Libris Britannia 4/science library(b).zip/science library(b)/ELECTRON/3645.ZIP/README.DOC

Text File | 1990-04-02 | 20KB | 459 lines

              Supplemental Information (README.DOC file)


     The "Switchers Made Simple" program is an expert system that
     automates the design of Simple Switcher (TM) based basic switching
     regulators.  The following topologies can be designed with the
     present version: buck and buckboost (invert) regulators with the
     LM2575 family, and boost and multiple-output flyback regulators
     with the LM2577 family.

     The first module of the program lets you input the specification
     of the converter you wish to design.  It is done via a menu, with
     editing capability.  The feasibility of the specification is
     checked and provision is made for editing input parameters should
     the program alert the user to an unsuitable parameter set.

     The second module calculates the limit values of the external
     components of the circuit based on the specification and loop
     stability requirements.  The stability analysis calculation is
     based on the state-space-averaged model introduced by
     Dr. Middlebrook.

     Next, the program chooses the actual components, according to the
     calculated limits, from a built in data-base of standard
     components.  There are three manufacturers providing standard
     inductors and tranformers for the Simple Switcher (TM) product family:
     AIE, Pulse Engineering and Renco.  The capacitor data-base is built up
     using the Sprague, Nichikon, Cornell Dubilier and Panasonic lines.

     You can override (i.e. edit) most component values of the list.
     Some values can not be edited: e.g. the ESR of the capacitors,
     R1, the upper resistor in the voltage divider when adjustable
     regulators are used, the transformer's turns ratio, etc.
     Some values are adjusted automatically with another component;
     e.g. R1 if R2 is changed etc.

     After the components selection is completed, the program
     calculates the control loop's crossover frequency and phase
     margin.  The design procedure ensures that the resulting circuit
     is stable in worst-case conditions and has adequate phase margin.
     Should you require higher phase margin, you can change component
     values (increase Co in LM2575-based designs and increase Co and
     Cc in LM2577-based designs) and rerun the stability analysis to
     verify the improvement.

     Finally, a junction temperature check is performed, based on our
     proprietary thermal model.  It calculates the junction
     temperature of the chip in your application circuit at the
     specified maximum ambient temperature.  If the calculated
     junction temperature exceeds the thermal shutdown limit (160degC)
     the program specifies the maximum allowable thermal resistance of
     the heatsink that must be used.

     After completing the design, provision is made for saving the file,
     printing the schematic, and/or printing the components list complete









     with manufacturers' part numbers. To print the schematic, you must
     have an Epson compatible printer.  You may view the schematic with
     either Hercules, CGA, EGA, or VGA grahics capabilities.




                            WARNINGS


     The program displays warnings to alert you to certain conditions:

     1.   Current or voltage-limit exceeded.

          This message appears after the feasibility check if either
          the maximum current or voltage rating of the power switch in
          the IC is exceeded.   The specifications of the circuit have
          to be modified (e.g. load or Vin max decreased) or, in case
          of the flyback converter, the turns ratio of the transformer
          may have to be modified to stay within the IC's ratings.

     2.   Burst mode operation.

          This flag calls attention to the fact that due to the light
          load, the regulator skips switching cycles to maintain
          regulation.  Strictly speaking, the stability analysis is
          not valid in this operating mode.  Practice shows, however,
          that the regulators are stable under these conditions.  The
          only possible problem is the indeterminate emmitted spectrum
          of the circuit, due to the non-constant operating frequency.
          The burst mode warning will always appear if minimum load
          is not specified, because the default value is zero.


     3.   Short Circuit Runaway.

          This condition can occur in flyback regulators.  The problem
          is generic to flybacks and not unique to the LM2577.  It
          indicates that due to the non-zero minimum on-time of the
          power-switch the current limit can not be maintained in an
          output short circuit condition. Using a fast-recovery diode
          instead of a Sckottky sometimes solves the problem.  In general,
          either Vin max or N (the transformer's turns ratio) has to be
          decreased to avoid the problem and to make sure that the switch
          current is safely limited in case of an output short circuit.


