For the past 50 years, the forward converter has dominatedthe market for commercial power supplies in excess of 50 W.Despite this long history, there is still an ongoing flow ofarticles claiming “novel” and “new” variations of this circuit.Many of these prove to be reincarnations of old ideas. We'llexplore some of the milestones in the history of the forwardconverter in this article.

The Basic Forward Converter CircuitFigure 1 shows a classic forward converter circuit. There aremany other variations that share the same basic energy trans-fer mechanism: power is delivered to the output while theprimary switch is conducting.

The forward converter is single-ended. The transformer isdriven in one direction with a single switch. Since the trans-former windings must have zero volt-seconds onaverage, core reset is required whereby a reset volt-

age is applied to the transformer. The peak voltage thatappears across the switch in the off state, in many variations,is the sum of the line voltage and the reset voltage.

For most applications, the pulse width modulation (PWM) ofthe primary side switch varies with input line. The switchon-time regulates the output. For a constant output voltage,the volt-seconds applied to the primary will be constant butreset time varies, being relatively long at high line and shortat low line. The switch voltage is minimized when the switchoff-time is long and fully used for reset

There is, however, a competing requirement. The rms currentin the switch increases with shorter duty cycle. It is desirableto maximize on time, but this increases the reset voltage so atradeoff is needed between switch current and voltage. All ofthis was recognized early on, and many new improvementshave focused on the best way to reset the core.

The circuit of Figure 1 uses a tertiary winding on the trans-former for core reset. At low line, a well-designed converteris close to optimum. Almost half the time is used to deliverpower, and the remainder is used to reset the core. However,at high line, the power pulse and reset time are short. Thetotal off-time available for reset isnot fully used, and the peak switchvoltage is higher than necessary.

by Rudy Severns

HISTORY OF THEFORWARD CONVERTER

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Core reset, switch voltage and switch current are not the onlyissues in the design of a forward converter. The stress on theswitch during switching transitions and the accompanyingloss is also important, especially as switching frequenciesincrease. This has been the subject of many circuit improve-ments over the years.

Historical DevelopmentsSingle-ended converters have been around for a long time.Heinrich Hertz (1857-1894) demonstrated the existence ofelectromagnetic waves with a simple flyback circuit. TheFord Model T ignition system also used a flyback. Buck andboost converters appeared in the early 1920's, but a clearlyidentifiable forward converter didn't appear until 1956.

Figure 2 is a circuit introduced by Paynter [1] in March1956. It seems like another push-pull oscillator of that peri-od, but a closer look shows one transistor (T1) passes powerand the other (T2) is clearly labeled “reset”. Power is cou-pled to the output during the interval when T1 is conducting.The circuit is self-oscillating with the conduction of T1 ter-minating when the core saturates. T2 then conducts, resetting

the core to saturation in the other direction. The duty cycle iscontrolled by the transformer turns ratio (W1/W2).

In 1963, Dudley [2] continued using Paynter's circuit, how-ever, he used the output voltage for reset as shown in Figure3. Duty cycles in the range of 0.6-0.8 were possible with thiscircuit when operating from low-voltage power sources.PWM is not used, and the output filter is simply a capacitor.

The forward converter received little further attention untilthe mid 70s. In 1975, there was a sudden burst of activity,mainly from Europe.

Definitive discussion of the forward converter in modernform first appears in a March 1975 Philips applications note[3]. It is clear at this point that the circuit was well under-stood, and a two-transistor version of the forward converteris introduced as shown in Figure 4. There is a reference to anearlier application note [4] dated 1973. In addition to thetwo-transistor version where reset is done with two diodes,the applications note shows the use of a tertiary winding anda diode or an R-C-diode network for reset.

Rudy Severnsis a consultant in the design of powerelectronics, magnetic components and power conver-sion equipment. His 40 years of experience includeboth commercial and military designs with TRW,Lockheed, Hughes Aircraft, Magnavox, Intersil,Siliconix and the University Of California. Since1978 he has lectured extensively in the US, Asia andEurope on power conversion, magnetics design andpower semiconductor topics. He holds a B.S.Engineering from the University of California at LosAngeles and is the author of two books and over 70technical papers. He is a fellow of the IEEE.

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The two-transistor forward converter is a modern-day stan-dard for off-line power from 200 W to 1 kW. Many compa-nies use it as the basis of their product lines.

In June of 1975, La Duca and Massey [5] addressed theproblem of optimizing the reset by using a voltage clamp.The amplitude was adjusted to provide the minimum neces-sary voltage during the reset interval as shown in Figure 5.This marks the beginning of efforts to optimize the forwardconverter by changing the reset voltage with input line.

Reset using a current source and an auxiliary winding wasintroduced by Heinicke [6] in a 1975 patent, shown in Figure6. The resetting current source is formed from the sourcevoltage and a series combination of an inductor and resistor.Heinicke's circuit also shows the use of a forward converteras a non-isolated boost converter.

Before this, the transformer core energy was discharged intoa voltage source. Only a portion of the potential change incore flux density could be applied, forcing the use of a largercore with a small air gap. The air gap required more energystored in the core during reset.

