EP2315499A1 - LED power supply - Google Patents

Abstract

An LED power supply with a flyback converter (1), wherein the primary winding of a transformer (6) in series with a designed as a power transistor controlled switch (7) to a DC input voltage (U _E ) and the secondary winding is a diode rectifier (8) and a Charging capacitor (9) are connected downstream, and with a drive circuit for the controlled switch, which has a clock oscillator (13) and a driver stage (12) and is adapted to turn the controlled switch at least approximately in the minimum voltage of the parasitic vibrations of the drain-source voltage , and with a regulation of the secondary current (i _D ) by an LED arrangement with at least one LED, wherein the output signal ( i _LED ) of a current sensor (17) of the secondary side and a reference signal (i _{LED, ref} ) to the inputs of the control amplifier (18 ) are fed, the output of the drive circuit is supplied, wherein the clock oscillator (13) as r ckgekoppelter Schmitt trigger is formed, the drain-source voltage of the power transistor (7) via a voltage divider (R _d, R _m) is fed to the input of a comparator (26) and the output of the comparator (26) via a coupling network (28 ) is fed to the feedback point (A) of the clock oscillator (13), the output signal of a variable gain amplifier (18) is added to a signal proportional to the switch current (i _T ), and a voltage value corresponding to this sum signal is also applied to the feedback point of the clock oscillator (13) ,

Description

The invention relates to an LED power supply, comprising a flyback converter having a transformer with a primary winding and a secondary winding, wherein the primary winding is connected in series with a power transistor designed as a controlled switch to a DC input voltage and the secondary winding followed by a diode rectifier and a charging capacitor are, as well as with a drive circuit for the controlled switch, which is supplied by a derived from the input DC voltage auxiliary voltage and having a clock oscillator and a driver stage and is adapted to turn the controlled switch at least approximately in the voltage minimum of the parasitic vibrations of the drain-source voltage, and with a regulation of the secondary current by an LED arrangement having at least one LED, wherein the output signal of a current sensor of the secondary side and a reference signal to the inputs of the control amplifier ers are supplied, the output of the drive circuit is supplied

Power supplies for supplying light-emitting diodes (LEDs), such as these e.g. for emergency lighting equipment or for general lighting tasks are increasingly used and are usually designed as power electronic converter (switching power supply), which adapt the DC or AC input / mains voltage to the load characteristics of the LEDs, for safety reasons in general a potential separation between Input and output side is required. Since smaller powers in the range of typically 5... 20 W are sufficient, in particular for emergency lighting, flyback converters are often used as converters in the current state of the art. These can be realized with very few components and are therefore very inexpensive to produce. In addition to low production costs, on the other hand, the highest possible efficiency of the converter is important, since this directly determines the efficiency of the entire lighting device and low losses facilitate the cooling of the device or improve its installation conditions.

An object of the invention is therefore to provide an LED power supply with minimal component complexity while maximizing the efficiency.

This object is achieved with an LED power supply of the type specified, in which according to the invention the clock oscillator is designed as a feedback Schmitt trigger, the drain-source voltage of the power transistor via a voltage divider network is fed to the input of a comparator and the output of comparator is fed via a coupling network to the feedback point of the clock oscillator, the output signal of a variable gain amplifier is added to a signal proportional to the switch current and a sum signal corresponding voltage value is also supplied to the feedback point of the clock oscillator.

Thanks to this solution, switching on at minimum voltage is only approximate, but very effective and practically without significant additional costs.

In a development of the invention, it is provided in the sense of a cost-effective solution that the comparator is designed as a Schmitt trigger.

Advantageous, since it is cost-effective if the coupling network is a parallel RC element.

