Voltage converter

0001The present invention relates to a voltage converter the forward converter type according to the preamble of claim 1.

0002Such voltage converters have a secondary side Synchronous rectifier on which to reach a possible low efficiency typically MOSFETs is realized. From the prior art are numerous Topologies for forward converter known, such as single-ended, push-pull, Half-bridge, push-pull, full bridge or phase shift forward converter.

0003However, in particular the principle of self-control Synchronous rectifiers, namely the use of the Converter circuit itself existing or occurring Voltage profiles to enable or disable the active Switching elements in synchronization with the primary-side switches, may in some forward converter topologies, particularly those If no single-ended forward converters are, in principle problematic be: if, with a push-pull, half-bridge Full-bridge or phase shift forward converter primary side all Semiconductors in the off state, must be on the Secondary side at this time of the freewheeling current through the active (rectifier) ​​switching elements are passed, and with an implementation of these elements by MOSFETs there must be a corresponding control signal. This however, directly from the transformer, about by means of a provided for the drive signal auxiliary winding, produced not readily available.

0004Although this reason is basically the principle of Known self-control, but it is due to this and other topology-related problems especially with forward converters said topologies adversely.

0005Rather, in these forward converter topologies (push-pull, Half-bridge, full-bridge or phase shift forward converters) or other topologies a controlling power semiconductors a secondary-side synchronous rectifier, as in a similar manner as in the German utility model described 299 01 322, by means of an external control dissolved as the prior art in Fig. 6 is described schematically: A PWM control unit 10 thereby controls both a primary side (otherwise known) Switch assembly itself, as well as a secondary side from a transmitter 16 disposed control unit 14, a pair acting as a synchronous rectifier power semiconductor 18, 20, the converter output voltage signal via a load resistor R_L drops.

0006The voltage curves of Fig. 7 illustrate the switching behavior the rectifier elements 18 and 20 relative to the Transformer voltage (first curve), which, due to the primary side control, in the manner shown by the Zero voltage swings: own In contrast, the control signals for the transistors 18, 20 (second and third curve) a Level greater than zero to activate the respective semiconductor for the freewheeling current.

0007However, as already with reference to FIG. 6 clearly such foreign control of the active switching elements the synchronous rectifier on the secondary side and consuming expensive.

0008Object of the present invention is therefore, a driving of active switching elements of a synchronous rectifier in a generic Voltage Converter To simplify mentioned topologies, in particular the circuitry to reduce expenses, the Principle of self-control, that regeneration of the control signals from existing in the converter circuit Signals without the need for external logic signal, is to be used.

0009The object is achieved by the device with the features of claim 1 dissolved; Advantageous developments of the invention are described in the subclaims.

0010So a capacitance device, the invention provides, which with their charge the activation energy (or drive voltage) caches for the active switching elements such that in particular at those times in the operating cycle at which the secondary side to the invention Auxiliary winding is not applied voltage signal, the control operation and consequently the proper functioning to ensure the synchronous rectifier can.

0011In accordance with the invention advantageously utilizes this, that the signal generated by the auxiliary winding in synchronism is the transformer voltage, so that the on- and Off of the active switching elements of the rectifier with high accuracy and thus carried with low losses. The invention for the control of the synchronous rectifier (More specifically: the control terminal of a respective Rectifier switching element) used semiconductor element, Diode or transistor, thereby allowing in circuitry an extremely simple manner, the signal generation and signal applying through the interaction of auxiliary winding and capacity.

0012It is particularly preferred here, a capacitance value for to select the capacitance device, which is significantly higher as a driving capacity (z. B. gate capacitance in the case of a MOSFET) of the control terminal, so that the relevant Charge ratios safe driving and stable ensure switching. It has proven particularly useful, for the capacitance of the capacitance device at least the five to ten times the existing for the active switching element to choose driving capacity.

0013In principle, the present invention for any suitable Topologies on the primary side and secondary side; as more preferably would be the primary side-pull, full bridge, Half-bridge or phase shift topologies, and the secondary side Current-Doubler- or the simple midpoint configuration regarded with choke.

