N1 Page 1 of 8 N1 he Elusive apped Output Inductor y Colonel Wm.. McLyman Designers of forward voltagefed dc/dc converters are always facing the possibility of transformer core saturation and the resultant current and voltage spikes that put undue stress on the switching transistors, which can lead to premature failure. here is a simple modification of the output filter that can eliminate or greatly reduce core saturation caused by asymmetrical currents on the secondary. Current Probe I pc I o N 1 N 2 V o Q2 Figure 1. typical pushpull primary, with a centertapped secondary converter. conventional pushpull voltagefed dc/dc converter, with a centertapped secondary and a LC output filter, is shown in Figure 1. he ideal current waveforms are shown in Figure 2, without parasitic influence. It is assumed the time intervals are the same. on off = Q2 on = Q2 he performance description of the pushpull converter in Figure 1 will be discussed using the waveforms in Figure 2. When transistor is turned on, (See Figure 2), diode is forwardbiased and delivers current to the load through. (See Figure 2D and 2E.) In the same time interval, is backbiased and turned off. Now, when is turned off, diode is still conducting, providing the commutating current X1 for the inductor. his same thing is repeated on the next half cycle. When transistor Q2 is turned on, (See Figure 2.) diode is forwardbiased and delivers current to the load through. (See Figure 2C and 2E.) In the same time interval, is backbiased and turned off. Now, when Q2 is turned off, diode is still conducting, providing the commutating current X2 for the inductor. here are many designs that show a diode, like, in Figure 1, as an alternate path for the commutating current for. his diode,, does not conduct, nor does it prevent the commutating current from flowing in diodes, and. It will be noted that currents X1 and X2 are flowing at the same time. his is the result of transformer trying to balance the ampereturns, due to the commutating current X1 flowing in. he currents X1 and X2 are not always balanced. off
N1 Page 2 of 8 he H loop, in Figure 3, shows how the unbalanced current will cause the core to ratchet to one end or the other of the H loop and saturate the core. I Q2 X2 C X1 D E t off t off Figure 2.ypical currents waveforms of a pushpull converter. H H H Ideal Operating H Loop Positive Saturation Negative Saturation Figure 3.ypical H loop driven into saturation. he commutating current flowing through diodes and is from a current source. Engineers have tried to balance this current through the diodes by screening and matching components, but this is both costly and limited in effectiveness because it is difficult to match
N1 Page 3 of 8 components over a wide temperature range. he transformer should have tightly coupled, secondary windings for current sharing. he result of this unbalanced current can be seen in Figure 4, using the current probe in Figure 1. he waveforms in Figure 4 have been viewed many times. he waveform, in Figure 4, shows the primary current, I pc, when the commutating current, X1, and X2, has only a small imbalance. he waveform, in Figure 4, shows the primary current, I pc, when the commutating current, X1 and X2, has a large imbalance forcing the transformer core into saturation. I pc I pc Q2 Q2 Q2 Q2 t Figure 4.Primary centertapped current. t apped Inductor Description If the diode,, were forced to conduct and provide an alternate path for the commutating current, while at the same time backbiasing diodes and, this commutating problem could be solved. he need is for a small bias voltage that could turn on, while at the same time or Q2 is turned off. he standard output filter inductor and the tapped inductor are shown in Figure 5. When either transistor, or Q2, is on, the current in the inductor is increasing, with the polarity, as shown in Figure 5, as Charge. Now, when either transistor, or Q2, is turned off, the current in the inductor will start to decrease, with the polarity, as shown in Figure 5, as Discharge. 1 Charge Discharge 2 1 2 3 Charge Discharge Figure 5. Comparing the standard output inductor with the tapped inductor. Such a bias voltage source is readily available righ the inductor. his can be done by adding just a few turns to the output inductor as shown in Figure 5. he selection of the tap ratio is greatly dependen the transformer secondary leakage inductance and the output voltage V o [See Reference 1]. Normally, it requires only a few turns.
