MIME-Version: 1.0 Content-Location: file:///C:/964AA912/Switchingcurrentpumpmakesbettercharger.htm Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset="us-ascii" EDN Access--01.16.97 Switching current pump makes better charger</ti= tle> <!--[if gte mso 9]><xml> <o:DocumentProperties> <o:Author>Chatham College Student</o:Author> <o:LastAuthor>Chatham College Student</o:LastAuthor> <o:Revision>2</o:Revision> <o:TotalTime>1</o:TotalTime> <o:Created>2011-03-29T04:28:00Z</o:Created> <o:LastSaved>2011-03-29T04:28:00Z</o:LastSaved> <o:Pages>1</o:Pages> <o:Words>1381</o:Words> <o:Characters>7874</o:Characters> <o:Company>SGH</o:Company> <o:Lines>65</o:Lines> <o:Paragraphs>18</o:Paragraphs> <o:CharactersWithSpaces>9237</o:CharactersWithSpaces> <o:Version>11.9999</o:Version> </o:DocumentProperties> </xml><![endif]--><!--[if gte mso 9]><xml> <w:WordDocument> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState=3D"false" LatentStyleCount=3D"156"> </w:LatentStyles> </xml><![endif]--> <style> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; 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Unfortunately, this design approach is far from optimal because it leads to unnecessary hardware redundancy and complexity. This = type of charger also raises a stability problem because of the additional acti= ve device in the feedback loop. </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>The alternative des= ign in </span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIG= URE 1"><strong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'>, which is based on a switching-inductive current pump, is much simpler and easier to build bec= ause it uses common discrete devices. Also, the primary design goal of a switc= hing voltage regulator is to keep the constant output voltage under the differ= ent load conditions, but a current pump is designed to maintain constant load current under varying terminal voltages. Thus, a current pump is inherent= ly suited to the charging requirements of NiCd and NiMH batteries, and even = the constant-current requirements of Peltier Coolers. </span></p> <div align=3Dcenter> <table class=3DMsoNormalTable border=3D1 cellpadding=3D0 width=3D504 style=3D'width:5.25in;mso-cellspacing:1.5pt;border:outset navy 2.25pt; mso-padding-alt:3.75pt 3.75pt 3.75pt 3.75pt'> <tr style=3D'mso-yfti-irow:0;mso-yfti-firstrow:yes'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal align=3Dcenter style=3D'text-align:center'><a name= =3D"FIGURE_1"><b><span style=3D'font-size:13.5pt;color:navy'>FIGURE 1</span></b></a></p> </td> </tr> <tr style=3D'mso-yfti-irow:1'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal align=3Dcenter style=3D'text-align:center'> <= img border=3D0 width=3D504 height=3D370 id=3D"_x0000_i1031" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Document%3d.html s\Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20m= akes%20better%20charger_files\02d19751.gif"></p> </td> </tr> <tr style=3D'mso-yfti-irow:2;mso-yfti-lastrow:yes'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal><span style=3D'font-size:10.0pt;font-family:Arial'= >Combining a switching-inductor current pump with a temperature-sensing circuit produces a battery charger that's simpler and more inherently suited for charging requirements than are switching-voltage-regulator designs. </s= pan></p> </td> </tr> </table> </div> <p><span style=3D'font-size:10.0pt;font-family:Arial'>The design in </spa= n><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 1"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'> combines a switching curren= t pump with a temperature-monitoring design formerly proposed in Kerry Lacanette= 's Design Idea, </span><strong><span style=3D'font-size:10.0pt;font-family:A= rial; color:red'><a href=3D_http_/www.edn.com/reg/1996/080196/16di3.html>"= ;Temperature sensor doesn't discharge battery"</a></span></strong><span style=3D'font-size:10.0pt;font-family:Arial'> (<em><span style=3D'font-fa= mily: Arial'>EDN</span></em>, Aug 1, 1996, pg 106). The current pump in </span>= <a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 1"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'> aims to keep the current co= nstant while the battery voltage varies. The circuit uses a typical switching inductor similar to the boost topology in voltage regulators; thus, the design formulas are similar. The main design formula for discontinuous mo= de is </span></p> <p align=3Dcenter style=3D'text-align:center'><img border=3D0 width=3D246= height=3D40 