0014In a particularly preferred embodiment of the invention is the secondary side with two circuit branches as Bridge rectifier implemented so that for each circuit branch an active switching element is present, from which an associated auxiliary winding of the pair of auxiliary coils is supplied with charge or voltage. In this case, one hand, be it low, a common for both branches provide capacitor as capacitance device; alternative provides a further, preferred embodiment of the Invention provides, for each branch a series connection of inductance (Ie respective auxiliary winding) and capacitor provide, as said, in this case, the semiconductor element Transistor (particularly preferably: MOSFET) is implemented and its control signal from a node between the inductor and capacitor of a respective opposite branch receives.

0015The advantage compared to an embodiment with diodes as Semiconductor element or only a capacitor is that by such a circuit of the capacitor charged by the associated auxiliary winding both, as are also discharged and the control terminals of the active Switching elements active during the scheduled switch-off are drawn to a zero level, so that in particular after fast voltage capacitive effects the power semiconductor itself is not accidental to a can cause switching.

0016A further preferred embodiment of the invention is is to the branches each with a voltage limiter combine, since the voltage used over a in each branch Capacitor as a capacitor element of the input voltage the converter is dependent and therefore, at about wide input voltage variations, a maximum drive voltage exceed the active switching element can. To this end, it is particularly appropriate, suitable arranged and controlled MOSFET channel side to Voltage limiting the respective control terminals of the upstream active switching elements.

0017As a result produced by the present invention in circuitry surprisingly simple manner a voltage converter with self-directed synchronous rectifier, extending through precise switching behavior of the secondary side Rectifier elements and thus high loss levels distinguished. At the same time minimizes the small number of Circuit elements used to manufacture technical Effort, so that in particular from a manufacturing View of the present invention has great advantages.

0018Further advantages, features and details of the invention result from the following description of preferred Embodiments and with reference to the drawings; these show in<dl tsize="7"><dt>Fig. 1:</dt><dd>a basic circuit diagram for clarification generating a drive signal for an active Rectifier switching element by means of auxiliary winding and capacitor;</dd><dt>Fig. 2:</dt><dd>a signal diagram for comparing the voltage across the auxiliary winding and the driving voltage;</dd><dt>Fig. 3:</dt><dd>a schematic diagram of a first embodiment of the present invention with two secondary-side active switching elements and in each case associated auxiliary winding in common capacitor;</dd><dt>Fig. 4:</dt><dd>a further development of the principle of Fig. 3 (Only segment of the control of the power semiconductors) by means of transistors as switching elements and separate for each branch capacitor;</dd><dt>Fig. 5:</dt><dd>yet a further development of the circuit diagram gem. Fig. 4 additionally provided with, as a voltage transistors connected in front of the respective control terminals of the active switching elements (Best mode);</dd><dt>Fig. 6:</dt><dd>a schematic diagram of a prior known technique, externally controlled voltage converter the flow transducer type and </dd><dt>Fig. 7:</dt><dd>a signal diagram for comparison of the various Control signals generated by additional circuitry for the secondary-side active Rectifier switching elements in Fig. 7.</dd></dl>

0019Fig. 1 and Fig. 2 show how with the aid of a capacitor C₁ as capacitance device and a diode D₁as a semiconductor element by means of a transformer Auxiliary winding W₁ produced transformer voltage signal U_W1in the form of a voltage doubler to generate a trigger voltage U_V1 for the power semiconductor component V₁ can be raised so that, from a high (ie Inrush) level in synchronization and in otherwise known manner the power semiconductor element in the rectifier operation periodically is turned off. The by the voltage doubling the signal U_V1 Signal peaks produced are for a Operating the power semiconductor V₁ harmless. The Auxiliary winding is in otherwise known manner on the (Not explained in detail) mounted main transformer and has a close link with the power windings on.