In terms of an often desired electrical isolation, it is recommended that the output of the control amplifier is fed via an optocoupler of the drive circuit. In this case, a favorable development of the invention is characterized in that the output of the control amplifier is connected to the input of an opto-transistor of the optocoupler, wherein the base of the optotransistor fed to a signal derived from the switch current and the collector-emitter path of the opto-transistor Series with a resistance between the feedback point of the clock oscillator and ground is located. Alternatively, it is possible that a control transistor is provided whose collector-emitter path between the feedback point of the clock oscillator and ground is located, wherein the base of the control transistor on the one hand via a resistor and the collector-emitter path of an opto-transistor of the optocoupler with the input the controlled switch is connected and on the other hand, the base of the control transistor, a signal derived from the switch current signal is supplied, as a result, a cheaper optocoupler can be used, the cost of the additional control transistor hardly matter.

In order to better define the switching hysteresis, it can be provided that between the feedback point and the actual input of the Schmitt trigger gate of the clock oscillator two antiparallel diodes and from the gate input to ground, an additional capacitor are connected.

In terms of a significant reduction in electromagnetic interference, it is advisable if an existing from a feedback Schmitt trigger RC oscillator is provided with respect to the clock frequency low frequency whose output signal is fed to the feedback point of the clock oscillator to the frequency modulation.

To protect against excessive input voltages, it is recommended that the input of another Schmitt trigger gate is connected via a voltage divider to the input DC voltage and the output of the gate is connected via a diode to the feedback point of the clock oscillator.

Switching losses, in particular at high input voltages, can be reduced if a burst-mode oscillator, consisting of a Schmitt trigger gate fed back via an RC network and having the series connection of a resistor and a diode in parallel with the feedback resistor, is provided. which is coupled via a diode to the feedback point of the clock oscillator and releases this periodically for a certain time interval.

An extremely effective protection against mains overvoltage results if between the common ground and the secondary ground of a power rectifier a series transistor is connected, whose gate is activated via a series resistor by the auxiliary voltage, wherein between the input DC voltage and the base of the auxiliary transistor, the series connection of a Zener diode and a series resistor is so that the locking of the series transistor its gate voltage by means of an auxiliary transistor is substantially short-circuited when the input DC voltage exceeds a value defined by the Zener diode value.

If the base of the auxiliary transistor is connected via the series circuit of a Zener diode and a resistor to the auxiliary voltage, so that the inrush current of the series transistor is limited, a soft run-up can be achieved, which is particularly desirable in emergency lighting. In this case, it is advantageous if, for detecting the current through the series transistor, a measuring resistor is provided and the measuring voltage lying on this is guided via a series resistor and a transverse capacitor to the base of the auxiliary transistor.

A power supply according to the invention can be made very cost effective despite many functionalities, if all Schmitt trigger are implemented as gates of a single CMOS device.

• Fig. 1a den prinzipiellen Aufbau eines Sperrwandlers, dessen Schalter unter Verwendung von Schmitt-Trigger Gattern, z.B. auf CMOS-Basis angesteuert wird,
• Fig.1b zugehörige Spannungs- bzw. Stromverläufe in einem Diagramm,
• Fig. 2a eine Schaltung nach der Erfindung mit Regelung des Sekundärstroms und Definition der Hysteresebreite durch zusätzliche Dioden,
• Fig. 2b zugehörige Spannungs- bzw. Stromverläufe in einem Diagramm,
• Fig. 3 eine erweiterte Schaltung nach der Erfindung mit einer Sicherung gegen Überspannungen am Eingang und einem Burst-Mode-Oszillator,
• Fig. 4 einen Ausschnitt aus der Schaltung nach Fig. 3, betreffend eine alternative Regelung unter Verwendung eines einfachen Optokopplers und
• Fig. 5 eine Variante einer erfindungsgemäßen Schaltung mit einer besonders wirksamen Sicherung gegen Überspannungen und Überströme.