The current source allows a larger flux swing and eliminatesthe need for an air gap. In principle, the core could be resetto negative saturation, and the entire range of differentialflux density up to positive saturation could be used. A latervariation on this idea was to incorporate a ferrite permanentmagnet in the core structure to provide reset bias.

In 1976 we see a continued flurry of forward converterdevelopments. Kamata and Katou [7] introduced the second-ary inductor current in combination with the regulated outputvoltage for reset as shown in Figure 7. The reset current goesdown as the load current drops but some reset can still beaccomplished using the voltage clamp on winding N4.

Following the Massey and La Duca circuit, Peterson pub-lished the variation [8] shown in Figure 8. An error amplifierreferenced to the output controls the reset voltage clamp.Vermolen also introduced a variable clamp reset [9].

In 1976, Lilienstein and Miller [10] combined tertiary wind-ing current as shown in Figure 9. Their circuit first showedan interleaved forward converter. The transistor switchesemployed mag-amps controlling the base drive to providePWM (Note: the bias winding for the magamp is omitted forclarity). Interleaved forwards are still very popular and use-ful today, undergoing another cycle of rediscovery.

In an important development in 1981, Carsten [11] used anactive clamp circuit with several variations, shown in Figure10. This simple circuit provides optimum reset voltage as theduty cycle changes, recovery of most of the core energy,symmetrical core excitation, and low loss switching transi-tions. That's just about everything you could ask for in a for-ward converter reset circuit. Since that time this circuit hasbeen widely adopted.

The idea of a voltage clamp suggested by La Duca andMassey in 1975, has been revived many times over theyears. Another clamp circuit was suggested by Kuwabaraand Miyachika in 1987 [12].

Carsten's further contribution in 1992 used a snubber circuitto simultaneously provide low stress switching and core

reset. This idea appears in other literature. Figure 11 shows avariation, where a well-known energy recovery snubber cir-cuit has been designed to provide core reset. A number ofpapers have appeared with similar ideas [14], frequentlyusing the transformer leakage inductance in a resonant ener-gy recovery scheme.

The forward converter remains the subject of vigorous inno-vation and debate today. Although the basic energy transfermechanism is the same as Paynter's original circuit, thedetails of implementation are still being explored. The onlyreal change has been the increase in switching frequenciesfrom about 2 kHz to 500 kHz or more. This is due mostly todevice improvements, packaging advances, and reduced sizerequirements. The most recent versions of the forward con-verter have focused on the use of MOSFET synchronous rec-tifiers with various gate drive schemes and reset clamparrangements.

References[1] Paynter, D.A., AN UNSYMMETRICAL SQUARE-WAVEPOWER OSCILLATOR, IRE transactions on Circuit Theory,March 1956, pp. 64-65[2] Dudley, William, UNSYMMETRICAL LOW VOLTAGE CON-VERTER, 17th Power Sources Conference proceedings, 1963, pp.155-158[3] van Velthooven, C., PROPERTIES OF DC-TO-DC CONVERT-ERS FOR SWITCHED-MODE POWER SUPPLIES, PhilipsApplication Information #472, 18 March 1975, pp. 8-10[4] G. Wolf, MAINS ISOLATING SWITCH-MODE POWER SUP-PLY, Philips Electronic Applications Bulleting, Vol. 32, No. 1,February 1973[5] La Duca and Massey, IMPROVED SINGLE-ENDED REGU-LATED DC/DC CONVERTER CIRCUIT, IEEE Power ElectronicsSpecialists Conference (PESC) record, June 1975, pp. 177-187[6] Heinicke, Harald, APPARATUS FOR CONVERTING D.C.VOLTAGE, U.S. patent number 3,921,054, 18 November 1975(1973 German filing)[7] Hamata and Katou, DC-TO-DC CONVERTER, U.S. patentnumber 3,935,526, 27 January 1976 (1972 Japanese filing)[8] Peterson, W.A., A FREQUENCY-STABILIZED FREE-RUN-NING DC-TO-DC CONVERTER CIRCUIT EMPLOYINGPULSE-WIDTH CONTROL REGULATION, IEEE PESC proceed-ings, June 1976, pp. 200-205[9] Vermolen, J.V., NON-SATURATING ASYMMETRIC DC/DCCONVERTER, U.S. patent number 3,963,973, 15 June 1976 (1973Dutch filing)[10] Lilienstein and Miller, THE BIASED TRANSFORMER DC-TO-DC CONVERTER, IEEE PESC proceedings, June 1976, pp.190-199[11] Carsten, B., HIGH POWER SMPS REQUIRE INTRINSICRELIABILITY, Power Conversion International (PCI) proceedings,September 1981, pp. 118-133[12] Kuwabara and Miyachika, A VERY WIDE INPUT RANGEDC-DC CONVERTER, IEEE INTELEC proceedings, 1987, pp.228-233[13] Wittenbreder, Martin and Baggerly, A DUTY CYCLEEXTENSION TECHNIQUE FOR SINGLE ENDED FORWARDCONVERTERS, IEEE Applied Power Electronics Conference(APEC) proceedings, 1992, pp. 51-57

More references are available on-line at www.switchingpower-magazine.com/forwardconverters. These references, compiled byRudy Severns, do not include all research. Additions to the listwould be greatly appreciated, particularly those in Europe in the1960s and 1970s.

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