DE102010009870A1 - Switch power pack controlling circuit arrangement for switch power pack device, has rectifier control unit switching transistor to non-conductive state independent of switching frequency prior to completion of freewheel phase - Google Patents

Abstract

The arrangement has a rectifier control unit (31) to control two transistors of a rectifier circuit (13) i.e. synchronous rectifier, of a switch power pack, where the transistors are switched to a conducting state when the rectifier circuit is in a freewheel phase. The control unit switches one of the transistors to a non-conductive state independent of switching frequency prior to completion of the freewheel phase. A time control unit (32) adjusts a transistor down time during the freewheel phase, where the transistor is switched to the non-conductive state during the down time. Independent claims are also included for the following: (1) a method for controlling a switch power pack (2) a switch power pack device comprising a rectifier circuit.

Description

The present invention relates to a circuit arrangement for driving a switching power supply, wherein the switching power supply has a transformer for transmitting electric power from a primary side to a secondary side, a switching unit for switching a drive current for driving the primary side of the transformer according to a switching frequency, a transformer control unit for controlling Switching frequency and a rectifier circuit, in particular a synchronous rectifier having two transistors for rectifying a voltage applied to the secondary side of the transformer secondary voltage.

The invention further relates to a switching power supply device with a switching power supply and such a circuit arrangement and a method for driving a switching power supply with a synchronous rectifier.

Switching power supplies (often referred to as switching power supplies) are well known and serve to convert an electrical voltage, for example, to generate a DC voltage from a mains voltage (AC voltage). For the transmission of a predetermined electrical power in a switching power supply, a transformer (often referred to as a converter transformer) is used for potential separation and voltage conversion. The transmittable power is dependent on the core cross-section of the transformer and the selected frequency (the so-called switching frequency) of the primary current, that is, the driving current flowing through the primary winding of the transformer. By increasing the switching frequency, although a larger power can be transmitted, but at the same time increase the losses in the transformer and in the most commonly used for switching the drive current switching transistors of the switching unit. The efficiency of the switching power supply decreases overall. On the other hand, in order to be able to transmit a higher power, the transformer can be dimensioned larger, which, however, increases the space requirement and the costs. As a rule, the transformer is therefore dimensioned so that it can transmit the rated power with sufficient reserve.

In the DE 10 2007 007249 A1 is a circuit arrangement for driving a switching power supply to cause a short-term increase in power transmission of the switching power supply is described. The switching power supply has there a transformer for transmitting electric power from a primary side to a secondary side, a switching unit for switching a drive current for driving the transformer according to a switching frequency and a control unit for controlling the switching frequency.

For driving the switching power supply, a circuit arrangement is proposed with a load measuring unit for measuring a load variable proportional to a load current flowing through a load on the secondary side, a threshold value unit for checking whether the measured load variable exceeds a first threshold value, and a control signal generating unit for generating, if the measured Load measurement variable exceeds the first threshold, a frequency control signal for causing a short-term increase in the switching frequency of the drive current and for generating when the measured load variable exceeds the first threshold, a first current limit signal for effecting limiting the load current to a maximum continuous load current and for outputting the frequency control signal and first current limit signal to the control unit.

With a switching power supply device having a switching power supply described therein and a circuit arrangement described therein for driving, short-term power increases can be achieved to cover peak loads or trigger circuit breakers. In this case, the switching power supply to provide this additional power not or not significantly above the rated power must be dimensioned.

On the secondary side of such switching power supplies, a rectifier is required to provide the load with a DC voltage can. Among others, synchronous rectifiers with generally two actively driven transistors are used for this purpose. The control is usually carried out so that the transistors are not driven in the freewheeling phase of the rectifier, so are in non-conductive state, such as in the US 7,082,043 is described. But are also known methods, for example. From the US 6,859,373 in which fixed delay times are added to the drive times, so that both transistors are switched on in the freewheeling phase (also referred to as free-wheeling state).

The term "freewheeling phase" usually refers to the inductance of the basic existing output filter of the switched-mode power supply and influences in which branches of the rectifier the current flows. The inductance of the output filter is then "free" when no power is supplied from the primary side.

The invention has for its object to provide a circuit arrangement for driving a switching power supply, with which a switching power supply works as lossless as possible and with the switching power supplies with synchronous rectifier are safely controlled in all modes, especially if by increasing the switching frequency of Switching unit short-term increases in power transfer done.

The object is achieved by a circuit arrangement mentioned above, which is characterized in that the circuit arrangement comprises a rectifier control unit for driving the two transistors of the rectifier circuit, wherein in the freewheeling phase of the rectifier circuit at least temporarily both transistors are put in a conducting state and independent of the switching frequency before the end of the freewheeling phase, a transistor is set in a non-conductive state.

The object is further achieved by a specified in claim 15, according to ausgestaltetes method for driving a switching power supply.