The invention is explained in more detail below with reference to the drawings on the basis of exemplary embodiments, which are illustrated in the drawing. In this show

• Fig. 1a the basic structure of a flyback converter whose switch is controlled by using Schmitt trigger gates, for example based on CMOS,
• 1b shows associated voltage or current characteristics in a diagram,
• Fig. 2a a circuit according to the invention with regulation of the secondary current and definition of the hysteresis width by additional diodes,
• Fig. 2b associated voltage or current characteristics in a diagram,
• Fig. 3 an extended circuit according to the invention with a protection against overvoltages at the input and a burst-mode oscillator,
• Fig. 4 a section of the circuit after Fig. 3 concerning an alternative regulation using a simple optocoupler and
• Fig. 5 a variant of a circuit according to the invention with a particularly effective protection against overvoltages and overcurrents.

As in 1a shown, there is a simple flyback converter 1 for feeding light emitting diodes 2 from an input voltage U _E from a transformer 6 having a primary winding 4 and a magnetically coupled thereto with this secondary winding 5. The primary winding 4 can by turning on a designed as a MOS-FET power transistor controlled switch 7 are applied to the input DC voltage U _E , wherein a secondary-side diode rectifier, here a rectifier diode 8 blocks. According to u = L di / dt and W _m = L i ^2/2 while magnetic energy W _{m is} stored in the transformer 6. This energy flows after switching off the controlled switch 7 from the secondary winding 5 via the then conductive rectifier diode D ₈ in a charging capacitor C ₉ at the output of the LEDs to be fed 2 are connected as a load. Thus, the energy W _{m is} transmitted to the output side per switching cycle. To maintain a permanent power flow, therefore, the controlled switch 7 must be controlled periodically via a drive circuit 3. This can be done approximately so that the control terminal 11 of the switch 7 is connected to the output of a driver stage 12, the input of which is driven by a clock oscillator 13, as fed back by means of an RC element R ₁₃ , C ₁₃ Schmitt trigger for generating a Signal is realized with the clock frequency f ₀ . With a suitable choice of frequency and duty cycle arise, as in 1b shows shown, ultimately triangular current waveforms i and _T i _D in the power transistor or switch 7 and the diode D ₈ (discontinuous operation). With U ₁₁ and U _T , the voltages at the control terminal of the switch 7 and its longitudinal voltage are designated.

The self-supply power supply of the drive circuit 3 can very easily but efficiently via a series resistor R ₁₅ and a Zener diode D ₁₆ and a smoothing capacitor C ₁₅ are made, which generate an auxiliary voltage Vcc.

In the in 1a shown simplest basic form, the supply of LEDs is unregulated. The self-adjusting current through the LEDs is hereby relatively inaccurate and determined by the circuit parameters and component characteristics.

In the Fig. 2a shown circuit has a control loop in the sense of a significant improvement. The actual current i _LED is thereby detected by a current sensor 17 (eg, a current measuring resistor) and fed to a control amplifier 18, at whose reference input the setpoint value i _{LED, ref} for the LED current is applied. With the output signal of the control amplifier 18, an optocoupler 19 is driven with an opto-transistor T ₁₉ whose collector terminal is connected to the feedback point A of the clock oscillator 13. The optocoupler 19 acts as a current source, which deduct from the feedback point A streams of variable size and so can influence the frequency and duty cycle of the clock oscillator 13. To set the gain or to determine the operating point of the emitter terminal of the opto-transistor T _{19 is} connected via a resistor R ₁₉ to the circuit ground. As a further control path, the base terminal of the optotransistor T _{19 is} connected via a resistor R ₂₂ to the control terminal 11 of the transistor 7, as well as a resistor R ₂₃ to the source terminal of the power transistor 7, which via a current measuring resistor R _24th connected to the circuit ground. As a result of the described arrangement, in addition to the secondary-side load current control (in which the duty cycle of the clock oscillator 13 is ultimately modified until the required LED current is reached), a current limitation for the power transistor 7 is also achieved. Between the base terminal of the optotransistor 19 and the circuit ground is still a capacitor C _{25 is} connected, which acts in conjunction with the resistors R ₂₃ and R ₂₄ as a low-pass filter or average value filter.

