PowerWise® Design University

Efficiency and Thermal Response of Buck Regulators

Efficiency and Thermal Response of Buck Regulators



Objective: Identify the major sources of power loss in a buck regulator. Learn to use WebTHERM to simulate the thermal response of a buck regulator pc board. Identify techniques to reduce the temperature of components on the regulator pc board.

Length: 60 minutes

Published Date: Jul 30, 04

Step(s) to complete

  1. Complete the suggested assignments
  2. Complete the online test to receive a certificate from Bob Pease

About the Instructor: Wanda Garrett is the Technical Communications Manager in the Web Business Group at National Semiconductor Corp. in Santa Clara, California. With 20 years experience in the design and application of DC/DC converters and related power management products, as well as being a key trainer on this subject, she is now a primary developer of National's Knowledge Base, the Online Seminar series, and other online technical tools. Before joining National in 1983, she received a BSEE from Washington State University.
Suggested Assignments

Reading Assignment:

  1. AN-1028, "Maximum Power Enhancement Techniques for Power Packages"             
  2. AN-1229, page 4- 9, "Placing Components 'acap'" through "Copper Filling: When to Stop", focusing on the last subsection in this range, c) Thermal Management.             
  3. Power WEBENCH and Buck Regulator Design, Jeff Perry, May 2002 (pdf 441KB)

For additional information: consult the Knowledge Base  using the following queries:

Research Assignment:

Using the approach described in the Power WEBENCH training material, design a step-down switching regulator for the following criteria: Vin=8-40V, Vout=3.3V, Iout=2A, using the Power WEBENCH.

1. Check Operating Values (WEBENCH-calculated Efficiency and Power Dissipation) at full load

 
Vin=8V
Vin=40V
Iout=2A Diode Pd = 0.60W
IC Pd = 0.39W
Total Pd = 1.3W
Efficiency = 83.4%		 Diode Pd = 1.1W
IC Pd = 0.77W
Total Pd = 2.1W
Efficiency = 76%

Add up the power dissipations, calculate the estimated efficiency: Eff = Pout/(Pout + Pdiss). Compare to estimated steady state efficiency in Operating Point list.

2. Analyze a Design: Thermal Simulation
a. Your initial thermal simulation has already been started, using VinMax and IoutMax as an operating point. Refresh the browser window until the status shows as "completed". Click on the link for your simulation. See the Diode and IC temperature for this condition.

b. Do another simulation: Click the button with those words, and a thermal simulation setup window will appear. Set Vin=8V, and Submit the simulation request. When the simulation is completed, check the Diode and IC temperatures again for this new condition.

 
Vin=8V
Vin=40V
Iout=2A Diode Temp = 55C
IC Temp (die) = 53C		 Diode Temp = 74C
IC Temp (die) = 70C

c. Check thermal resistance of simulated board, at Vin=40V. Use Ta = 30C, from Thermal Simulation setup. ThetaJA = (Tj-Ta)/Pd = 52degC/W. Note that the diode's heating of the IC may give a different thermal resistance for Vin=8V.


3. Improve the thermal results
Do another simulation, this time add fan at 200LFM. When the simulation is completed, check the Diode and IC temperatures, and compare to those seen previously. Also calculate thermal resistance of simulated board with a fan.

 
Vin=40V
Iout=2A Diode Temp = 64C
IC Temp (die) = 61C
Thermal resistance = 40degC/W

4. For further investigation, try the following:
- Use a heat sink (this creates a new design using a TO-220 packaged regulator IC).
- Change copper weight (make it less or more).
- Change the temperature of one edge of the board.
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