     4.   Possible Subharmonic Oscillation.

          In current-mode controlled DC/DC converters, like  all  the
          LM2577-based converters, subharmonic oscillation can occur if
          the compensating ramp slope is not high enough. The built-in,
          fixed compensating ramp of the LM2577 is designed so that it
          ensures subharmonic oscillation free operation in most practical
          applications. However, in some cases the stability criteria
          may be violated. In these rare cases the program issues a
          warning.  Although the regulator maintains control of the output









          voltage even under these circumstances, you may want to avoid
          this operating mode, because of increased noise and output
          ripple. The value of the inductor or the primary inductance of
          the transformer has to be increased to avoid this condition.


     5.   Maximum Duty-cycle Exceeded.

          The duty-cycle of the LM2577 is limited to 90%. If the
          specification requires higher duty-cycle, the program issues this
          warning.  The output - input voltage difference has to be
          decreased, or the transformer's turns ratio increased to avoid
          this error condition.



     ESR AND OUTPUT VOLTAGE RIPPLE

     The ESR, equivalent series resistance, of the output filter
     capacitors is a very important parameter in switching regulator
     applications.  It introduces a zero in the regulator's control
     loop.  To maintain good phase margin, the ESR has to be smaller
     than a limit value in boost and flyback converters and has to be
     between a high and a low limit in buck and buck-boost regulators.
     The program outputs these limits in the "Limit Values" section
     of the screen.  They can be met using so called "high frequency"
     or "low ESR" capacitors, like the ones used in the internal
     database.  If you choose your own capacitor from a different
     manufacturer, make sure you use a product line that specifies
     the ESR at higher frequency than 120Hz, e.g. 10KHz or 100KHz.
     Capacitors without high frequency ESR specifications are
     not intended for use in switching regulators.  Also, ESR is
     temperature dependent, it can increase substantially at low
     temperature.  If you design a circuit that has to operate below
     0 degC, you should consult a detailed capacitor data-sheet.

     It is good practice to use a solid tantalum capacitor (at
     least 10% of the output capacitor's value) in parallel with the
     output capacitor if operation below 0 degC is required.

     The ESR has also a strong effect on the output ripple voltage of
     the converter.  The ripple voltage can be calculated as the
     product of the current ripple of the inductor or transformer
     winding feeding the capacitor, and the ESR.  In boost and flyback
     converter applications the output ripple (Vripple) can be
     specified as an input parameter.  The program ensures that the
     specification is met by the final circuit.  The program both alerts
     and disallows user choice of Vripple less than 0.005Vout.  This is
     because very low values of ripple can only be achieved in basic
     converter designs via impractically large output capacitors with
     extremely low ESR.  If Vripple is not specified, the program assumes a
     ripple voltage of 0.01Vout by default.

     In buck and buck-boost applications the ripple voltage can not be
     explicitly specified.  It will be calculated by the program and
     listed under "Limit Values".  The ripple voltage can be
     decreased, if needed, by optionally selecting larger inductors in









     these applications.



     STANDARD INDUCTORS AND TRANSFORMERS

     In each regulator you have the choice to use standard
     inductors/tranformers or to design your own.

     The standard inductors are chosen for 30% current ripple.  The
     standard transformers for flybuck converters cover the input
     voltage range 4.5V to 15V and the output voltage range of +/-10V
     to +/-15V.

     If you decide to use nonstandard inductors, you can specify
     either the inductance or the required maximum current ripple
     (in Amperes).  The program automatically calculates the other
     parameter.

     In the case of nonstandard flyback transformers you have two
     choices: either you can use a "full custom transformer", which means
     you can input the value of the primary inductance (Lp), and turns
     ratio of the main output (N1), or you can let the program design your
     custom transformer. In the latter case the transformer is optimized to
     deliver the maximum output power in the given application.


