Objective:

Understand how a switching regulator delivers a regulated output voltage. Understand how to select the regulator IC and the necessary external components, circuit operation (switch action, inductor, duty cycle), efficiency.

Online Seminar:

Magnetic Buck Converters for Portable Applications, January 22, 2003, Frank DeStasi & Matt Jacob
System designers often look at switching buck converters as strange and problematic devices and for this reason end up designing in lower efficiency alternatives. However, the characteristics of modern buck converters, including the set of features and ease of component selection, makes the design process straightforward and rapid.

Reading Assignment:

1. AN-556, Introduction to Power Supplies ("Introduction" through "Buck Regulator" sections, pages 1-3)
2. AN-711, LM78S40 Switching Voltage Regulator Applications ("Analysis-Step-down Operation" section, pages 3-4)             
3. AN-1253, Wide-Input, High Voltage Buck Converter

Additional Resources:

To learn how to effectively use National's Power WEBENCH online design environment to design switching regulators, see the following training material (broken into four parts for convenience:

• Introduction (pdf 182KB)
• Step 1: Choose a Part; Step 2: Create a Design (pdf 349KB)            
• Step 3: Electrical and Thermal Analysis (pdf 441KB)
• Step 4: Build It! (pdf 157KB)

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

• Buck Converter      
• WebSIM      
• Buck Inductor Current      
• What is Efficiency

Research Assignment:

Using the approach described in the Power WEBENCH training material listed above, design a step-down switching regulator for the following criteria: Vin=8-24V, Vout=3.3V, Iout=2A, using the Power WEBENCH.

1.  Estimate Duty Cycle. Using the equation in the Inductor section of AN-1253, estimate duty cycle (D) for this regulator at Vin=8V and Vin=24V, at Iout=2A and Iout=0.5A. Assume a forward diode voltage drop Vd of 0.5V, and a switch voltage drop Vsw of
Vsw = Rds(ON) x Iout, where Rds(ON) = 0.15ohm (typical)

3.  See waveforms: When the Control Panel gives the status as Ready, it’s time to view the waveforms. On the schematic, click the following nodes:

• Output voltage V_OUT
  • See the average output voltage. Notice the ripple voltage superimposed on this average. To a large extent, the ripple voltage is proportional to the series resistance (ESR) of the output capacitor. A lower ESR will give less ripple voltage, but may increase the capacitor cost and size, and may reduce the stability of the regulator.   
• Switching voltage at Node A (at center of schematic)
  • From the switching voltage we can determine duty cycle. Duty cycle D = ton/T,
    where ton is the period of time the switch is on and T is the switching period. For a buck regulator, ton is the time the switch is high. For the LM2676, the period T is 1/260kHz, or 3.85 usec.
  • Zoom in on the waveform to measure the duty cycle: Using your mouse, draw a rectangle from upper-left corner to lower-right corner; this is the zoom window. By selecting a little more than one switching cycle (from one low-to-high transition to the next), it’s relatively easy to determine the value of ton, and confirm the value of T.
  • Calculate D=ton/T. Compare this to your results in FIND OUT Part 1 and FIND OUT Part 2 (for Vin=8V, Iout=2A). If the results are different, why would that be? Consider the assumptions made in FIND OUT Part 1.   
• Inductor current I_L
  • Click on the current probe I_L to see inductor current. Why does it have a triangular shape? (See AN-711)       
  • Is the peak inductor the same or different vs the peak switch and diode currents? Why? (See AN-711)

Program Coordinator/Professor

Wanda Garrett
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.

Quiz:

Take a quiz to check your understanding of this subject. Click here.
Answer all questions correctly to receive a certificate from Dean Bob Pease.