FORWARD CONVERTERS
HISTORY OF THE
FORWARD CONVERTER
by Rudy Severns
For the past 50 years, the forward converter has dominated
the market for commercial power supplies in excess of 50 W.
Despite this long history, there is still an ongoing flow of
articles claiming “novel” and “new” variations of this circuit.
Many of these prove to be reincarnations of old ideas. We'll
explore some of the milestones in the history of the forward
converter in this article.
The Basic Forward Converter Circuit
Figure 1 shows a classic forward converter circuit. There are
many other variations that share the same basic energy transfer mechanism: power is delivered to the output while the
primary switch is conducting.
The forward converter is single-ended. The transformer is
driven in one direction with a single switch. Since the transformer windings must have zero volt-seconds on
average, core reset is required whereby a reset volt-
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July 2000
age is applied to the transformer. The peak voltage that
appears 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) of
the primary side switch varies with input line. The switch
on-time regulates the output. For a constant output voltage,
the volt-seconds applied to the primary will be constant but
reset time varies, being relatively long at high line and short
at low line. The switch voltage is minimized when the switch
off-time is long and fully used for reset
There is, however, a competing requirement. The rms current
in the switch increases with shorter duty cycle. It is desirable
to maximize on time, but this increases the reset voltage so a
tradeoff is needed between switch current and voltage. All of
this was recognized early on, and many new improvements
have focused on the best way to reset the core.
The circuit of Figure 1 uses a tertiary winding on the transformer for core reset. At low line, a well-designed converter
is close to optimum. Almost half the time is used to deliver
power, and the remainder is used to reset the core. However,
at high line, the power pulse and reset time are short. The
total off-time available for reset is
not fully used, and the peak switch
voltage is higher than necessary.
FORWARD CONVERTERS
Core reset, switch voltage and switch current are not the only
issues in the design of a forward converter. The stress on the
switch during switching transitions and the accompanying
loss is also important, especially as switching frequencies
increase. This has been the subject of many circuit improvements over the years.
Historical Developments
Single-ended converters have been around for a long time.
Heinrich Hertz (1857-1894) demonstrated the existence of
electromagnetic waves with a simple flyback circuit. The
Ford Model T ignition system also used a flyback. Buck and
boost converters appeared in the early 1920's, but a clearly
identifiable forward converter didn't appear until 1956.
Figure 2 is a circuit introduced by Paynter [1] in March
1956. It seems like another push-pull oscillator of that period, but a closer look shows one transistor (T1) passes power
and the other (T2) is clearly labeled “reset”. Power is coupled to the output during the interval when T1 is conducting.
The circuit is self-oscillating with the conduction of T1 terminating when the core saturates. T2 then conducts, resetting
the core to saturation in the other direction. The duty cycle is
controlled by the transformer turns ratio (W1/W2).
In 1963, Dudley [2] continued using Paynter's circuit, however, he used the output voltage for reset as shown in Figure
3. Duty cycles in the range of 0.6-0.8 were possible with this
circuit 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 until
the mid 70s. In 1975, there was a sudden burst of activity,
mainly from Europe.
Definitive discussion of the forward converter in modern
form first appears in a March 1975 Philips applications note
[3]. It is clear at this point that the circuit was well understood, and a two-transistor version of the forward converter
is introduced as shown in Figure 4. There is a reference to an
earlier application note [4] dated 1973. In addition to the
two-transistor version where reset is done with two diodes,
the applications note shows the use of a tertiary winding and
a diode or an R-C-diode network for reset.
Rudy Severns is a consultant in the design of power
electronics, magnetic components and power conversion equipment. His 40 years of experience include
both commercial and military designs with TRW,
Lockheed, Hughes Aircraft, Magnavox, Intersil,
Siliconix and the University Of California. Since
1978 he has lectured extensively in the US, Asia and
Europe on power conversion, magnetics design and
power semiconductor topics. He holds a B.S.
Engineering from the University of California at Los
Angeles and is the author of two books and over 70
technical papers. He is a fellow of the IEEE.
July 2000
Switching Power Magazine
21
FORWARD CONVERTERS
The two-transistor forward converter is a modern-day standard for off-line power from 200 W to 1 kW. Many companies use it as the basis of their product lines.
In June of 1975, La Duca and Massey [5] addressed the
problem of optimizing the reset by using a voltage clamp.
The amplitude was adjusted to provide the minimum necessary voltage during the reset interval as shown in Figure 5.
This marks the beginning of efforts to optimize the forward
converter by changing the reset voltage with input line.
Reset using a current source and an auxiliary winding was
introduced by Heinicke [6] in a 1975 patent, shown in Figure
6. The resetting current source is formed from the source
voltage and a series combination of an inductor and resistor.
Heinicke's circuit also shows the use of a forward converter
as a non-isolated boost converter.
Before this, the transformer core energy was discharged into
a voltage source. Only a portion of the potential change in
core flux density could be applied, forcing the use of a larger
core with a small air gap. The air gap required more energy
stored in the core during reset.