Furthermore, the invention also relates to a specified in claim 14 switching power supply device with a switching power supply and a circuit arrangement according to the invention for driving the switching power supply.

The task is thereby completely solved. Preferred embodiments of the invention are specified in the subclaims, wherein the switching power supply device according to the invention and the method according to the invention are defined in a corresponding manner as in the subclaims related to the circuit arrangement.

The invention is based on the finding that in each case it must be prevented that both transistors of the rectifier circuit are driven simultaneously outside of the freewheeling phase of the rectifier circuit, ie that they are simultaneously in a conducting state. As a result, the transformer of the switched-mode power supply would be short-circuited on the secondary side, which would lead to damage or destruction of the switched-mode power supply after a few switching cycles, given customary dimensioning of the components of the switched-mode power supply.

According to the invention, it is therefore proposed, irrespective of the respective switching frequency, that is to say also in the case of a short-term increase in the switching frequency, to achieve an increase in the power transmission of the switched-mode power supply, to adapt the control of the two transistors of the rectifier circuit such that in each case before the end of the freewheeling phase at least one of the two transistors is set in a non-conductive state. Unlike in known circuit arrangements, therefore, no fixed, adapted to a specific switching frequency delay time for driving the transistors is added, but the control of the transistors is directly adapted to the respective switching frequency and changed accordingly. Would, however, as in the US 6,859,373 described in the control times of the transistors added a fixed delay time, and this in a switching power supply device, as described in the above DE 10 2007 007 249 A1 is described, then be used in a phase of short-term increase in power by increasing the switching frequency of the drive current both transistors are also driven outside the freewheeling phase and thus be conductive, whereby the switching unit would be destroyed or at least damaged. On the other hand, such a case can not occur in the solution according to the invention.

The circuit arrangement according to the invention can not only be used in switching power supplies in which an increase of the power transmission is to be achieved by a short-term increase in the switching frequency, but can in principle be used in all switching power supplies in which a rectifier circuit with two (or more) actively controlled transistors is provided.

In a preferred embodiment, a time control unit is provided for setting the transistor turn-off time during the freewheeling phase, to which the one transistor is set by non-conducting state by appropriate control.

This time control unit is designed in particular for setting the transistor turn-off time as a function of the switching frequency or a switching frequency signal derived from the switching frequency. In such a time control unit, for example, if this is designed to be programmable, the relationship between the transistor turn-off time or a delay of the drive times and the switching frequency or in a switching frequency signal derived from the switching frequency can be programmed (eg programmed as a fixed algorithm or as a table or fixed relation), so that when changing the switching frequency immediately and the transistor drive reacts accordingly. However, the time control unit may, for example, also be a simple variable timer in which the discharge time of a capacitor is adjustable. This makes it particularly easy to implement the time control unit with analog components.

When using such a variable timing element is provided according to the invention in a further embodiment that the timing unit has means for testing whether the voltage applied to the timer timer voltage falls below a switch-off threshold, and that the rectifier control unit is configured for displacement of the transistor during the freewheeling phase of Rectifying circuit in non-conducting state when the voltage applied to the timer timer voltage falls below the switch-off. The turn-off threshold is preferably fixed, while the timer voltage is dependent on the switching frequency in the sense that the timer voltage drops faster, the higher the switching frequency, so that even at higher switching frequency, the turn-off threshold is reached faster. Of course, it is conceivable to design the test and switch-off logic in the opposite sense such that means are provided for checking whether the timer voltage applied to the timer exceeds a switch-off threshold and that the transistor is then rendered non-conductive when the switch-off threshold is exceeded. Both solutions can basically be implemented in a simple manner by analog circuit elements or in digital logic.

In a further development, it is provided that the rectifier control unit is configured to drive the two transistors of the rectifier circuit such that only one of the two transistors is put in a conductive state outside the freewheeling phase of the rectifier circuit. This ensures the proper operation of the switching power supply and serves as further assurance that outside the freewheeling phase at any time both transistors of the rectifier circuit are conductive, which would lead to damage or destruction of the switching power supply.

Preferably, the circuit arrangement according to the invention is used in a switching power supply device, as described in the aforementioned DE 10 2007 007 249 A1 is described, in which by increasing the switching frequency of the drive current of the primary side of the transformer, a short-term increase in the power transmission of the switching power supply can be effected, for example, to cover peak loads or trigger a fuse. Accordingly, in preferred embodiments of the circuit arrangement according to the invention, a frequency control signal generating unit for generating a frequency control signal for causing a short-term increase in the switching frequency of the drive current of the primary side of the transformer, which is designed to output the frequency control signal to the transformer control unit of the switching power supply to a short-term increase in power transmission of the switching power supply to effect. This frequency control signal generating unit preferably has a load measuring unit, a threshold value unit and a control signal generating unit, as is provided according to a further embodiment. The circuit arrangement may further be developed as described in the cited DE 10 2007 007 249 A1 described and shown in detail.