A circuit eg after 1a has the disadvantage of relatively high turn-on losses, which arise in the power transistor or switch 7 and affect the efficiency of the entire power supply. The reason for this is that the stray inductance of the transformer 6 together with its winding capacitances and in particular the output capacitance C _{DS of} the power transistor 7 forms a parasitic resonant circuit. After switching off the diode 8, there are pronounced free vibrations of the transistor voltage u _T (see 1b shows ). At the time of the next turn-on of the transistor 7, its effective output capacitance C _{DS can} therefore have a relatively high voltage value. When switching is now connected to this voltage according to Wc = C _DS ( u _T ) ^2/2 connected, stored in the transistor 7 energy is lost (ie, it is converted into heat, there are switching losses). It should be noted that modern power MOSFETs only have a very low on-resistance in the conducting state, the conduction losses in relation to the switching losses described above are therefore almost negligible in the case of low-power converters.

The switching losses can be substantially reduced (even with state-of-the-art semiconductor components) by virtue of the fact that the power transistor 7 is always switched on in a voltage minimum of the parasitic oscillation ("minimum-voltage switching"). Thus, the parasitic resonant circuit is used as a "relief network", the flyback converter operates in a quasi-resonant mode. Although this basic principle has long been known from the relevant power electronics literature, it requires a relatively complex evaluation circuit for determining the voltage minimum. To facilitate the applicability of this principle, special flyback converter control modules with integrated minimum detection have therefore been developed. However, these control components are rather expensive and because of the more complex signal processing also require a relatively high supply current, so that the described simple internal power supply by means of series resistor and Zener diode is no longer applicable. Alternatively, the voltage minimum could also be determined more easily by means of an additional winding on the transformer 6, but this means (in particular for flyback converters of low power) a considerable additional outlay.

Although the circuit according to the invention achieves the described switching on in the voltage minimum only in an approximate form, it is nevertheless very effective and practically without significant additional costs. According to 2b For this purpose, another Schmitt trigger gate 14.3 of a CMOS device 14 is used as a comparator 26 whose input voltage via a voltage divider 27 consisting here of two resistors R _D , R _m from the drain-source voltage U _{T of} the power transistor. 7 is derived. The voltage divider 27 is dimensioned in conjunction with the technologically predetermined switching threshold of the comparator 26 so that the switching threshold Us (see Fig. 2a ) of the entire arrangement is just above the voltage minimum of the parasitic oscillation. The output of the comparator 26 is subsequently coupled via a parallel RC element 28 - resistor R ₂₈ and capacitor C ₂₈ - to the feedback point A of the clock oscillator 13. Via the RC element 28, a "synchronization" of the clock oscillator 13 takes place, so that it always starts again (ie, the power transistor 7 turns on again), when u _{T is} the voltage minimum achieved as in 2a shown. Although the comparator 26 does not detect the exact point of the voltage minimum of u _T in the circuit according to the invention, this point is approximately detected. However, since the loss energy stored in the output capacitance C _{DS of} the transistor 7 is quadratically dependent on u _T , a significant reduction of the turn-on losses is achieved even with only approximate detection using a single, additional gate. The additional gate is advantageously one of, for example, six gates / inverters integrated in a single device 14, present on a device which are commercially available as CMOS integrated circuits at extremely low cost, with extremely low power consumption due to CMOS technology results.

Due to the immediate reconnection of the power transistor 7 without any significant pause, the regulation of the LED current can not be achieved directly via the duty cycle but only via a variation of the amplitude of the transistor current i _T. In effect, namely, the output signal of the control amplifier 18 with a derivative of the transistor current signal (formed by the components R ₂₂ , R ₂₃ , R ₂₄ , C ₂₅ ) functionally added in the optocoupler 19 and ultimately the clock oscillator 13 is adjusted via the optocoupler output transistor in that the desired current flows through the LEDs in steady-state operation, but also guarantees the desired switching of the power transistor 7 in the voltage minimum ( 2b ).