The current source allows a larger flux swing and eliminates
the need for an air gap. In principle, the core could be reset
to negative saturation, and the entire range of differential
flux density up to positive saturation could be used. A later
variation on this idea was to incorporate a ferrite permanent
magnet in the core structure to provide reset bias.
In 1976 we see a continued flurry of forward converter
developments. Kamata and Katou [7] introduced the secondary inductor current in combination with the regulated output
voltage for reset as shown in Figure 7. The reset current goes
down as the load current drops but some reset can still be
accomplished using the voltage clamp on winding N4.
Following the Massey and La Duca circuit, Peterson published the variation [8] shown in Figure 8. An error amplifier
referenced to the output controls the reset voltage clamp.
Vermolen also introduced a variable clamp reset [9].
In 1976, Lilienstein and Miller [10] combined tertiary winding current as shown in Figure 9. Their circuit first showed
an interleaved forward converter. The transistor switches
employed mag-amps controlling the base drive to provide
PWM (Note: the bias winding for the magamp is omitted for
clarity). Interleaved forwards are still very popular and useful today, undergoing another cycle of rediscovery.
In an important development in 1981, Carsten [11] used an
active clamp circuit with several variations, shown in Figure
10. This simple circuit provides optimum reset voltage as the
duty cycle changes, recovery of most of the core energy,
symmetrical core excitation, and low loss switching transitions. That's just about everything you could ask for in a forward converter reset circuit. Since that time this circuit has
been widely adopted.
The idea of a voltage clamp suggested by La Duca and
Massey in 1975, has been revived many times over the
years. Another clamp circuit was suggested by Kuwabara
and Miyachika in 1987 [12].
Carsten's further contribution in 1992 used a snubber circuit
to simultaneously provide low stress switching and core
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July 2000
reset. This idea appears in other literature. Figure 11 shows a
variation, where a well-known energy recovery snubber circuit has been designed to provide core reset. A number of
papers have appeared with similar ideas [14], frequently
using the transformer leakage inductance in a resonant energy recovery scheme.
The forward converter remains the subject of vigorous innovation and debate today. Although the basic energy transfer
mechanism is the same as Paynter's original circuit, the
details of implementation are still being explored. The only
real change has been the increase in switching frequencies
from about 2 kHz to 500 kHz or more. This is due mostly to
device improvements, packaging advances, and reduced size
requirements. The most recent versions of the forward converter have focused on the use of MOSFET synchronous rectifiers with various gate drive schemes and reset clamp
arrangements.
References
[1] Paynter, D.A., AN UNSYMMETRICAL SQUARE-WAVE
POWER OSCILLATOR, IRE transactions on Circuit Theory,
March 1956, pp. 64-65
[2] Dudley, William, UNSYMMETRICAL LOW VOLTAGE CONVERTER, 17th Power Sources Conference proceedings, 1963, pp.
155-158
[3] van Velthooven, C., PROPERTIES OF DC-TO-DC CONVERTERS FOR SWITCHED-MODE POWER SUPPLIES, Philips
Application Information #472, 18 March 1975, pp. 8-10
[4] G. Wolf, MAINS ISOLATING SWITCH-MODE POWER SUPPLY, Philips Electronic Applications Bulleting, Vol. 32, No. 1,
February 1973
[5] La Duca and Massey, IMPROVED SINGLE-ENDED REGULATED DC/DC CONVERTER CIRCUIT, IEEE Power Electronics
Specialists 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. patent
number 3,935,526, 27 January 1976 (1972 Japanese filing)
[8] Peterson, W.A., A FREQUENCY-STABILIZED FREE-RUNNING DC-TO-DC CONVERTER CIRCUIT EMPLOYING
PULSE-WIDTH CONTROL REGULATION, IEEE PESC proceedings, June 1976, pp. 200-205
[9] Vermolen, J.V., NON-SATURATING ASYMMETRIC DC/DC
CONVERTER, U.S. patent number 3,963,973, 15 June 1976 (1973
Dutch filing)
[10] Lilienstein and Miller, THE BIASED TRANSFORMER DCTO-DC CONVERTER, IEEE PESC proceedings, June 1976, pp.
190-199
[11] Carsten, B., HIGH POWER SMPS REQUIRE INTRINSIC
RELIABILITY, Power Conversion International (PCI) proceedings,
September 1981, pp. 118-133
[12] Kuwabara and Miyachika, A VERY WIDE INPUT RANGE
DC-DC CONVERTER, IEEE INTELEC proceedings, 1987, pp.
228-233
[13] Wittenbreder, Martin and Baggerly, A DUTY CYCLE
EXTENSION TECHNIQUE FOR SINGLE ENDED FORWARD
CONVERTERS, IEEE Applied Power Electronics Conference
(APEC) proceedings, 1992, pp. 51-57
More references are available on-line at www.switchingpowermagazine.com/forwardconverters. These references, compiled by
Rudy Severns, do not include all research. Additions to the list
would be greatly appreciated, particularly those in Europe in the
1960s and 1970s.