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Issue 290: EQ Answers

Problem 1—What is an R-C snubber, and what is a typical application for one?

Answer 1—An R-C snubber is the series combination of a resistor and a capacitor that is placed in parallel with a switching element that controls the power to an inductive load in order to safely absorb the energy of switching transients.

The problem is that a load that has an inductive component will produce a brief very high-voltage “spike” when the current through it is interrupted quickly. This spike can cause semiconductor devices to break down or even mechanical contacts to arc over, reducing their lifetime. The snubber absorbs the energy of the spike and dissipates it as heat, without ever allowing the voltage to rise too high.

Problem 2—How do you pick the resistor value in an R-C snubber?

Answer 2—To pick the resistor value, you first need to know what the maximum voltage you want to allow is. For example, if you have a MOSFET that has a drain-to-source breakdown rating of 400 V, you might choose to limit the snubber voltage to 200 V. Call this VMAX. Next, you need to know the maximum current that will be flowing through the load (and the switching element). Call this IMAX. At the instant the switching element opens, this current will be flowing through the resistor, and this will determine the initial voltage that appears across the switching element. Therefore pick the resistance: R = VMAX/IMAX.

Question 3—How do you pick the capacitor value in an R-C snubber?

Answer 3—Picking the capacitor can be more tricky. The key concept is that you need to pick a capacitor that can absorb the energy stored in the inductance of the load while keeping its terminal voltage under VMAX. Since loads don’t often specify their values of inductance, this may require some experimentation. Let’s call the load inductance LLOAD. The energy that it stores at the maximum current is: E = 0.5 IMAX2 LLOAD.The energy that a capacitor stores is: E = 0.5 V2C.

So, if we say that we want the capacitor to store the same energy that’s in the inductance when its terminal voltage is at VMAX, we can combine the twe equations and then solve for C:



This value will actually be somewhat conservative, because some of the initial energy of the inductance will be dissipated in the resistor during the initial transient, before it even gets to the capacitor. After that, the inductance and the capacitor will behave as a series-resonant circuit, with the current oscillating back and forth until all of the energy is gone.

Problem 4—What additional concern is there with regard to an R-C snubber when switching AC power?

Answer 4—When switching DC, the snubber absorbs the energy stored in the load’s inductance, and after a while, no current flows and the capacitor is charged to the supply voltage. However, when switching AC, the snubber has a finite impedance at the AC frequency, which means that it “leaks” a certain amount of current even when the main switching element is open. While this may or may not cause a problem for the load (usually not), there is also the issue of the continuous power being dissipated in the snubber resistor. The resistor must be rated to withstand this leakage power in addition to the energy of the switching events.


Issue 288: EQ Answers

Problem 1—When designing a pair of band-splitting filters (for, say, an audio crossover), why is it important to match frequencies of the –3-dB points of the low-pass and high-pass responses?

Answer 1—The cutoff frequencies of the two filters should be the same so that the overall frequency response when the filter outputs are recombined is flat and has no phase shift. For example, if you feed the cutoff frequency into both filters and then combine the results again, the output will be the same level as the input (0-dB overall gain). As long as the “order” of the two filters is the same (they have the same roll-off slope), the gain will be flat across the entire transition band of frequencies.

What’s really going on is this: A filter’s –3-dB point is where the output has half the power of the input signal, which means that the output voltage is 1/sqrt(2) times the input voltage. The –3-dB point is also where the output signal is phase shifted by 45°. It lags by 45° in the low-pass filter and leads by 45° in the high-pass filter. This means that the outputs of the two filters have a total phase shift of 90° relative to each other.

When you add two sinewaves that have the same amplitude and a 90° phase shift, you don’t get double the voltage. You get sqrt{2} times the voltage. You also get a waveform that has a phase midway between the two signals being added.

So, the final amplitude is sqrt{2}/sqrt{2} times the original input voltage, and the final phase is midway between 45° and –45°, or 0°. In other words, you get the original sinewave back exactly.

Problem 2—A certain portable stereo unit runs for about 12 h on a set of LR20 (D-size alkaline) batteries. If you want to extend the stereo’s run-time, is it better to simply use multiple sets of batteries sequentially, or to connect them all in series-parallel to create one big battery pack?

Answer 2—In general, batteries provide greater capacity at lower average currents. This is partly due to the battery’s internal chemistry, but largely due to the simple fact that less power is wasted in the internal resistance of the battery.

Here are two graphs taken from two different datasheets that illustrate this.eq0659_fig1eq0659_fig2

If the stereo is running for 12 h on a set of batteries, based on eyeballing these graphs, it’s probably getting about 8 A-h of capacity out of one set, so it’s drawing about 660 mA on average. Putting three sets of batteries in parallel would drop the current in each set to about 220 mA, and it will get something closer to 12 A-h from each set.

In other words, if you use, say, three sets of batteries sequentially, you’ll get 36 h of playing time (24 A-h total), but if you use them in parallel together, you’ll get something closer to 54 h of playing time (36 A-h total).

Problem 3—If you wanted to make a capacitor from scratch, what common household materials might you use?