     PRIME OUTPUT AND TURNS RATIO IN MULTIPLE OUTPUT FLYBACK CONVERTERS.

     The output designated by "1" subscript (V01) is always the "prime"
     or directly regulated output of the flyback converter. The turns ratio
     of the auxiliary secondary (output) windings are designated by N1, N2,
     and N3.  N1 = the number of secondary turns of output #1 divided by
     the number of primary turns; N2 and N3 are defined similarly. N2 and
     N3 are not variables you can change, they are always chosen by the
     program, based on N1 and the output voltages given in the input
     specifications section.


     IC OPTIONS

     If the specified output voltage is 5, 12 or 15V in a buck
     converter or -5, -12 or -15V in an buck-boost converter, the
     program automatically chooses and displays the appropriate fixed
     output voltage versions of the Simple Switcher family.

     The same is true for boost and flyback regulators with 12V or
     15V output voltage.

     For other output voltages the adjustable versions are specified
     with appropriate voltage divider resistors of 1% tolerance.

     If the specified ambient temperature limits are within the
     temperature range: -40degC to 125degC, the program
     automatically specifies the LM2575 or LM2577 families in 5-pin
     TO-220 packages.  For wider temperature range the LM1575 and
     LM1577 families in 4 lead TO-3 package are indicated and









     specified. It should be noted, however, that the IC itself
     is guaranteed to meet specifications in the -40 to 125 degC (LM2575 &
     LM2577) and the -55 to 150 degC (LM1575 & 1577) JUNCTION temperature
     range, with an approximate shutdown temperature of 160 degC.


     OUTPUT  ACCURACY

     The accuracy of the output voltage of the buck and boost
     converters using the fixed output voltage versions of the LM2575
     and LM2577 families is guaranteed to be better than +/-5% over
     the entire line and load range, worst case component variations
     and over the full temperature range.  If the adjustable versions
     are used, the accuracy can be somewhat worse depending on the
     tolerance of the external divider resistors used.  If "perfect"
     divider resistors are used, the adjustables actually have an accuracy
     specification slightly superior to that of the fixed versions.

     In buck-boost and flyback converters the accuracy of the main
     (directly regulated) output can be expected to meet the same
     specification.  The auxiliary outputs of flyback converters are
     not regulated and usually have lower output voltage accuracy due
     to the load-regulation being much worse than on the prime
     regulated output.


     CROSSOVER FREQUENCY AND PHASE MARGIN

     The program calculates the unity - gain crossover frequency and phase
     margin of the converter's control loop. They characterize the loop's
     stability. The calculation is done using the final (edited) component
     values. This feature enables the user to fine tune the design and/or
     experiment with different component values.

     The phase margins of converters using the LM2577 are typically high
     (60 - 80 deg), due to the chip's current mode control design.
     The phase margin calculations are performed under worst case
     conditions, i.e. minimum temperature (-55 degC), and maximum error
     amplifier transconductance. As a consequence, the crossover frequency
     is typically twice as high and the calculated phase margin is lower
     than the value you can measure using typical parts at room
     temperature.

     The buck and buck-boost converters, using the LM2575, have typically
     lower phase margins. It should be noted however, that their loop gain
     is extremely stable, practically constant over process variations
     and operating temperature. This means that the usual design reserves
     that account for loop gain variations need not be applied, and
     a phase margin of 20 to 40 deg is perfectly acceptable. Remember that
     the value the program calculates is worst case.  The phase
     margin can be increased, if needed, by increasing the value of the
     output capacitor, or in the case of the buck converter, also by
     increasing the value of the inductor.  This can be done when editing
     in the "Component Values" column. The program calculates the crossover
     frequency and phase margin using the edited component values.
     Whenever Cout is edited this calculation is performed using the ESRmax
     value in the "Limit Values" column.











     STANDARD UNITS

     Throughout the program the following standard units are used:
     V, A, OHM, H, F. Any input quantity entered without unit designation
     is understood in these units. E.g. a current "300" is understood as
     300A, if you want to input 300mA, you should type "300m" or "300mA" or
     ".3".  "Micro" is understood by using the suffix, "u".


