LOUVEL, Jean-Paul (Am Boegle 30, Brigachtal, 78086, DE)
MEITZNER, Michael (Neuer Weg 17, Villingen-schwenningen, 78052, DE)
HERMANN, Wolfgang (Berneck 97, Tennenbronn, 78144, DE)
LOUVEL, Jean-Paul (Am Boegle 30, Brigachtal, 78086, DE)
MEITZNER, Michael (Neuer Weg 17, Villingen-schwenningen, 78052, DE)
Claims
1. Switched mode power supply comprising an active PFC circuit with an inductor (L) coupled to an input voltage (U) , a switching transistor (T) coupled to the inductor (L) , an integrated controller circuit (IC) for switching the switching transistor (T) with a switching frequency, rectifying means (D) coupled to the inductor (L) for providing a rectified output voltage (U) and a feedback loop (FL) , which couples the output voltage (U) to the integrated controller circuit (IC), characterized in that an external modulation circuit (MC) is coupled to the integrated controller circuit (IC) for modulating the switching frequency.
2. The switched mode power supply according to claim 1, wherein the modulation circuit (MC) delivers a modulating voltage having a frequency in the range of 20 and 500 Hz.
3. The switched mode power supply according to claim 1 or claim 2, comprising further a bridge rectifier (BR) for coupling with an AC mains supply voltage, and the external modulation circuit (MC) comprising a resistor (R M ) , which is coupled to the bridge rectifier (BR) via a voltage divider (Rl, R2) .
4. The switched mode power supply according to one of the claims 1 to 3, comprising further an integrated controller circuit (IC), which generates an internal modulation of the switching frequency, wherein the external modulating circuit (MC) generates a further modulation of the switching frequency, which is 1,5 to 3 times higher than the internal modulation. |
SWITCHED MODE POWER SUPPLY
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention is related to a switched mode power supply according to the preamble of claim 1. Switched mode power supplies with active power factor correction circuits, mentioned as active PFC circuits, are used, for example, within television sets. In particular, the invention is related to an active PFC circuit, which comprises an up- converter circuit and is applicable as a power factor correction pre-converter .
DESCRIPTION OF THE PRIOR ART
A requirement in several countries is to equip switched mode power supplies with PCF circuits, as a means to reduce the harmonic content, of the waveform of the mains input voltage, to permissible values. An active PFC circuit, which is generated as a step-up pre-converter, may be used for this purpose. The active PFC circuit, which comprises an integrated controller circuit, offers a compact and very effective PFC solution.
An integrated controller circuit of this kind is e.g. the NCP1650, which is manufactured by OnSemiconductor (http://onsemi.com) . In figure 1 a block diagram of the integrated controller circuit NCP1650 is shown. The operation of this controller circuit is known for persons skilled in art. A detailed description of the controller circuit NCP1650 as well as application circuits is described in a datasheet, available via internet or from OnSemiconductor, which is herewith incorporated by reference.
In addition figure 2 shows, as an example, a simplified circuit diagram of an active PFC circuit with the integrated controller circuit NCP1650. The active PFC circuit further comprises an inductor coupled to an input voltage, a switching transistor driven by the integrated controller circuit and coupled to the inductor, rectifying means, coupled to the inductor and providing a rectified output voltage, and a feedback loop, the PFC circuit operating as a step-up pre-converter .
In this appliance, the switching transistor is driven with a constant switching frequency, which switching processes itself causes interferences at the switching frequency and at its harmonics. As a result, at a mains terminal still a considerable amount of harmonics with regard to desired limits of EMC standards can be seen, figure 5a, which may even exceed the limits.
In order to achieve a reduced harmonics content some protective means, such as larger coils, large ferrite beads or multilayer printed circuit boards, are known. These protective means, which have to be implemented in the power supply of the product, increase the cost and size of the product .
An integrated power switch is known, FSDH0265 from Fairchild Semiconductor Corp., which includes an internal frequency modulation for reducing electromagnetic interference. The FSDH0265 comprises an internal oscillator which cannot be influenced by outside circuit components. The modulation range of the FSDH0265 varies between 2,0 kHz at 67 kHz
switching frequency and 3,0 kHz at 100 kHz switching frequency.
SUMMARY OF THE INVENTION
It is therefore desirable to improve a switched mode power supply according to the preamble of claim 1, with respect to its harmonics content.
According to the invention this can be achieved by the features of claim 1.
A switched mode power supply according to the invention has an active PFC circuit arranged as a step-up pre-converter . The active PFC circuit comprises an inductor, which is coupled to an input voltage, a switching transistor, which is coupled to the inductor, and an integrated controller circuit for switching the switching transistor with a switching frequency. The active PFC circuit further comprises rectifying means, which are coupled to the inductor, for providing a rectified output voltage, and a feedback loop, which couples the output voltage to the integrated controller circuit, and comprises an external modulation circuit for modulating the switching frequency with a modulating frequency, which is coupled to the integrated controller circuit. The external modulation circuit can be coupled to an internal oscillator circuit of the integrated controller circuit, which generates the switching frequency.
The external modulation circuit generates a signal with the modulating frequency, which is to be used for modulating the switching frequency. Alternatively, the external modulation
circuit can be coupled to a source with a frequency suitable for modulation and can change this signal, if necessary.
The modulation of the switching frequency results in lower interference peaks, at the switching frequency and at its harmonics, of the mains terminal voltage as can be seen in the figures 5a and 5b.