0020In the actual circuit implementation shows FIGS. 3, the operation of a self-control of a secondary-side synchronous rectifier by means of a capacity: A pair of auxiliary windings W₁, W₂ for each Power semiconductor V₁ or V₂ produced in otherwise known manner an anti-phase voltage signal. By effect associated diodes D₁ or D₂ and the common, between the connection point of the diodes and the Auxiliary windings connected capacitor C₁ is in the in shown Fig. 2, bottom curve, the drive signal for the power semiconductor V₁, V₂ each raised so that sufficient to the desired driving the semiconductor Charge and thus voltage level is available. adopted is in this case that the capacitor C₁ dimensioned is that he is the one (parasitic) gate capacitance MOSFETs V₁ or V₂ exceeds by several times, so that by driving the semiconductor via C₁ falling Voltage drops only insignificantly.

0021The result produced by this behavior an almost ideal voltage waveform for driving the active Rectifier, as the driving voltage, as shown in Figure 7, not only on the Switch is constant but active during the phase in which the anti-phase Rectifier element is off, the drive voltage is further increased. This is advantageous because in this Phase of the total load current by an active rectifier element flows and increased by a driving voltage its resistance (and thus its heat dissipation) is further reduced.

0022Convenient is also the provision of auxiliary windings no substantial disadvantage, small as the face to be created Output voltages (typically &lt;3.3 V) direct Controlling the active rectifier elements anyway would not be possible.

0023While the circuitry of FIG. 3 on the primary side a called. half-bridge configuration, and a secondary side called. Current-Doubler configuration shows is here only to suitable embodiments. In the However, practical implementation are numerous primary and secondary-side topologies, such as the primary side Push-pull, half-bridge, full-bridge or phase shift configurations and on the secondary side Current Doubler or the simple center configuration with a choke.

0024With reference to FIGS. 4 and 5 is a development of the basic idea of ​​Fig. 1 to 3 explained, by the use of (MOSFET) transistors instead of diodes inherent disadvantages of FIG. 3 can be improved. The main disadvantages are explained above: The problem is that the capacitor voltage a Image of the input voltage, and when it varies, then the capacitor voltage changes with. Since the Capacitance value C₁ but must be selected to be relatively large, can take some time to adapt to the capacitor voltage. Particularly in the case of diode connection is in principle only fast charging possible the necessary unloading the capacity for a large (negative) input voltage jump can not take place through the diode and would therefore have to take place via a resistor, but would lead to a permanent loss of capacity. Therefore is in accordance with (MOSFET) transistors the solution. Fig. 4, Fig. 5 is advantageous because the transistors 30, 32 in Fig. 4, Fig. 5 (particularly MOSFET) principle in both "current direction" are driven low resistance and therefore the Condenser quick to assemble and can also unload; thus follows the voltage on this capacitor is always the input voltage, regardless of how it changes. (If this is not the case, there is a risk that the Synchronous rectifier with a negative step input is not properly shutdown.)

00254 is as in Fig. (And also in the further development of FIG. 5) the sequence of capacitance and inductance (Auxiliary winding) reversed, with the result that each branch of the fragmentary shown in Fig. 4 Circuit a separate capacitor C₁ or C₂ having. In addition, a respective diode by a MOSFET 30, 32 is replaced, such that instead of the diode of Fig. 3 the corresponding channel of the MOSFET extends, and the (Gate) controlling a respective MOSFET by the signal is tapped at each opposite branch, ie, the MOSFET 32 is in between the signal at node W₁ and C₁ tapped off, while the control signal for MOSFET 30 at the node between W₂ and C₂ is tapped.

0026This ensures that a relevant capacitor now charged by an associated auxiliary winding both and can be discharged. Additionally, makes about MOSFET 30 that in an off state of the Power semiconductor V₁ the gate electrode of low resistance is pulled to zero potential (and accordingly applies this for MOSFET 32, or V₂), With the effect that, in particular even during rapid changes in voltage at the drain electrode the active rectifier elements V₁, V₂, themselves these parasitic (feedback) capacities between Drain and gate can not even turn it on again.

0027Another addition undergoes the embodiment shown in Fig. 4 in that in the branch between the capacitor and respective control electrode of the Leistungshalbleiterelments even a semiconductor 34 and 36, preferably MOSFET, is connected for voltage limitation, which, in FIG. 5 symbolically, with a selected voltage U_G is preset and therefore maximum drive voltage for V₁ or V₂ defines or limits.