In a further advantageous refinement, the rectifier control unit is designed to displace the transistor during the freewheeling phase of the rectifier circuit in a nonconducting state as a function of a message signal received from the load measuring unit that the measured load variable exceeds the first threshold value or in dependence on the frequency control signal. This signaling signal or the frequency control signal thus serves, as it were, as a switching frequency signal derived from the switching frequency in the above sense, on the basis of which the transistors are driven or the control is changed, and which, for example, serves to set the transistor turn-off time when using a time control unit. Of course, but also the switching frequency of the rectifier control unit can be supplied directly for this purpose.

In a preferred embodiment, it is provided that the time control unit is configured to set the transistor turn-off period during which the one transistor remains in a non-conducting state. In particular, the transistor turn-off period is set in such a way that the one transistor remains in a non-conducting state for at least a period not exceeding 25%, in particular 20%, of the switching period and / or in the range of 20 ns to 2 μs, in particular in the range of 50 ns to 100 ns, lies. Otherwise, the (synchronous) rectification would lose too much efficiency. For example, at a switching frequency of 100 kHz, the transistor turn-off period would be a maximum of 2 μs.

Furthermore, it is preferably provided in one embodiment that the rectifier circuit is a synchronous rectifier.

The invention will be explained in more detail with reference to embodiments shown in the figures, the scope of protection is not limiting in principle. Show it:

1 a circuit diagram of a diode rectifier;

2 a circuit diagram of a synchronous rectifier;

3 Signal diagrams for different drives of the transistors in the in 2 shown synchronous rectifier;

4 a block diagram of a first embodiment of a switching power supply device according to the invention;

5 a circuit diagram of a variable timer;

6 a block diagram of a second embodiment of a switching power supply device according to the invention;

7 a block diagram of a frequency control signal generating unit; and

8th a block diagram of a third embodiment of a switching power supply device according to the invention.

On the secondary side of switching power supplies, a rectifier is basically necessary in order to be able to provide the load with a DC voltage. Often this rectifier is realized with diodes. A circuit diagram of such a diode rectifier is in 1 shown, in this case a mid-point two-way diode rectifier. It is on the secondary side of the transformer 10 of the switching power supply at the end points of the secondary winding in each case a diode D ₁ , D _{2 connected} in the same direction of passage, the load-side connection points are interconnected. At a center tap of the secondary winding of the transformer 10 a coil L _{0 is} connected, whose load-side connection point is connected via a capacitance C to the common load-side connection point of the diodes D ₁ , D ₂ . About the capacitor C, the rectified output voltage U _{L is} tapped to supply the load. By the two diodes D ₁ , D ₂ , the current of the two half-waves of the secondary side of the transformer 10 applied transformer output voltage U ₂ over a common point of the circuit. Through the common line with the coil L ₀ , the currents are then back to the transformer 10 recycled. The structure and operation of such a diode rectifier are basically known, so that is dispensed with further comments at this point.

However, the diodes D ₁ , D ₂ of such a diode rectifier produce undesirably high forward losses and reduce the achievable efficiency. As an improvement, active rectifiers with transistors are therefore also known, in which therefore the diodes are replaced by transistors, in particular MOSFETs. A circuit diagram of such a synchronous rectifier is in 2 shown. There are the transistors Q ₁ , Q ₂ , the diodes D ₁ , D ₂ of in 1 replace shown diode rectifier, shown with its equivalent circuit diagram of an idealized transformer T ₁ and T ₂ and the parasitic diodes D _T1 and D _T2 . Otherwise corresponds to the circuit structure in 1 shown circuit structure.

A non-energized active rectifier (synchronous rectifier) thus behaves like a diode rectifier. When the transistors Q ₁ , Q _{2 are driven} , however, the forward losses considerably decrease. However, the blocking effect in the reverse direction is lost. Therefore, the transistors Q ₁ , Q ₂ must be replaced at appropriate times by an additional rectifier control unit (not shown in FIG 2 ).

• a) D₁ leitet, D₂ leitet nicht;
• b) D₂ leitet, D₁ leitet nicht;
• c) D₁ und D₂ leiten (sog. Freilaufzustand).

The invention explained below basically relates to symmetrically operated switching power supply topologies, in particular the half-bridge topology and the push-pull topology. There, starting from the diode rectifier ( 1 ) three switching states on the secondary-side rectifier, namely:

• a) D ₁ conducts, D ₂ does not conduct;
• b) D ₂ conducts, D ₁ does not conduct;
• c) D ₁ and D ₂ conduct (so-called free-wheeling state).

• a) Q₁ wird angesteuert (ist in leitendem Zustand), Q₂ wird nicht angesteuert (ist in nichtleitendem Zustand);
• b) Q₂ wird angesteuert, Q₁ wird nicht angesteuert;
• c) Q₁ und Q₂ werden angesteuert.

