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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: 408-732-7549. 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.