The clock oscillator 13 is according to 1a in its simplest form from an inverting Schmitt trigger gate, which is fed back via an RC element. The oscillation frequency is determined by the hysteresis width of the Schmitt trigger module. Since the hysteresis width is heavily dependent on manufacturer or series, the oscillation frequency is not precisely defined, which makes the design of the entire converter more difficult. This adverse effect can be achieved by the in 2b shown circuit extension can be significantly improved. In this case, between the feedback point A and the actual input of the gate two antiparallel connected diodes D ₁ , D ₂ and the gate input to ground, an additional capacitor C _{b is} connected. As a result of this circuit expansion, the hysteresis width is now essentially determined by the defined threshold voltage of the diodes D ₁ , D ₂ and no longer dominant by the (inaccurately lower) hysteresis of the gate itself.

An additional advantage results from the fact that a dimensioning is possible in which as a result of the temperature dependence of the diode voltages, the oscillation frequency of the clock oscillator increases with increasing temperature, which increases with increasing temperature counteracts sinking current threshold of the current limit and is thus ensured for an approximately constant maximum power.

Now up Fig. 3 It will be explained that a further improvement of the system properties (once again without practical additional costs) can be achieved if the circuit is supplemented by an additional RC oscillator 29, which in turn uses a (already existing) Schmitt trigger gate of the CMOS Blocks 14 is realized. The feedback network of the RC oscillator 29, namely a resistor R ₂₉ and a capacitor C ₂₉ , is dimensioned so that an oscillation frequency f ₁ sets, which is low compared to the operating frequency f _{0 of} the clock oscillator 13 (eg 1 kHz compared to 100 kHz ). The RC oscillator 29 is now, for example via a coupling resistor R ₃₀ connected to the return point A of the clock oscillator 13, whereby it comes to a kind of frequency modulation (jitter) of the clock oscillator 13 with the operating frequency of the RC oscillator 29 as the modulation frequency. The so-called "spread spectrum" mode of the flyback converter, the line or radiation-bound electromagnetic compatibility (EMC) is significantly improved, thus facilitating the filtering of interference voltage components at the input and output of the converter.

By using a further Schmitt trigger gate 14.5, whose input is connected via a voltage divider R _p + Rq to the supply voltage U _E and whose output is connected via a diode D ₃ to the feedback point A of the clock oscillator 13, can be achieved that the clock oscillator 13 and as a result the entire power supply is switched off at higher voltages. By selecting the switch-off threshold (via dimensioning of the voltage divider), it can be achieved that the power supply does not operate in the event of overvoltages, so that overvoltage protection for the power transistor 7 is provided. Alternatively, but this also an operation is possible in which - strong ripple of the supply voltage U _E assumed, eg due to low capacitance of the DC link capacitor Cz - the power supply only in a portion of the mains voltage period clocks ("quasi-burst mode"), whereby the Switching losses can be further minimized.

The above-explained burst mode can also be enforced explicitly if, with another Schmitt trigger gate 14.6 with a corresponding feedback network, namely an RC network R _r and C _e with an additional resistance diode path of a resistor R _d in Series with a diode D _r parallel to R _r a separate burst-mode oscillator is realized, which is also coupled via a diode D _k to the feedback point A of the clock oscillator 13 and this periodically, but always releases only for a short time interval.

Fig. 4 shows the relevant detail of an embodiment of the circuit after Figure 3 , In this variant, the function of the current sink at the feedback point A of the clock oscillator 13, which in the Fig. 2a and 3 is realized by the optocoupler 19, executed by an additional control transistor T _f . Here, the emitter of the opto-transistor is passed through a resistor R4 to the base of the control transistor T _f , and from this base, a capacitor C4 leads to ground. In addition, the base is connected to the current measuring resistor R ₂₄ . The necessary for potential separation optocoupler 19 is only used to control the control transistor T _f . The advantage of this variant is that the optocoupler 19 now no longer requires a running base terminal of the opto-transistor, thus a low-cost type can be used.