When using a synchronous rectifier with transistors ( 2 ) result in the best case, the states:

• a) Q ₁ is driven (is in a conducting state), Q ₂ is not driven (is in a non-conducting state);
• b) Q ₂ is activated, Q ₁ is not activated;
• c) Q ₁ and Q ₂ are activated.

Simple method of control, as for example. In the US 7,082,43 are described implement only the states a) and b), so that in the freewheeling state, the transistors Q ₁ , Q _{2 are} not driven and conduct the parasitic diodes. This leads to an undesirable increase of the forward losses in the freewheeling state and generally no advantage over a diode rectifier is achieved.

However, methods are also known in which the transistors Q ₁ , Q _{2 are driven} in the freewheeling state. However, most of these methods require additional windings on the transformer 10 of the switching power supply. Other methods do not require these additional windings, but add delay times to the drive times (cf. US 6,859,373 ), so that both transistors Q ₁ , Q ₂ are also activated in the freewheeling phase and are thus in a conducting state. The delay times are constant there and must be adapted to the switching frequency of the switching power supply.

3 shows signal diagrams for different drives of the transistors Q ₁ , Q ₂ at the in 2 shown synchronous rectifier. 3a shows the time course of the transformer output voltages U ₂₁ , U _{22 of} the two partial windings of the secondary winding of the transformer 10 , These voltages are from the primary side of the transformer 10 given and basically used to generate the drive signals of the transistors Q ₁ , Q ₂ . When Freewheeling phase is called the phase in which both transformer output voltages U ₂₁ , U ₂₂ do not provide a signal, ie have amplitude 0.

3b shows ideal to be aspired waveforms of the drive signals S _{Q1, ideal} , S _{Q2, ideal} for the transistors Q ₁ , Q ₂ . The drive signal S _{Q1, ideal of} the transformer Q ₁ is the inverse signal of the transformer output voltage U ₂₂ , and the drive signal S _{Q2, ideal} is the inverse signal of the transformer output voltage U ₂₁ . In practice, however, this inversion can not be used directly in principle, as between the switching off of the transistor Q ₁ and the rising edge of the transformer output voltage U ₂₂ (and vice versa) a short dead time is observed.

A very simple implementation of drive _signals S _Q1a , S _Q2a is in 3c shown. Thus, as mentioned above, only the states a) and b) are implemented. In the freewheeling phase, however, the transistors Q ₁ , Q ₂ are not driven, with the result that in the freewheeling phase the parasitic diodes conduct and lead to undesirable passage losses.

3d shows drive _signals S _Q1b , S _Q2b for driving the transistors Q ₁ , Q ₂ , when one of the two transistors, for example, the transistor Q ₁ , a timer is assigned. The time signal T _Q1b shows by way of example the profile of the output voltage at such a, the transistor Q ₁ associated timer. The transistor Q ₁ is turned off when a predetermined voltage _threshold k from this time signal T _{Q1b is exceeded} . A simple implementation of such a timer consists, for example, of a simple RC element with a resistor and a capacitor connected in series, via the dimensioning of which the fall time of the timing element is determined.

At the in 3d shown waveforms, the fall time of the timer is fixed and adapted to a specific switching frequency of the switching power supply. As from the synopsis in the 3a and 3d shown signals is ensured that the transistor Q _{1 is} no longer driven, that is placed in a non-conductive state before the freewheeling phase ends. So if the transformer output voltage U ₂₂ increases, then only the transistor Q _{2 is} driven.

But if now the switching frequency is increased, as based on the transformer output voltages U _{21 '} , U _22' in 3e (for comparison, the in 3a shown transformer output voltages U ₂₁ , U ₂₂ shown), leads a fixed delay time or a control as in 3d shown to problems. This is based on the in 3f shown drive _signals S _Q1e , S _Q2c recognizable. The timing _signal T _Q1c is, as stated, unchanged since the fall time of the timer is not changed. This results in an increased switching frequency (transformer output voltages U _{21 '} , U _22' ) to the fact that even outside the freewheeling phase, both transistors are partially driven. In this case, this occurs while the transformer output voltage U _{22 'is} at a high potential, because the (fixed) fall time of the timer prevents, despite the increased switching frequency, the first transistor Q _{1 being turned off} fast enough and before the freewheeling phase has expired. This would lead to damage or destruction of the switching power supply within a very short time at an increased switching frequency.

According to the invention, it is proposed to modify the driving of the transistors Q ₁ , Q ₂ such that at least temporarily both transistors are driven during the freewheeling phase, but that one of the two transistors is definitely brought into the non-conducting state before the freewheeling phase has ended. According to the invention, it is thus prevented that both transformers are driven outside the freewheeling phase, in order to avoid the mentioned negative consequences for the switched-mode power supply.