N1 Page 4 of 8 nother benefit in using the tapped inductor is when the commutating current is flowing through, diodes, and, is that it has plenty of time to recover. he anodes for both diodes and are tied to the tap (#2) on. When and Q2 are turned off, the inductor becomes a voltage divider and backbiases and. his prevents the currents X1 and X2 from from flowing through the secondary of the transformer and biasing the transformer core. his full recovery of both diodes and reduces the stress on the switching transistors and Q2 requiring the transistors to turnon into a diode, recovery short. he new waveforms, using the tapped inductor, are shown in Figure 6. gain, these are idealized waveforms without parasitic influence. he number of turns required on the over wind (#12) should be minimized in order to keep the current step, I step, to a minimum (See Figure 6F). With the tapped inductor installed, the primary center tap current is returned to normal, as shown in Figure 7. I Q2 C D E F t off I step t off Figure 6. PushPull converter current waveforms using the tapped inductor. I pc Q2 Q2 Figure 7.Primary, center tapped current, using the tapped inductor. t
N1 Page 5 of 8 Single Forward Converter he tapped inductor is also beneficial to the singleended, forward converter. conventional, singleended forward dc/dc converter and a LC filter are shown in Figure 8. he ideal current waveforms are shown in Figure 9, without parasitic influence. he singleended forward converter has the same commutating problem as the pushpull converter. he performance description of the forward converter, in Figure 8, will be discussed, using the current waveforms in Figure 9. Under ideal conditions, when transistor in Figure 9 is turned on, diode in Figure 9C is forwardbiased and delivers current to the load through the inductor. When is turned off, diode will cease to conduct, and, in Figure 9D, will then conduct the commutating current in inductor. t the same time, when is turned off, diode in Figure 9 is conducting the demagnetizing current to reset the core. In actuality, when looking at the diode recovery and forward conduction, along with the transformer s many parasitics, the true waveforms found on the actual converter are far from ideal. here are times when and are sharing the commutating, as shown in Figure 1, using the current probe in Figure 8. here are two demagnetizing currents, shown in Figure 1. he current waveform, in Figure 1, is the normal, reverse, saw tooth waveform. he current waveform, in Figure 1, is a current that would be expected if some of the commutating current were flowing through and at the same time. I o N 1 N 2 N mag Current Probe I m V o Figure 8. typical singleended, forward converter.
N1 Page 6 of 8 I c on on t I m t C t D t t reset t off Figure 9. Idealized currents for a singleended, forward converter. I m I m I (demag) t t I (demag) t t Figure 1. Examples of the demagnetizing current through the demag winding. his extra current flowing in demag winding can only come from the secondary winding, being energized by the commutating current. When there is current flowing in the secondary winding during the off time, the transformer will try to balance the ampereturns by adding current to the demag winding, as shown in Figure 1. his can really cut into the overall converter efficiency. he same tapped inductor approach, used on the pushpull, can be used on either the singleended forward converter, or the two series transistor forward converter. he tapped inductor for the singleended forward converter is shown in Figure 11. he same design guidelines for the tapped inductor can be used for both the singleended forward converter and the pushpull converter.
N1 Page 7 of 8 I o N p N s V o N mag Figure 11. singleended, forward converter using the tapped, output inductor. he tapped inductor can be used to improve the performance, in designs that incorporate the magnetic amplifiers. pushpull converter incorporating a magnetic amplifier design and a tapped inductor is shown in Figure 12.. singleended forward converter incorporating a magnetic amplifier design and a tapped inductor is shown in Figure 13. M1 N 1 N 2 V c M2 CR4 CR5 Q2 Figure 12. pushpull magnetic amplifier design using the tapped, output inductor. Conclusion: he use of the tapped, output inductor and its benefits have been shown for both the pushpull converter and the singleended, forward converter. here is very little to add to get exceptional circuit performance. he author incorporates the tapped inductor in all designs, when feasible. he performance of a converter, using the single or pushpull magnetic amplifiers, can also be improved with the tapped inductor.
N1 Page 8 of 8 M1 I o N p N mag N s CR4 V c V o Figure 13. forward converter magnetic amplifier design using the tapped, output inductor. REFERENCES 1. Power Converter, NR1455, echnology ransfer & Commercialization, Jet Propulsion Laboratory, 48 G.W. Wester, n Improved PushPull Voltage Fed Converter Using a apped OutputFilter Inductor, IEEE Power Electronics Specialists Conference, 1983 Record. 2. C.W.. McLyman, Elimination of Currents Spikes in uck Power Converters, U.S. Patent No. 4,245,288, January 13, 1981. 3. C.W.. McLyman, Elimination of Current Spikes in uck Oak Grove Drive Pasadena, California 9119899, 1(818) 3542577. ILIOGRPHY Colonel William. McLyman, ransformer and Inductor Design Handbook, Second Edition, Marcel Dekker Inc., New York, 1988. Colonel William. McLyman, Magnetic Core Selection for ransformers and Inductors, Second Edition, Marcel Dekker Inc., 1997. Colonel William. McLyman, Designing Magnetic Components for High Frequency, dcdc Converters, Kg Magnetics, Inc., 1993. Software For information regarding the above ooks and Companion Software for Windows 95', 98' and N, contact: Kg Magnetics, Inc. 38 West Sierra Madre lvd, Suite J Sierra Madre, Ca. 9124 Phone: (626) 8367233, FX: (626) 8367263 Web Page: www.kgmagnetics.com Email: sheassoc@pacbell.net