Fig. 5 shows an additional overvoltage protection of the LED power supply. In the case of electronic equipment connected directly to the public power supply network there is an increased risk of damage / destruction due to mains overvoltages. Short-term transient overvoltages, often simulated by so-called surge test pulses, can usually be controlled well by the use of parallel-acting voltage-limiting networks based on Zener diodes and / or VDR resistors. For longer acting overvoltages, as can be caused by a break of the neutral conductor, for example in three-phase systems, above all the DC link capacitor must be protected against overvoltage. Such a fault can only survive the device without damage if it is disconnected from the mains by a series-connected electronic disconnecting device in the event of an overvoltage. The inventive, in Fig. 3 designated as a whole by the reference numeral 1 LED power supply is in Fig. 5 indicated only by a box and here extended by a series transistor T _g , which is connected between the common ground GND and the secondary ground pole of a mains rectifier Glr, and which is fully turned on in normal operation, because its gate via a series resistor R _h by the auxiliary voltage ( internal supply voltage) V _{CC of} the power supply 1 is activated. However, the control terminal of the transistor T _g can now be deactivated by an auxiliary transistor T _i , which short-circuits the gate voltage of the transistor T _g when its base is driven. This occurs when the supply voltage U _{E of} the LED power supply 1 exceeds a limit defined by a zener diode D _j (typically eg 440V), which is connected from the operating voltage U _E via a series resistor R _k to the base terminal of the auxiliary transistor T _i is.

By a further series connection of a Zener diode D1 and a resistor R1, which is connected from the base terminal of the auxiliary transistor T _i to the auxiliary voltage Vcc of the LED power supply, can be achieved that during startup of the circuit the transistor T _g only turns on when the voltage applied to it falls below a defined value (typically 39 V, for example). This results in a current-limited soft start, which reduces the inrush current to a very low value, which is a significant advantage especially for emergency lights.

To protect against overcurrents, it may be expedient if the current flowing through the series transistor T _g detected by a measuring resistor R _n and the voltage across this resistor is fed to the base terminal of the auxiliary transistor T _i , wherein for the suppression of interference an RC filter, consisting of a series resistor R _o and a transverse capacitor C _{o is} interposed. When applying the mains voltage of the DC link capacitor C _z via a diode D _y , the resistor R _h and the auxiliary transistor T _i is charged until the auxiliary transistor T _i via the Zener diode D _l and the resistor R _{1 is} released and the series transistor Tg the Charging current takes over. Only when the difference of the rectified mains voltage to the voltage at the intermediate circuit capacitor C _z falls below the zener voltage of the diode D ₁ , the transistor Tg turns on properly. A Zener diode D _z limits the gate voltage of the transistor Tg. With this circuit expansion, the series transistor T _{g is switched off} not only in the event of overvoltages but also in the event of overcurrents, whereby the robustness of the entire power supply can be further increased.

Claims (14)