Corresponding drive _signals S _Q1d , S _Q2d and the time _signal T _{Q1d of} a timer are in 3g shown. In this case, the decay time of the timer was changed (depending on the switching frequency of the switching power supply) that the amplitude falls faster below the threshold K, as can be seen in the time _signal T _Q1d . This also turns off the first transistor Q ₁ faster, and it is effectively prevented that outside the freewheeling phase, so if the transformer output voltage U _{22 '} goes to high potential, the first transistor Q _{1 is} still conductive (the second transistor is then conductive) , Alternatively or additionally, it could also be provided that the threshold K is increased when the switching frequency is increased in order to achieve a faster turn-off of the first transistor Q ₁ .

The shutdown of the second transistor Q ₂ is controlled in one embodiment in that the transistor Q _{2 is} also associated with a variable timer. Alternatively, a single timer may be provided, which is then always assigned to the transistor just turned on. These solutions effectively prevent both transistors from being driven when U _{21 'is} at high potential, if Q _{2 were} not turned off fast enough.

Of course, the logic shown may also be designed to be complementary, such that not a drop of a voltage of a timer under a threshold is checked, but that exceeding a threshold is checked. In addition, this function can of course be solved by other elements (analog and / or digital) as a timer.

4 shows a block diagram of a first embodiment of a switching power supply device according to the invention. The switching power supply device includes a switching power supply 1 and a circuit arrangement according to the invention 2 for controlling the switching power supply 1 , The switching power supply 1 includes a converter transformer 10 which converts an input voltage U ₁ provided on the primary side into an output voltage U ₂ output on the secondary side. The switching power supply 1 further comprises, on the primary side of the transformer 10 , a switching unit 11 by a control unit 12 of the switching power supply 1 is driven and from one of a voltage source 3 Provided supply voltage U ₀ (for example, a mains voltage of the electrical supply network), the input voltage U _{1 of} the transformer 10 generated. For this purpose, the supply voltage U ₀ is switched on and off according to a switching frequency, preferably the supply voltage U _{0 is} first rectified and smoothed, so that a switched drive current I1 and a switched drive voltage U ₁ at the output of the switching unit 11 is produced. The control of the switching unit 11 done by means of a transformer control unit 12 , Such a switching power supply 1 is often referred to as primary switched mode power supply.

On the secondary side of the transformer 10 includes the switching power supply 1 a rectifier circuit 13 for rectification of the output voltage U ₂ , so that a rectified load current I _L or a rectified load voltage is provided at the load L. The basic structure and operation of such a switching power supply are well known and should therefore not be further explained at this point.

For controlling the rectifier circuit 13 , which is turned off according to the invention as a synchronous rectifier (eg. As in 2 shown) is in the circuit arrangement according to the invention 2 a rectifier control unit 31 and a timer unit 32 intended. By means of the rectifier control unit 31 The signals are used to control the synchronous rectifier 13 generates, so in particular the Transistoransteuersignale (S _Q1d , S _Q2d in 3g ). To control the transistors (Q ₁ , Q ₂ in 2 ) to adapt to the respective switching frequency is as a timing unit 32 For example, a variable timer provided, as, based on 3g has been described, operates and its time delay variable, so adaptable to the respective switching frequency is. Such variable timing elements are known in principle.

An embodiment of such a variable timer 32 is exemplary in 5 which essentially comprises two capacitors C ₁ , C _2, a transistor Tr and a resistor R. About the transistor Tr, the fall time of the timer is variably adjustable. Of course, other embodiments of timers can be used, which have this functionality and with which the fall time can be varied. With such a timing unit 32 In principle, the switching times and thus the durations of certain switching states of the transistors T1, T2 can be set.

In the in 4 The embodiment shown receives the time control unit 32 , constantly or only on change, information from the transformer control unit 12 which immediately notifies the current switching frequency, an information signal derived from the switching frequency relating to the switching frequency or a change in the switching frequency. Based on this, the timing unit changes 32 then the switch-off of at least one transistor during the freewheeling phase, for example. By changing the fall time of the timer and / or changing a switch-off, which is checked in falling below for switching off at least one of the transistors.

When using a variable timing element according to the invention, the delay time in the synchronous rectifier with an increase in the switching frequency, for example. To effect a short-term increase in power of the switching power supply is achieved. For this purpose, as soon as the criteria for the short-term power increase or the increase of the switching frequency have occurred, the timer is switched over to a shorter activation time. This switching occurs simultaneously with the increase in the switching frequency or a short time before.

This ensures that not both transistors of the synchronous rectifier are driven, even if the switching power supply is not in the freewheeling phase. Outside the freewheel phase, power is transmitted from the primary side through the transformer. If both transistors of the synchronous rectifier were then activated, the transformer would be short-circuited on the secondary side, which would lead to the destruction of the power supply in the case of customary dimensioning of the components after a few switching cycles.

After the switching frequency increase has been reversed, is switched back to a longer activation time. This can also take some time after the reduction of Switching frequency, since not directly threatened by the destruction of the power supply, but only more power loss occurs.

