Electrical transmission or interconnection systems – Miscellaneous systems – Conversion systems
Reexamination Certificate
2002-06-12
2004-02-24
Sircus, Brian (Department: 2836)
Electrical transmission or interconnection systems
Miscellaneous systems
Conversion systems
C307S149000
Reexamination Certificate
active
06696772
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power source technology. In particular, the invention relates to a DC/DC converter which uses controlled synchronous rectification.
BACKGROUND OF THE RELATED ART
Almost all electronic circuits require a direct-current power source (DC/DC converter). A direct-current converter as described e.g. in the publication “Soft switched PWM DC/DC converter with synchronous rectifiers”, Li Xiau, Rames Oruganti, converts an input voltage into an output voltage by switching or modulating the input voltage into a wave-mode pulse using high-power MOSFET switches. The pulse is further connected across a power transformer to the secondary side of the transformer and rectified to produce an output voltage. The output voltage is regulated e.g. by the width of an asymmetrically modulated pulse.
In DC converters, as in other electronic components as well, increasing the power density and improving the efficiency are subjects under continuous development. increasing the power density by increasing the component density is difficult because this leads to heating of components unless the power dissipation remaining in the structure is reduced at the same time, in other words, unless the efficiency is improved. The current trend toward lower operating voltages (5 V, 3.3 V, 2.5 V, 2.8 V . . . ) with the power demand remaining the same or even increasing has led to increasing heat problems in DC converters.
For the rectification of low voltages, the transformer secondary is often provided with a Schotky diode, in which, even in an optimal case, there remains a voltage drop of about 350 mV, which e.g. in a 2.5-V output voltage means a 14-% power loss.
At present, no techniques are known that could be used to improve the situation by reducing the power dissipation remaining in the Schottky diode, so the best way to improve the efficiency is to replace the diode rectifier with a MOSFET synchronous rectifier. Low-voltage MOSFETs have undergone rapid development as their channel resistance and gate charge have been reduced, the variety of enclosure alternatives has been increased and the number of manufacturers has grown, which has led to competition and lower prices. This has accelerated the transition to MOSFET technology, although that again produces a whole lot of new problems.
In synchronous rectification, the rectifier diodes (Schottky diodes) after the isolating transformer used in the DC converter are replaced by MOSFETs having a low channel resistance (R
DS(on)
). In this arrangement, two different operating principles are in use, which are described in the above-mentioned publication “Soft switched PWM DC/DC converter with synchronous rectifiers”.
The first operating principle is self regulation, whereby the regulating circuit of the DC converter only controls one or more power switches on the primary side of a power transformer while the rectifier switches on the secondary side are controlled by the voltages of the secondary coils of the power transformer. In another arrangement, a regulating circuit controlling both primary and secondary side switches is used. The regulating circuit may be placed either on the input voltage side (primary side) or on the output voltage side (secondary side), involving different circuit solutions and properties in practical implementations.
The self-regulated synchronous rectifier for the forward topology has been thoroughly investigated and is a much favored solution, as is also suggested by the numerous articles written about this subject. The push-pull topology is ill adapted for a self-regulated synchronous rectifier because its power transformer does not provide a suitable control voltage to the rectifier switches during the off phase but the output current flows via the body diodes of the MOSFET switches, thus “spoiling” the efficiency. The solutions used at present are based on two-stage topology with a buck regulator placed first to take care of voltage regulation and current limitation and a 50%/50% push-pull stage placed after it to produce isolation. The problem of this solution is encountered in a current limitation situation where the output voltage falls to zero and the MOSFETs lose their control voltage. Therefore, Schottky diodes need to be connected in parallel with the MOSFETs, thus reintroducing the problems described above.
The self-regulated forward topology also involves problems. First of all, this topology is only suited for use with certain output voltages when secondary coil control is used. Typically, these voltages are 5 V and 3.3 V. With other output voltages, separate control windings are required in the transformer. The large range of variation of the input voltage involves an obvious risk of the rectifier and flywheel FETs receiving an insufficient or excessive control voltage at the extremities of the input voltage range. Likewise, the large range of variation of the load current involves problems regarding control. In an open-circuit condition, the topology involves a tendency to self-induced oscillation of the rectifier, which confuses the control of the regulating circuit and the primary switch. In the cases of current limitation and short circuit, the flywheel FET loses its control voltage and the current starts flowing via a body diode unless a Schottky diode has been connected in parallel with the FET.
Furthermore, the control of the flywheel FET is defective at reset of the power transformer unless an active reset circuit or a corresponding auxiliary circuit is used. At changes of state, the output current is instantaneously forced to flow via the body diodes, involving additional losses and reducing the efficiency. The use of converters in parallel without isolating diodes causes problems at start-up and shutdown of the converters and in situations where the power tends to circulate internally between the converters.
Synchronous rectification controlled by a regulating circuit is discussed e.g. in the above-mentioned article “Soft switched PWM DC/DC converter with synchronous rectifiers”, and it involves certain timing errors which give rise to defects of a certain order.
FIG. 1
presents a diagram illustrating the principle of push-pull topology and showing the measuring points for the measurement of the associated curve forms.
FIG. 1
shows an input voltage source U
i
and an output voltage U
o
. A power transformer T is placed between the input voltage and the output voltage. Connected to the power transformer are primary side MOSFET switches swA and swB and secondary side MOSFET switches srA and srB, which in this circuit function as synchronous rectifiers. Connected to the secondary side is also an output filter for filtering the output voltage to remove any extra noise signals from it. Moreover,
FIG. 1
shows the measuring points at which the curve forms presented in
FIGS. 2 and 3
are measured, the voltage across the primary switches being indicated by arrows U
swA
and U
swB
and the voltage across the secondary switches by arrows U
srA
and U
srB
. The current through the primary winding and switches is indicated by arrows I
swA
and I
swB
, and the current through the secondary winding and switches by arrows I
srA
and I
srB
~
FIG. 1
also shows the control signals A, B,
A
and
B
controlling the switches.
FIG. 2
presents the ideal curve forms in the operation of the topology illustrated in FIG.
1
.
FIG. 3
presents a more detailed illustration of instants t
2
and t
3
included in
FIG. 2
, which correspond to the instants of change of the control voltage
B
applied to secondary side power switch srB. Using the control signal B of primary side power switch swB as a reference, the figure illustrates the consequences that will follow if the control signal
B
driving power switch srB lags behind or leads the control signal B driving power switch swB.
FIG. 2
presents the ideal curve forms representing the operation of the circuit in
FIG. 1
, showing eight instants of time which are focused on in the analysis. In particular, the changes occurring at
Antonelli Terry Stout & Kraus LLP
DeBeradinis Robert L.
Nokia Corporation
Sircus Brian
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