This document discusses optimizing power supply design through simulation. It covers simulating an ideal buck converter, modeling a PWM controller and its components, capturing parasitics, optimizing capacitors, checking RMS currents, selecting components, testing for input ripple and short circuits, and calculating heat loss. The goal is to optimize efficiency and reliability through iterative simulation that accounts for real-world effects.

1. Optimizing Power Supply Design
6/20/2019 Optimizing Power Supply Design 1
A Webinar By EMA Design Automation
Presenter:
Roger Chin

2. 3
• How do we ensure we can convert a (potentially variable)
input voltage into a fixed output voltage?
• Let’s simulate:
1. Build ideal converter to meet desired input and output criteria.
2. Model PWM Controller
3. Capture effects of parasitics
4. Optimize selection of input and output capacitors.
5. Check RMS currents
What Is Power Supply Design…
Optimizing Power Supply Design6/20/2019

3. The Effect of Power Supply Problems
4
Reliability Issues
& Field Failures
Reduced Yields Costly Re-spins
Project Delay
Ripple Effect
Increase MFG
Costs
Missed Time to
Market Schedules
Optimizing Power Supply Design6/20/2019

4. Buck Converter
5
Most common DC-DC converter:
• steps down the voltage
• Avg output current = Avg inductor current
Optimizing Power Supply Design6/20/2019

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Simulating an Ideal Buck
Optimizing Power Supply Design 6
• Input voltage: Vin=V6=25Volt
• Output voltage: Vo=VR1=12.5Volt

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Modeling the PWM Controller
• Difficult to find unencrypted model of switching controller:
use SPICE behavioral blocks instead
– Transistors are a trade-off of simulation time vs. accuracy
– Model with switches with On/Off resistances and threshold/hysteresis
voltages
• Features to model:
– Pulse generation
– Error amplifier (usually modeled w/ opamp
with feedback sense R)
– Soft start can be modeled w/ another switch
– Input under/overvoltage protection
modeled w/ switches
– Output current limiter modeled w/ another
switch

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Quick Aside: Averaged Switch Model
• If you find a macro-model for the switch network, that is great
news!
• How does an averaged switch model work:
– Assumes CCM (continuous conduction model)
– Averages the circuit behavior to eliminate time-variant L’s and C’s
– Provides a subcircuit model that is:
• Large-signal
• Nonlinear
• Time-invariant
– Which means it is more flexible for AC simulation and loop analysis than
the transient methods we show today
Image source - https://ecee.colorado.edu/~rwe/references/COBEP.pdf

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PWM Pulse Generation
How to replicate the switching IC output:
• Create a Sawtooth Ramp voltage source
• Use Switch as a comparator block for feedback loop

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Input Voltage Protection
• Most controllers have circuit protection against input voltage
too high or too low
• We can model with switches to cut voltage supply when the
input crosses a threshold

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Soft Start Mechanism
• Again we can use a voltage-controlled switch with a capacitor
to replicate the soft start function

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Modeling Current Limiter
• Use a Gain block to measure the current, then a switch
to trigger off that measurement

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Selecting the Right Components
• Beyond satisfying equations for input/output ripple, we need to identify in our circuit how variations in component
values will affect our power supply
– Sensitivity analysis to weigh impact of each component parameter
– Monte Carlo to predict how component tolerances affect our goals
– Optimize component values to better meet our goals

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Modeling Parasitics
• We should take into account the equivalent series inductance and
equivalent series resistance of the capacitors and the DC
resistance and parallel capacitance of the inductors
• TIP: some capacitors lose capacitance as DC bias increases and this
can be modeled with a voltage-controlled capacitor in SPICE
• TIP: be sure to give source and drain areas for all MOSFETs, so that
the junction capacitors and overlap capacitors are modeled.

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Accurately Modeling Magnetics
• Capture shape & core type effects of inductors/transformers
• Ideal models will not reflect the behavior in saturation
• A flyback converter stores magnetic energy in inductor air gap

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Calculating Hysteresis Loss
• The tool automatically calculates the inductor current frequency
and loop area of hysteresis B-H curve.
This solves for total core loss of the inductor as the core charges
from - to + saturation and back

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Testing Input Ripple
• Add peak-to-peak noise to our previously ideal DC source
• We can also add another source to trigger a change in input voltage
to show how our circuit would respond

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Testing Short Circuit at Output
• Test how the circuit responds to load changes
• The circuit should stop supplying pulses if the load is shorted

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Power Supply Convergence Issues
• Tip: loosen ABSTOL or use Autoconvergence when simulating power circuits
• As a rule of thumb: the absolute tolerances should not be more than 9
orders of magnitude smaller than typical signals present in the circuit.

19. 20
Selecting and Simulating Heat Sinks
• Selecting appropriate heat sink is a key aspect of any
power circuit design
• We should also simulate effect of heat sink
• Associate appropriate heat sink with semiconductor device
Define
Derate
specification
Associate
Derate
specification
with
component
Select
desired
Derate
specification
Run SMOKE
analysis
Optimizing Power Supply Design6/20/2019

20. 22Optimizing Power Supply Design6/20/2019
Tackling RMS Currents
• Root Mean Squared
measurements of current is
the best way to check power
dissipation and thus heat
• In real-life, it is often a
challenge to measure RMS
currents accurately on each
device
• We want to know how much
the rating is exceeded for
each device

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Digital Power Supplies
Optimizing Power Supply Design6/20/2019
• Digital power supply technology uses analog-to-digital
converters which will also need to be modeled
• If your PWM controller is modeled in C or MATLAB, it can be
a challenge to simulate along a SPICE circuit

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With Good Power Supply You Can:
See & Fix Violations
In Real-Time
Streamline Design
Release Process
Eliminate Multiple
Validation Iterations
Improve Time
To Market

23. Demo Time…
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Thank you for joining us today.
Questions?
Next Webinar:
Serial Links
EMA Design Automation
800-813-7494
edc@ema-eda.com
www.ema-eda.com
Optimizing Power Supply Design6/20/2019