According to the invention, the advantage of the higher efficiency of synchronous rectifiers (compared to diode rectifiers) for power supplies with variable switching frequency, ie, for example, with short-term power increase, usable, without requiring a major additional effort, ie, for example, a complex drive circuit with auxiliary windings are required.

A block diagram of a further embodiment of a switching power supply device according to the invention is shown in FIG 6 shown. In this embodiment, the circuit arrangement according to the invention 2 for controlling the switching power supply 1 a load measuring unit 21 with which a load variable S _L proportional to the load current I _L is measured. In the simplest case, this load measuring unit 21 a simple load measuring resistor, which measures a directly to the load current I _L load measuring voltage in the secondary circuit as a load measurement S _L.

This load measurement variable S _L is considered as an input of a threshold unit 22 supplied, which receives as a further input variable, a first threshold S ₁ . In the event that the load measurement variable S _{L is} a load measurement voltage, the first threshold value S _{1 is} a voltage threshold value. In this threshold unit 22 it is checked whether the measured load measured variable S _L exceeds the first threshold value S ₁ . If this is the case, this will be according to a downstream control signal generating unit 23 reported, for example via a corresponding message signal M or by a corresponding level on the connecting line between the threshold value unit 22 and the control signal generation unit 23 is significantly changed, for example, is set high.

The control signal generation unit 23 , which are described below on the basis of 7 will be explained in more detail, then generates a frequency control signal F, which via a potential separation unit 24 , For example, an optocoupler, the control unit 12 is supplied. This frequency control signal F causes the control unit 12 , the switching unit 11 To control so that the drive frequency is increased, resulting in that over the transformer 10 a larger power is transmitted and provided to the load L.

When the measured load quantity S _L exceeds the first threshold value S ₁ , the control signal generating unit generates 23 Furthermore, a first current limiting signal B ₁ , which also via the potential separation unit 24 the control unit 12 is supplied. This will be the control unit 12 causes the switching unit 11 to control so that a maximum Daueransteuerfrequenz is not exceeded, thus limiting the load current to a maximum continuous load current and thus limit the power transmitted over a longer period of time to the secondary side to prevent damage to components of the switching power supply or the load , As will be explained below, this first current-limiting signal B _{1 is} output with a delay, so that for a short time a load current flowing above the maximum continuous load current can flow in the load.

With this embodiment of the switching power supply device according to the invention, it is thus possible without the transformer 10 in its dimensioning, to transmit for a short time a significantly greater power than the rated power of the transformer. The cost and the additional space required for any necessary larger dimensioning of other components of the switching power supply device, in particular the switching unit 11 (Thyristors or transistors are preferably used at high switching frequencies up to, for example, 100 kHz), which are significantly lower than when using a larger-sized transformer 10 ,

For controlling the rectifier circuit 13 has the circuit arrangement 2 Furthermore, the basis of the 5 already explained units 31 . 32 on. That from the control signal generation unit 23 output frequency control signal F or that of the threshold value unit 22 output message signal M is the time control unit 32 supplied as information about the current switching frequency or a switching frequency change. Further, the voltage drop across the rectifier element Q1 and Q2, respectively, becomes abstract in the rectifier control unit 31 contained, measured, which basically takes place in the same manner as in known rectifier drive circuits.

In 7 is a detailed embodiment of the control signal generating unit 23 shown. For generating the frequency control signal F, the control signal generating unit 23 first a frequency control signal generating unit 231 on. This signal F may be a simple control command involving the control unit 12 instructs to increase the switching frequency. Furthermore, the control signal generation unit has a first current limit signal generation unit 232 on, which generates the current limiting signal B ₁ . However, this signal is not sent immediately to the control unit 12 but is output by means of a delay unit 233 Only after a delay, for example after a few seconds, until then the limitation of the load current (or the transmitted power) on to cause the maximum continuous load current, but before allowing a larger load current or a larger transmitted power. The first current limiting signal B ₁ has for this purpose the control unit 12 to reduce the switching frequency again or to limit a maximum continuous switching frequency to prevent damage to any components of the switching power supply.

In an example embodiment, the first threshold unit is 22 designed so that it indirectly checks whether the load current I _L exceeds 1.2 times the rated current. If this is the case, this is done by means of the message signal M of the control signal generating unit 23 displayed. This then generates immediately the frequency control signal F, which also directly to the control unit 12 is passed on, which then the switching unit 11 controls so that the switching frequency is up-regulated. The switching frequency can be increased, for example, to twice the switching frequency at rated current.

However, it is preferably provided that the switching frequency is regulated up depending on the current intensity of the load current I _L. This is the first threshold unit 22 configured so that the message signal M also contains information about how much the current load current exceeds the rated current, so that the frequency control signal generating unit 231 generates the frequency control signal F accordingly, for which it contains information about how much the control unit should raise the switching frequency.