LED power supply comprising a flyback converter (1) having a transformer (6) with a primary winding (4) and a secondary winding (5), the primary winding being connected in series with a controlled switch (7) designed as a power transistor to an input DC voltage ( U _E ) and the secondary winding, a diode rectifier (8) and a charging capacitor (9) are connected downstream, and with a drive circuit for the controlled switch, which is supplied by a derived from the input DC voltage auxiliary voltage (Vcc) and a clock oscillator (13) and a driver stage (12) and is adapted to turn on the controlled switch at least approximately in the minimum voltage of the parasitic vibrations of the drain-source voltage, and with a regulation of the secondary current (i _D ) by an LED array with at least one LED, wherein the Output signal (i _LED ) of a current sensor (17) of the secondary side and a reference signal (i _LED , _ref ) are fed to the inputs of the control amplifier (18) whose output is fed to the drive circuit,
characterized in that
the clock oscillator (13) is formed as a feedback Schmitt trigger, the drain-source voltage of the power transistor (7) via a voltage divider (R _d , R _m ) is supplied to the input of a comparator (26) and the output of the comparator (26 ) is fed via a coupling network (28) to the feedback point (A) of the clock oscillator (13), the output signal of a variable gain amplifier (18) is added to a signal proportional to the switch current (i _T ), and a voltage value corresponding to this sum signal is also applied to the feedback point of Clock oscillator (13) is supplied. LED power supply according to claim 1, characterized in that the comparator (26) as Schmitt trigger (14.3) is formed. LED power supply according to claim 1 or 2, characterized in that the coupling network (28) is a parallel RC element. LED power supply according to one of claims 1 to 3, characterized in that the output of the control amplifier (18) via an opto-coupler (19) is supplied to the drive circuit. LED power supply according to claim 4, characterized in that the output of the control amplifier (18) is connected to the input of an opto-transistor of the opto-coupler (19), wherein the base of the opto-transistor is supplied from the switch current (i _T ) derived signal and the collector -Emitter path of the opto-transistor in series with a resistor (R ₁₉ ) between the feedback point (A) of the clock oscillator (13) and ground is located. LED power supply according to claim 4, characterized in that a control transistor (T _f ) is provided whose collector-emitter path between the feedback point (A) of the clock oscillator (13) and ground is located. wherein the base of the control transistor on the one hand via a resistor (R ₄ ) and the collector-emitter path of an opto-transistor of the opto-coupler (19) to the input of the controlled switch (7) is connected and on the other hand, the base of the control transistor one of the switch current (i _T ) derived signal is supplied. LED power supply according to one of claims 1 to 6, characterized in that between the feedback point (A) and the actual input of the Schmitt trigger gate (14.2) of the clock oscillator (13) comprises two antiparallel diodes (D ₁ , D ₂ ) and the gate input to ground an additional capacitor (C _b ) are connected. LED power supply according to one of claims ₁ to 7, characterized in that an existing from a feedback Schmitt trigger RC oscillator (29) with respect to the clock frequency (fo) of the clock generator (13) of low frequency (f ₁ ) is provided Output signal to the feedback point (A) of the clock oscillator (13) is supplied to the frequency modulation. LED power supply according to one of claims 1 to 8, characterized in that the input of another Schmitt trigger gate (14.5) via a voltage divider (R _p , R _m ) to the input DC voltage (U _E ) is placed and the output of the gate is connected via a diode (D ₃ ) to the feedback point (A) of the clock oscillator (13). LED power supply according to one of claims 1 to 9, characterized in that a burst-mode oscillator (s), consisting of a fed back via an RC network and parallel to the feedback resistor (R _r ), the series connection of a resistor (R _d ) and a diode (D _r ) having Schmitt trigger gate (14.6) is provided, which is coupled via a diode (D _k ) to the feedback point (A) of the clock oscillator (13) and this periodically releases for a certain time interval , LED power supply according to one of claims 1 to 10, characterized in that between the common ground (GND) and the secondary ground pole of a mains rectifier (Glr) a series transistor (Tg) is connected, whose gate via a series resistor (R _h ) by the auxiliary voltage (V _CC ) is activated, wherein between the input DC voltage (U _E ) and the base of the auxiliary transistor, the series circuit of a Zener diode (D _j ) and a series resistor (R _k ) is located, so that for blocking the series transistor its gate voltage by means of an auxiliary transistor (T _i ) is substantially short-circuited as soon as the input DC voltage (U _E ) exceeds a value defined by the zener diode (D _j ). LED power supply according to claim 11, characterized in that the base of the auxiliary transistor (T _i ) is connected via the series connection of a Zener diode (D ₁ ) and a resistor (R ₁ ) to the auxiliary voltage (Vcc), so that the inrush current of the series transistor ( T _g ) is limited. LED power supply according to claim 12, characterized in that for detecting the current through the series transistor (Tg) a measuring resistor (R _n ) is provided and the measuring voltage lying across this via a series resistor (R _o ) and a transverse capacitor (C _o ) the base of the auxiliary transistor (T _i ) is guided. LED power supply according to one of claims 1 to 13, characterized in that all Schmitt triggers (14.1 .... 14.6) are implemented as gates of a single CMOS chip (14).

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