or eliminating costly and time
consuming design iterations. The result is a higher quality and more robust
design which is tailored to the designer’s needs.
Design tools are also useful for more experienced power supply design
engineers. With access to a database of thousands of components provided by
a good design tool, engineers can visualize tradeoffs using real part numbers
with accompanying parametric, price and footprint information, a task which
might otherwise involve extensive research.
A power supply design environment should be easy to use and provide an end to
end on-line enabled design and prototyping process. The design flow can be
broken out as follows:
1) User enters design parameters and selects from a listing of recommended
topologies/regulators.
2) Software calculates passive components and power supply characteristics.
3) User optimizes the design using graphs to visualize tradeoffs.
4) Designer conducts real time electrical simulation to view dynamic circuit
behavior and further refine the design.
5) User performs thermal simulation to identify and correct heat problems.
6) Software facilitates easy on line ordering of samples, demo boards and
custom prototype kits with overnight processing.
Figure 1: National Semiconductor’s WEBENCH Power Supply Design Tool
Power Supply Design Specifications
Before using a design tool, it is important for the designer to determine in
advance basic specifications such as input voltage range, output voltage, load
current and ambient temperature. Other inputs include performance factors such
as Vout ripple and desired transient response. In addition, the user should have
a good idea of the footprint and efficiency budgets. Design tools prompt the
designer for these specifications as part of the initial process. Then the user is
presented with a list of recommended components and topologies. The first
decision to be made is choosing the topology. For example in the case of a step
down application, a buck solution is normally the most cost effective due to the
low component count, but a flyback topology may be required if isolation is
needed or if the output voltage is very close to the input voltage. Another key
parameter is the switching frequency of the regulator. If the user requires high
efficiency, it is best to have a regulator that runs at low frequency (down to 100
kHz). On the other hand, a regulator with high switching frequency (up to 1MHz
or more) is best if small footprint is desired. A regulator with user selectable
frequency allows for more optimization possibilities. Other attributes to weigh
include features such as soft start, variable current limit, and on/off capability.
The next step is the calculation of component values by the software. To
achieve this, first the voltages and currents at each node in the circuit are
calculated and safety margins are added to determine minimum component
limits. Targets for other component parameters are then determined by the
desired performance. For example, the target for inductance is based on the
ripple current goal which affects peak current and Vout ripple. Input capacitance
is determined based on the desired input voltage deviation. Output capacitance
is determined by the targeted output transient response. Components are thus
automatically selected based on the component selection algorithms in the
software.
Modeling and reducing power loss is essential to achieve high efficiency in a
design. For an asynchronous integrated FET buck converter circuit, as shown in
figure 2, the primary sources of loss occur in the main current path and consist of
the regulator with internal FET, the catch diode, and the output inductor. Of
these components, power dissipation is dominated by the FET and/or diode
depending on the duty cycle, with high duty cycle designs having more loss in the
FET and low duty cycle designs being dominated by losses in the diode. Power
dissipation in the FET consists of DC losses determined by the RdsOn
characteristic of the FET and AC losses which are a function of the capacitance
of the FET which affects the rise and fall time of the switch. Power loss in the
diode is determined by the forward voltage drop and by the capacitance of the
diode. Losses in the inductor are a function of the DC resistance and core
material/geometry.
Input
Load
Current Path with Switch On
Current Path with Switch Off
Figure 2: Current Path through a Typical Buck Switching Regulator
AC Losses vs. Frequency
AC losses in the switch follow the following equation:
PswAC = ½ * Vds * Ids * (trise + tfall)/tsw
As shown in figure 3, AC switching losses occur when the switch is turning on
and off and both current and voltage in the switch are non zero. These losses
are relatively fixed in duration. But we have control over the switching time
period (tsw) for regulators which have user selectable frequency. Using low
switching frequency/long switching period causes the rise/fall time to be a smaller
percentage of each switching cycle and efficiency is raised. However, this
increases the on time of the switch (dt) which requires increased inductance to
keep the ripple current (dI) constant as per the basic inductance equation:
L = V * dt/dI
This results in more turns on the inductor and therefore a larger footprint.
Conversely, increasing the switching frequency reduces the footprint. Higher
frequency reduces the switch on time (dt) and thus lowers the inductance
required to get the same ripple current (dI). This will reduce the number of turns
required and make for an inductor with smaller footprint. Unfortunately, high
frequency designs lead to higher AC losses in the switch and worse power
dissipation, the opposite of what was described above.
Regions of power loss (V*I)
Vsw
Miller Plateau
Miller Plateau
Vdriver
Vth
Vth
Isw
Vg
Vsw = -Vds
Switch Off
On
Trise
High Freq = High Loss
Off
Tfall
Low Freq = Low Loss
Figure 3: Symbolic View of AC Switching Power Losses
Optimizing for Efficiency vs. Footprint
A tool such as the WEBENCH Design Environment provides an easy method to
optimize a buck controller design for efficiency or footprint or a balance between
the two. A software knob control, as shown in figure 4, allows the user to select
the desired outcome of optimization. If the user picks a high optimization setting
to emphasize high efficiency, the frequency is lowered, which causes lower AC
switching losses but higher footprint as described above. Conversely, if the user
picks a low optimization setting to get a small footprint for the design, the
frequency is raised which results in a smaller inductor and lower overall footprint,
but worse efficiency. There are additional factors which are considered in the
optimization process including parasitic resistance (ESR, DCR), component cost
and availability. As can be seen the graph, intermediate results may offer good
compromises between efficiency and footprint, such as the optimization 4 setting
shown in figure 4 with an efficiency of 91% and a footprint of 350 square
millimeters vs. the design in option 5 with 93% efficiency but a footprint over 100
square millimeters.