The delay unit 233 is exemplary designed such that it outputs the first current limiting signal B ₁ only after a delay time of, for example, 10 seconds. After that, the control unit 12 By means of the first current limiting signal B ₁ instructed to reduce the switching frequency back to a value that a maximum continuous load current, which can not lead to damage of components, is not exceeded. For example, this reduces the switching frequency to a value at which load current I _L corresponds to only 1.2 times the rated current.

The above-mentioned values, in particular for the first threshold value, the delay time and the maximum continuous load current, are only to be understood as examples. These values are preferably predefined by appropriate dimensioning of components or appropriate programming depending on the intended use. However, it is also conceivable that these values are individually variable for the user, for example, by means of a corresponding controller are reprogrammable or by the use of appropriate variable components. Preferred values for the first threshold are in the range between 1.0 and 2.2 times the rated current, in particular in the range between 1.2 and 1.5 times the rated current. Preferred values for the delay time of the delay unit are in the range between 1 and 20 seconds, in particular in the range between 5 and 10 seconds. Preferred values for the maximum continuous load current are in the range between 1.0 and 1.5 times the rated current.

In a further development, it can also be provided that the delay unit is designed so that it does not output the first current limiting signal B ₁ after a fixed predetermined delay time, but that the delay time is dependent on the degree of increase of the load current I _L. In particular, the delay unit 233 preferably designed such that the transmitted power multiplied by the time duration of the increased power transmission remains approximately the same. This means, for example, that if the transmitted power is increased by 100%, the delay time can be 5 seconds, while if the transmitted power is increased by only 50%, the delay time 10 Can be seconds. The delay time is thus determined in this embodiment, depending on the current load current I _L and is not fixed. For this purpose, information about the current load current I _L to the control signal generating unit is also in this development with the message signal M. 23 transfer.

The control signal generation unit 23 is also preferably designed so that a repeated increased load current is available only after a certain time. This will be the switching power supply 1 protected against thermal overload.

The for the transmission of the signals from the control signal generating unit 23 to the control unit 12 provided optocouplers 24 is often available in switching power supplies for a voltage regulation independent overvoltage protection in case of voltage control failure. Advantageously, therefore, no separate optocoupler is used for the purpose of the invention signal transmission to the control unit, but it can be used for the original purpose and additionally for the transmission of the signals, the existing optocoupler.

Another embodiment of a switching power supply device according to the invention is in 8th shown. In this embodiment, the circuit arrangement 2 for controlling the switching power supply 1 in addition to the in 1 shown elements an overcurrent protection unit 25 on. This contains a second threshold unit 251 , which is also that of the load measuring unit 21 gemessehe Load measured variable S _{L is} supplied and checks whether this load measurement exceeds a second threshold S ₂ . This indirectly (or directly, if the load is measured directly the load current) is checked whether the load current I _L exceeds a certain threshold, for example, in the range between 2 to 3 times the rated current, preferably 2.0 to 2 , 3 times the rated current. If so, a corresponding overcurrent indicator signal V is applied to a second current limit signal generation unit 252 the overcurrent unit 25 output, which then generates a second current limiting signal B ₂ and the potential separation unit 24 to the control unit 12 outputs. By means of this second current limiting signal B _{2 of} the load current is limited to an overload current, which is generally greater than the maximum continuous load current, but may not be exceeded at any time, as this could damage components. The control unit 12 So is instructed by the second current limiting signal B ₂ , not to increase the load current over a maximum load current, ie, the load current would be limited to this maximum load current, which corresponds, depending on the application, for example, 2 to 3 times the rated current. This also ensures effective short-circuit protection.

Further, in this embodiment, there is no timing unit 32 provided, but only a rectifier control unit 31 , which can be configured, for example, as a digitally programmable unit. Based on a signal associated with the switching frequency, for example, the message signal M, this can directly the transistors of the rectifier circuit 13 control or corresponding control elements for the control accordingly control to achieve the effect of the invention.

Other embodiments with regard to the embodiment of the circuit arrangement according to the invention are in the DE 10 2007 007 249 A1 , especially there in the 4 to 7 shown. The inventive concept can also be used in such embodiments. The description of these embodiments in this document is hereby incorporated by reference.

According to the invention is thus achieved that regardless of the switching frequency damage or destruction of a switching power supply with a synchronous rectifier is avoided.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102007007249 A1 [0004, 0015, 0021, 0021, 0076]
• US7082043 [0007]
• US 6859373 [0007, 0015, 0040]
• US 708243 [0039]

Claims (15)