     CONTINOUS/DISCONTINOUS OPERATION

     Every DC/DC converter can operate in either continous or discontinous
     operating mode. In continous mode the inductor current (or the
     Magneto-Motive Force in transformers) never falls to zero. The
     converters operating in continous mode are able to deliver higher
     output power with the same power switch limits. However, they also
     generate more radiated noise and and need more input filtering as well
     as more careful layout.

     For flyback converters you may elect to implement your
     specifications with a discontinous mode converter. The program
     warns you if this is not feasible, and switches to continous mode
     automatically.

     The program designs the boost converters ensuring continous mode
     operation, so as to maximize the available output power. You can force
     discontinous operation by choosing  a low value custom inductor.

     Whether in buck or buck-boost converters, the program checks the input
     parameters to decide whether the converter operates in continous or
     discontinous mode. The model, used to calculate crossover frequency
     and phase margin, is automatically adjusted to fit the operating mode.
     You can recognize whether the converter is running in continous or
     discontinous operating mode from the Limit Value list: if L<"value"
     is shown, the converter operates in discontinous mode,  L>"value",
     on the other hand, indicates continuous mode operation.


     COMPONENTS NOT FOUND IN THE DATA-BASE.

     Components, not found in the data-base, are printed without vendor
     callout in the component list. In the case of output capacitors, the
     limit values are printed.


     EXTREME LOW POWER CONVERTERS

     If very low output power is specified (Po<0.1W), discontinous mode
     operation yields superior results. In these cases it is recommended
     that you use custom inductors or full custom transformers with
     adequately low primary inductance. Should you not use this preferred
     mode, the resulting continous mode design (although functional and
     stable) will have inductors and capacitors with extremely high, and
     thus impractical, values.











     MULTIPLE ITERATIONS

     If you iterate by changing component values many times in the same
     circuit, you may  reach your memory limit, and could be dumped back
     to DOS.  To avoid this, it is recommended that after 5 changes you
     save your circuit, and recall it if you wish to experiment more
     with it.

     A WORD OF CAUTION:

     Although we made every effort to ensure that the program is
     bug-free and yields circuits that correspond to the
     specifications, we strongly urge you to build the regulators and
     test them thoroughly before using them in production.  National
     offers a small PCB for evaluation purposes to assist you in evaluating
     actual hardware.


     QUESTIONS OR SUGGESTIONS

     Our Design and Application staff have put a great deal of effort into
     creating both a true Expert System and a user friendly tool.  We
     sincerely hope that you will find this diskette useful in applying our
     new Simple Switcher (TM) family of high performance, yet easy-to-use
     voltage regulators.

     "Switchers Made Simple" represents a breakthrough in design-aid tools
     offered by semiconductor vendors. Unlike modeling tools that ANALYZE
     circuits the user must first create, this program undertakes the more
     difficult task of SYNTHESIZING the design and then calculating
     performance parameters.  It then takes the additional step of
     identifying the actual components needed to physically realize the
     circuit.  "Switchers Made Simple" is but one part of a complete
     package of support that National offers you, our customers.

     Should you have any questions or suggestions regarding this
     program call Linear Applications at 408-721-5608.  You can also
     reach us by Fax: [phone].  Please call your local National
     Semiconductor sales office or authorized distributor for samples
     and evaluation boards.



     SUGGESTED READING

     1.   LM2575 and LM2577 data-sheets.

     2.   T. Szepesi, J. Bittner, H. Suzuki: Simple Switchers: A new
          breed of power + control IC's for DC/DC converters.
          PCIM '89 Proceedings, 1989 Long Beach, pp:437-449

     3.   T. Szepesi, J. Bittner, H. Santo: On card DC/DC converters
          with the new "Simple Switcher" regulator family.
          PCIM '90 Europe Proceedings, 1990 Munich.


     To get a copy, call Linear Applications at the phone number above.