id=3D"_x0000_i1032" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Documents/%3d.html Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20mak= es%20better%20charger_files\02q19751.gif"></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>where I<sub>LOAD</s= ub> is the average load (charge) current; V<sub>BAT</sub> and V<sub>CC</sub> are= the battery and supply voltages, respectively; D is the duty cycle equal to T= <sub>ON</sub>/T<sub>ON</sub>+T<sub>OFF</sub>; F<sub>SW</sub> is the switching frequency; and h is the overall efficienc= y. The efficiency term is the source of most uncertainty in this equation. T= he efficiency of switching regulators based on currently available n-channel MOSFETs is typically 85 to 95%. So, using an efficiency of 0.9 when solvi= ng this equation produces acceptable results. </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>Note that the equat= ion assumes full inductive discharge during T<sub>OFF</sub>, which implies discontinuous-mode operation. The mC must provide the proper duty cycle to ensure that the energy stored in the inductor completely dissipates during the T<sub>OFF</sub> interval. This requirement means that </span></p> <p align=3Dcenter style=3D'text-align:center'><img border=3D0 width=3D160= height=3D29 id=3D"_x0000_i1033" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Documents/%3d.html Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20mak= es%20better%20charger_files\02q19752.gif"></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>where V<sub>F</sub>= is the forward voltage of catch-diode D<sub>1</sub>, approximately 0.5V. Continuous-mode operation is not recommended for this application. </span= ></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>Note also that for = this current-pump topology, V<sub>BAT</sub> has to be greater than V<sub>CC</s= ub> to prevent dc flows directly through the forward-biased catch diode. A V<= sub>CC</sub> of 5V well suits the charging of camcorder and other batteries. For lower voltages, you can modify the design in </span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 1"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'> for 3.3V operation or chang= e the topology of the current pump (</span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 2"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 2</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'>). In </span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 2"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 2b</span></strong></a= ><span style=3D'font-size:10.0pt;font-family:Arial'>, for boost-mode operation, = the control circuit has to turn on MOSFET Q<sub>2</sub> continuously; for buck mode, the control circuit has to turn off Q<sub>1</sub>. </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>In </span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 1"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'>, zener diode D<sub>Z</sub> protects the output from the short-term voltage spikes that may occur if = the load disconnects during the operation. The digital control algorithm shou= ld include a subroutine that checks the terminal voltage and stops the current-pump operation when the terminal voltage rises above the predetermined thresho= ld. In this design, CH<sub>0</sub> of the ADC10732 performs continuous monito= ring of the terminal voltage. Output capacitor C<sub>1</sub> improves stability and reduces ripples, which is an important criterion for driving a Peltier Cooler. </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>Use the following equations to select the inductor: </span></p> <p align=3Dcenter style=3D'text-align:center'><img border=3D0 width=3D246= height=3D28 id=3D"_x0000_i1034" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Documents/%3d.html Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20mak= es%20better%20charger_files\02q19753.gif"></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>and </span></p> <p align=3Dcenter style=3D'text-align:center'><img border=3D0 width=3D246= height=3D28 id=3D"_x0000_i1035" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Documents/%3d.html Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20mak= es%20better%20charger_files\02q19754.gif"></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>where I<sub>L(PEAK)= </sub> is the peak inductor current and E·T is one of the inductor's basic specifications. The inductor, D<sub>1</sub>, and Q<sub>1</sub> must be ab= le to carry the inductor's peak current. You can digitally adjust I<sub>LOAD= </sub> by controlling D, F<sub>SW</sub>, or both. It is a good design practice to keep F<sub>SW</sub> within 50 to 500 kHz because both