Circuit arrangement for controlling a switching power supply, wherein the switching power supply ( 1 ) a transformer ( 10 ) for transmitting electrical power from a primary side to a secondary side, a switching unit ( 11 ) for switching a drive current for driving the primary side of the transformer ( 10 ) according to a switching frequency, a transformer control unit ( 12 ) for controlling the switching frequency and a rectifier circuit ( 13 ) with two transistors for rectifying one on the secondary side of the transformer ( 10 ) has secondary voltage, characterized in that the circuit arrangement a rectifier control unit ( 31 ) for driving the two transistors of the rectifier circuit, wherein in the freewheeling phase of the rectifier circuit, at least temporarily both transistors are put in a conducting state and regardless of the switching frequency before the end of the freewheeling phase, a transistor is set in a non-conducting state. Circuit arrangement according to Claim 1, characterized by a time control unit ( 32 ) for setting the transistor turn-off timing during the freewheeling phase to which the one transistor is set by non-conducting state by appropriate control. Circuit arrangement according to Claim 2, characterized in that the time control unit ( 32 ) configured to set the transistor turn-off timing in response to the switching frequency or a switching frequency signal derived from the switching frequency. Circuit arrangement according to Claim 2 or 3, characterized in that the time control unit ( 32 ) has a variable timer. Circuit arrangement according to Claim 4, characterized in that the time control unit ( 32 ) Means for checking whether the voltage applied to the timer timer voltage falls below a switch-off threshold, and that the rectifier control unit ( 31 ) is configured for the displacement of the transistor during the freewheeling phase of the rectifier circuit in a non-conductive state when the voltage applied to the timer timer voltage falls below the switch-off. Circuit arrangement according to one of the preceding claims, characterized in that the rectifier control unit ( 31 ) is designed to drive the two transistors of the rectifier circuit such that outside of the freewheeling phase of the rectifier circuit only one of the two transistors is set in the conductive state. Circuit arrangement according to one of the preceding claims, characterized by a frequency control signal generating unit ( 20 ) for generating a frequency control signal for causing a short-term increase in the switching frequency of the drive current of the primary side of the transformer and for outputting the frequency control signal to the transformer control unit ( 12 ) of the switching power supply to cause a short-term increase in the power transmission of the switching power supply. Circuit arrangement according to Claim 7, characterized in that the frequency control signal generating unit ( 20 ): - a load measuring unit ( 21 ) for measuring a load variable proportional to a load current flowing through a load (L) on the secondary side, - a threshold value unit ( 22 ) for checking whether the measured load measured variable exceeds a first threshold value, - a control signal generating unit ( 23 ) for generating, when the measured load measure exceeds the first threshold, a frequency control signal (F) for causing a momentary increase in the switching frequency of the drive current and for generating, when the measured load measure exceeds the first threshold, a first current limit signal for effecting a limitation of the load current a maximum continuous load current and for outputting the frequency control signal and the first current limiting signal to the transformer control unit ( 12 ). Circuit arrangement according to Claim 8, characterized in that the rectifier control unit ( 31 ) is configured for offsetting the transistor during the freewheeling phase of the rectifier circuit in a non-conducting state as a function of one of the load measuring unit ( 21 ), that the measured load quantity exceeds the first threshold value or in dependence on the frequency control signal (F). Circuit arrangement according to Claims 2 and 8, characterized in that the time control unit ( 32 ) is configured to set the transistor turn-off timing in dependence on one of the load measuring unit ( 21 ), that the measured load quantity exceeds the first threshold value or in dependence on the frequency control signal (F). Circuit arrangement according to Claim 2, characterized in that the time control unit ( 32 ) is configured to adjust the transistor turn-off period during which the one transistor is left in a non-conductive state. Circuit arrangement according to Claim 2, characterized in that the time control unit ( 32 ) sets the transistor turn-off period such that the one transistor remains in a non-conducting state at most for a period not exceeding 25%, in particular 20%, of the switching period and / or in the range of 20 ns to 2 μs, in particular in the range of 50 ns to 100 ns, lies. Circuit arrangement according to one of the preceding claims, characterized in that the rectifier circuit ( 13 ) is a synchronous rectifier. Switching power supply device comprising: - a switching power supply with a transformer ( 10 ) for transmitting electrical power from a primary side to a secondary side, a switching unit ( 11 ) for switching a drive current for driving the primary side of the transformer ( 10 ) corresponding to a switching frequency, a transformer control unit ( 12 ) for controlling the switching frequency and a rectifier circuit ( 13 ) with two transistors for rectifying one on the secondary side of the transformer ( 10 ) adjacent secondary voltage, and - a circuit arrangement for controlling the switching power supply according to one of the preceding claims. Method for controlling a switching power supply, wherein the switching power supply ( 1 ) a transformer ( 10 ) for transmitting electrical power from a primary side to a secondary side, a switching unit ( 11 ) for switching a drive current for driving the primary side of the transformer ( 10 ) according to a switching frequency, a transformer control unit ( 12 ) for controlling the switching frequency and a rectifier circuit ( 13 ) with two transistors for rectifying one on the secondary side of the transformer ( 10 Having applied secondary voltage, characterized in that the two transistors of the rectifier circuit are driven such that in the freewheeling phase of the rectifier circuit at least temporarily both transistors are put in a conducting state and regardless of the switching frequency before the end of the freewheeling phase, a transistor is set in a non-conductive state.

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