Answer 3—A capacitor consists of two flat conductors separated by a dielectric. Aluminum foil is an obvious candidate for the conductors, and either waxed paper or plastic food wrap would be suitable dielectrics — they have similar characteristics.

Problem 4—How big would a 10-µF capacitor using these materials be?

Answer 4—You need to do some basic calculations first. The formula for capacitance is:Eq288eq1

  • εR is the relative permittivity of the dielectric
  • ε0 is the permittivity of free space
  • A is the area of one plate
  • d is the separation between the plates

Let’s say you want to use 1-mil (25.4 µm) waxed paper as a dielectric. Note that this will determine the voltage rating of the capacitor. The dielectric strength of waxed paper is about 35-40 MV/m, so this will give you a capacitor that can theoretically handle almost a kilovolt, but be conservative in how you use it!

The relative permittivity of waxed paper is about 3.7, the permittivity of free space is 8.854e-12 F/m. Solve for the area required:Eq288eq2

If you get aluminum foil and waxed paper that’s about 12″ (30 cm) wide, you can probably get an overlap of, say, 25 cm, which means that you’ll need a length of about 15.5 m to get the area you need.

If you then roll up your capacitor (using a second layer of waxed paper), the capacitance will be doubled, or about 10 µF. Obviously, this will be physically rather large, more than a foot long and several inches in diameter.

Plastic food wrap has a similar dielectric constant and dielectric strength as waxed paper, but typically comes in a 0.5-mil (12.7 µm) thickness. A capacitor using this would have about half the voltage rating and about half the overall volume.

2014 SoC Conference Early Bird Registration Now Open

Early Bird Registration is now open for the 12th International System on a Chip (SoC) Conference, which will take place at the University of California, Irvine (UCI) from October 22–23, 2014. Early Bird Registration ends October 10, 2014.

The conference will include technical presentations, exhibits, networking opportunities, panel discussions, and keynotes.

About the conference:

  • Keynotes
    • Dr. Takahiro Hanyu, New Paradigm VLSI System Research Group, Laboratory for Brainware Systems Research Institute of Electrical Communication, Tohoku University, Japan
    • Jim Aralis, Chief Technology Officer (CTO), and Vice President of R&D,  Microsemi
    • Dr. Peter L. Gammel, Chief Technology Officer (CTO), Skyworks Solutions, Inc.
    • Hughes Metras, VP, Strategic Partnerships CEA-LETI, France.
  • Special IC Technology Tutorial
    • “IC Technology at New Nodes Made Easy!,” Dr. Alvin Loke, IEEE Solid-State Circuits Distinguished Lecturer, Qualcomm Technologies, Inc.
  • Sessions
    • “Optical Computing with Silicon Photonics.” Yunshan Jiang, Peter DeVore, Jacky Chan, Bahram Jalali, UCLA
    • “Widely Tunable MMMB Wireless Front-Ends Using RF-CMOS MEMS,” Jeffrey L. Hilbert, CEO & Founder, WiSpry, Inc.
    • “Packaging and Assembly for Internet of Things Electronics: SoC Performance at SiP Cost,” Dr. Jayna Sheats, CEO, Terecircuits
    • “Full SoC Emulation from Device Drivers to Peripheral Interfaces,” Jim Kenney, Marketing Director for Mentor Mentor Graphics’ Emulation Division
    • And more.

Source: 2014 SoC Conference


Encapsulated 60-Watt AC-DC Power Supplies

XP Power recently announced the ECE60 series of single-output encapsulated 60-W AC-DC power supplies. The ECE60—which is available in either PCB mount, chassis mounted with screw terminals, or in a DIN rail mount configuration—is intended for applications requiring a slim profile, low no-load power consumption, and a high-power density.ECE60pr


  • The PCB-mounted model measures 91.4 × 38.1 × 28.0 mm and has a power density of greater than 10 W per cubic inch
  • The range comprises eight models providing all the popular nominal output voltages from + 3.3 to + 48 VDC.
  • The ECE60 meets EMC specification EN55022 for class B conducted and radiated emissions.
  • The supply operates from –25° to 70°C with no derating until 50°C.
  • Class II construction means no Earth or Ground connection is required.
  • The ECE60 series parts come with three-year warranty.

Source: XP Power

MPLAB Harmony Firmware Dev Framework for 32-Bit MCUs

Microchip Technology announced Monday the availability of MPLAB Harmony Version 1.0, which was described as a “fully integrated firmware development platform for all 32-bit PIC32 microcontrollers (MCUs).”  Microchip-MPLAB-Harmony

According to Microchp’s release, “It takes key elements of modular and object-oriented design, adds in the flexibility to use a RTOS or work without one, and provides a framework of software modules that are easy to use, configurable for specific design requirements and that are purpose built to work together.”


  • MPLAB Harmony Configurator for fast driver and middleware settings management
  • A pro graphics library in addition to functional and performance improvements across many of the Harmony driver libraries
  • IPv6 certification of the Microchip TCP/IP stack

The MPLAB Harmony Integrated Software Framework is supported by Microchip’s free MPLAB X Integrated Development Environment (IDE). The MPLAB Harmony basic framework is currently available as a free download.

Source: Microchip Technology, Inc.