analog and digital parts of the circuit can easily handle frequencies in this range, and the inductor does not "buzz." </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>Two control algorit= hms are possible for switching-inductor circuits: pulse-width modulation (PWM) and pulse-frequency modulation (PFM). For certainty and simplicity, this circuit uses the PFM control algorithm and discontinuous operation. First, the A/D converter samples V<sub>BAT</sub>, and the mC calculates the movi= ng average (10 to 100 samples are reasonable). The system also checks the lo= ad temperature. If there is no reason to terminate charging, the mC then calculates and adjusts F<sub>SW</sub> to produce the necessary load curre= nt. The system also must check that, for the calculated frequency, the induct= or's E·T and I<sub>L(PEAK)</sub> are still in the safe area. </span></p> <p><span style=3D'font-size:10.0pt;font-family:Arial'>The component value= s in </span><a href=3D_http_/www.edn.com/archives/1997/011697/02di_02.html#FIGURE 1"><st= rong><span style=3D'font-size:10.0pt;font-family:Arial'>Figure 1</span></strong></a>= <span style=3D'font-size:10.0pt;font-family:Arial'> produce a 6V/0.1A battery c= harger with D=3D0.2 and F<sub>SW</sub>=3D50 kHz. For higher currents, such as 0.= 25A, you can choose a smaller inductor, such as 15 µH, or reduce F<sub>SW</s= ub>. You can increase D, but keep D<D<sub>MAX</sub>, or you will convert the system into a continuous-conducting mode. You can upgrade the 10-bit ADC1= 0732 to the 12-bit ADC12032 for higher resolution. Q<sub>1</sub> features low = R<sub>ON</sub> providing higher efficiency under operation to V<sub>CC</sub> as low as 2= .7V. Any similar MOSFET is also applicable. This circuit uses V<sub>CC</sub> a= nd battery temperature for charge-termination criteria. If you use this circ= uit to drive a Peltier Cooler, you can use the LM235 temperature sensor to measure the surface temperature of both sides. (DI #1975)</span> </p> <div align=3Dcenter> <table class=3DMsoNormalTable border=3D1 cellpadding=3D0 width=3D332 style=3D'width:249.0pt;mso-cellspacing:1.5pt;border:outset navy 2.25pt; mso-padding-alt:3.75pt 3.75pt 3.75pt 3.75pt'> <tr style=3D'mso-yfti-irow:0;mso-yfti-firstrow:yes'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal align=3Dcenter style=3D'text-align:center'><a name= =3D"FIGURE_2"><b><span style=3D'font-size:13.5pt;color:navy'>FIGURE 2</span></b></a></p> </td> </tr> <tr style=3D'mso-yfti-irow:1'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal align=3Dcenter style=3D'text-align:center'><img bo= rder=3D0 width=3D332 height=3D312 id=3D"_x0000_i1036" src=3D_file_/C_/Documents%20and%20Settings/Dan%20Fraser/My%20Document%3d.html s\Schematics\Update\EDN%20Access--01_16_97%20Switching%20current%20pump%20m= akes%20better%20charger_files\02d19752.gif"> </p> </td> </tr> <tr style=3D'mso-yfti-irow:2;mso-yfti-lastrow:yes'> <td style=3D'border:inset navy 1.0pt;mso-border-alt:inset navy .75pt; padding:3.75pt 3.75pt 3.75pt 3.75pt'> <p class=3DMsoNormal><span style=3D'font-size:10.0pt;font-family:Arial'= >For low-voltage operation, you can modify the current pump to include a transformer (a) or create a universal buck/boost design (b). </span></p> </td> </tr> </table> </div> <div class=3DMsoNormal align=3Dcenter style=3D'text-align:center'> <hr size=3D2 width=3D"100%" align=3Dcenter> </div> <div class=3DMsoNormal align=3Dcenter style=3D'text-align:center'> <hr size=3D2 width=3D"80%" noshade color=3Dblack align=3Dcenter> </div> <p align=3Dcenter style=3D'text-align:center'><span style=3D'font-size:10= .0pt; font-family:Arial'>| </span><a href=3D_http_/www.edn.com/_.html><strong><span style=3D'font-size:10.0pt;font-family:Arial'>EDN Access</span></strong></= a><span style=3D'font-size:10.0pt;font-family:Arial'> | </span><a href=3D_http_/www.edn.com/feedback.html><strong><span style=3D'font-size= :10.0pt; font-family:Arial'>Feedback</span></strong></a><span style=3D'font-size:1= 0.0pt; font-family:Arial'> | </span><a href=3D_http_/www.edn.com/subscrib.html>= <b><span style=3D'font-size:10.0pt;font-family:Arial'>Subscribe to EDN!</span></b>= </a><span style=3D'font-size:10.0pt;font-family:Arial'> |</span></p> <div class=3DMsoNormal align=3Dcenter style=3D'text-align:center'> <hr size=3D2 width=3D"80%" noshade color=3Dblack align=3Dcenter> </div> <address style=3D'text-align:center'><span style=3D'font-size:10.0pt'>Cop= yright © 1996 </span><a href=3D_http_/www.edn.com/_.html><span style=3D'font-si= ze:10.0pt'>EDN Magazine</span></a><span style=3D'font-size:10.0pt'>. 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