Left side:
Higher frequency
Smaller footprint
Right side:
Lower frequency
Lower resistance
Figure 4: Optimzation Of A Power Supply Design Showing
Tradeoff between Efficiency and Footprint
Once a power supply has been optimized for performance, price must be taken
into account. There are likely a number of possible components with similar
parameters which meet the specifications for the design. If the designer is willing
to compromise slightly on the efficiency, a lower priced component may be used
with almost the same results. Figure 5 shows a graph of the design efficiency
calculated for each pair of FET solutions in a synchronous buck design vs. the
relative price of the FET pair. A user can save 10% on the FET cost by taking a
drop of about .4% in efficiency.
Figure 5: Pairs of FET Solutions for a Synchronous Controller
Comparing Efficiency with Price
Electrical Simulation
Why do electrical simulation if the design has already been configured for stable
operation? The user may want to observe dynamic circuit behavior, such as
transient response, which is not readily discernable from static operating values.
In turn, the designer may want to further tune the design for output ripple voltage,
loop stability and/or transient response. A good design environment allows a
user to run a number of different time domain simulations including start-up, input
line transient, output load transient and steady state. A Bode plot, useful for
confirming system stability, is also essential. A waveform viewer which allows
easy zooming and manipulation of waveforms is needed to analyze simulation
results.
As an example, the load transient simulation allows easy comparison of two
different regulators, one using a pulse width modulated (PWM) voltage control
mode and the other using a hysteretic control mode. As can be seen in figure 6,
the simulation shows that the hysteretic regulator circuit has much faster
transient recovery time but more output ripple thus allowing the user to quickly
select a regulator based on which attribute is most important.
Figure 6: Load Transient Simulations for Voltage Mode PWM (left)
vs. Hysteretic (right) Voltage Regulator Circuits
using a Current Pulse going from .5 to .1A.
Solving Thermal Issues
Thermal simulation is an excellent tool for identifying and solving temperature
problems in power supply circuits. Board level simulation takes into account co
heating of adjacent components which traditional thetaJA calculations do not
address. Thus if two components which dissipate significant amounts of power in
a regulator circuit are located close together, the net effect can be higher than
expected temperatures for each of them. To get good accuracy balanced with
fast simulation, the type of model used for each component should be
appropriate. For components dissipating small amounts of power, a lumped
cuboid model based on the volume of the component is adequate and speeds
simulation time. For components dissipating more power, a sophisticated model
taking into account the component geometry and materials must be used. The
copper traces on the board itself also affect the temperature and therefore should
be modeled.
When a thermal problem is encountered, there are many ways to solve it. This
might include moving components apart, increasing the copper thickness on the
board, and increasing the amount of copper attached to components. More
costly solutions include adding a fan and/or including an extruded heat sink. The
thermal simulation tool should allow the user to try out these options and quickly
see the results. In figure 7, a 5A power supply using an integrated switch voltage
regulator initially starts out hot. By running thermal simulations, it is found that
using a fan or an attached heat sink with larger board layout brings the
temperature down to acceptable levels.
Use a Fan
•No airflow
•Diode: 134C
•IC: 146C
•500 LFM airflow
•Diode: 95C
•IC: 106C
Or add a
Heat Sink
•No forced airflow
•Heat sink
•Diode: 91C
•IC: 52C
Figure 7: Thermal Simulation Results for a 5A Integrated Switch Power Supply
Building a Prototype and Documenting the Design
After using design tools to optimize a power supply design, it is still important to
build a prototype. This is because simulation, even after extensive verification of
models, may not be completely accurate or take into account all factors under all
conditions. The ability to quickly obtain samples, evaluation boards and
complete custom kits with on line ordering and overnight shipment capability is a
key attribute. In this way, a prototype can be quickly constructed and evaluated
to make sure the result is what was desired. Downloadable schematic, layout
and Gerber files ease the integration of the power supply into the designer’s
overall circuit.
Lastly, automatic documentation of the design with the schematic, bill of
materials, graphs of operating values and simulation results provides a record for
future reference and reuse. This greatly eases a task which can tend to be
skipped under the normal pressures of getting a product released.
In summary, design tools such as National Semiconductor’s WEBENCH Design
Environment, available at http://www.national.com, allow optimization of power
supplies through visualization of tradeoffs such as efficiency, footprint and price
using real world components. The result is a power supply which is better
tailored to the requirements of a customer’s proprietary circuit than simply using
a fixed reference design from a datasheet. In addition, end to end design and
prototyping is achievable in a very rapid manner which reduces the design cycle
time and brings designs to market faster.
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