The level of the radiated energy, which is generated by the active PFC circuit as electromagnetic radiation, is diminished. As a result, by widening the frequency spectrum, the active PFC circuit of the invention observes the legal limits defined by the EMC standards more clearly.
Advantageous developments of the invention are specified in the dependent claims.
The modulating frequency, which is generated by the modulation circuit, can constitute between 30 and 300 Hz and can be delivered by the supply voltage. Preferably, the external modulation circuit can be generated by a resistor, which is coupled to a bridge rectifier for the supply voltage via a voltage divider.
As an alternative, a switched mode power supply with an integrated controller circuit, which generates an internal modulation of the switching frequency, can further comprise an external modulating circuit. The external modulation circuit can generate a further modulation of the switching frequency, which is 1,5 to 3 times higher than the internal modulation .
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail using an embodiment, which is illustrated in figures 3 and 4.
The figures show:
Figure 1 a simplified block diagram of a known integrated controller circuit, Figure 2 a simplified circuit diagram of a known active PFC circuit with an integrated controller circuit as shown in figure 1,
Figure 3 a simplified block diagram of an active PFC circuit of an embodiment of the invention, Figure 4 a simplified circuit diagram of an active PCF circuit of the embodiment of the invention, Figure 5a a frequency spectrum of a voltage present at a mains terminal of the known active PFC as shown in figure 2, and Figure 5b a frequency spectrum of a voltage present at a mains terminal of an active PFC circuit of the embodiment of the invention.
DETAILED DESCRIPTION OF AN PREFERRED EMBODIMENT
The switched mode power supply of the embodiment of the invention has an active PFC circuit, which is generated as a step-up pre-converter . The switched mode power supply can be for example a known flyback converter as used in consumer electronics, the active PFC circuit being arranged ahead of the switched mode power supply.
Figure 3 shows the active PFC circuit, which comprises an inductor L, a switching transistor T and an integrated
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controller circuit IC. The inductor L is coupled to an input voltage U 1n with its one side, mentioned as input side, while the transistor T is coupled to the other side, e.g. the output side, of the inductor L. The integrated controller circuit IC, in particular its driver output, is coupled to the transistor T for switching it with a switching frequency.
The active PFC circuit further comprises rectifying means D and a feedback loop FL. The rectifying means D are coupled to the output side of the inductor L and provide an output voltage U out . The feedback loop FL couples the output voltage U out to the integrated controller circuit IC, in particular to its feedback input.
In addition, according to the invention the active PFC circuit comprises an external modulation circuit MC for modulating the switching frequency with a modulating frequency. By being coupled to the integrated controller circuit IC, the external modulation circuit MC provides the integrated controller circuit IC with a voltage having a modulation frequency. In particular, the external modulation circuit MC is coupled to an oscillator circuit OC of the integrated controller circuit IC, which generates the switching frequency. This is depicted in figure 3.
The integrated controller circuit NCP1650, also mentioned as power factor controller, is used for this embodiment of the invention. Its oscillator circuit is seen in its simplified block diagram, figure 1, and mentioned as oscillator. A simplified circuit of the active PFC circuit of the invention with this integrated controller circuit IC is seen in figure 4.
The external modulation circuit MC is coupled to an AC power supply circuit, namely to its bridge rectifier BR. In particular, the external modulation circuit MC is coupled to the bridge rectifier BR via a voltage divider with resistors Rl and R2. It only comprises one resistor R M , which, in this embodiment, has a resistance of 100 kOhm. The external modulation circuit MC, e.g. its resistor R M , is coupled to the oscillator input of the integrated controller circuit IC. In addition, an oscillator capacitor C 0 , which determines the switching frequency, is coupled to this oscillator input. The modulating frequency, generated by the supply frequency and delivered by the modulation circuit MC, constitutes about 50 Hz in this case, the mains frequency. The switching frequency generated by the controller circuit IC can be for example circa 70 kHz.
The switching transistor T can be an n-channel MOSFET, while the driver output of the integrated controller circuit IC is coupled to the gate of the transistor T. The drain and the source of the switching transistor T can be coupled to each other via a damping capacitor Ci.
The inductor L is coupled with one end to the bridge rectifier BR, which is provided with two smoothing capacitors C3 and C 4 and which delivers the input voltage U 1n . The input voltage U 1n is smoothed by a small capacitor C5 coupled in parallel with the bridge rectifier BR for damping of switching spikes.
The inductor L is connected with the opposite end to the rectifying means D and with the current input of the switching transistor T. The rectifying means D, which are
generated as a diode, deliver the output voltage U out . The output voltage U out is smoothed by a capacitor C6.
The feedback loop FL to the integrated controller circuit IC is coupled with the capacitor C6 and via a voltage divider with several resistors R3, R 4 , R 5 and R 6 . In particular, the feedback loop FL is coupled between the resistors R 5 and Re. It is provided with an RC circuit, which is arranged with resistor R 6 and a capacitor C7.
During operation, the integrated controller circuit IC drives the switching transistor T with the switching frequency. As a result, the input voltage U 1n is converted by the inductor L and by the driven switching transistor T into the output voltage U out , which is rectified by the rectifying means D. The output voltage U out , namely a certain part of the output voltage U out , is coupled via the feedback loop FL to the feedback input of the integrated controller circuit IC. The switching frequency, which is dependent on the value of the capacitor C 0 , is modulated with the modulating frequency as delivered by the modulation circuit MC, for providing a variation of the switching frequency, the modulation range being dependent on the value of the current of the modulation circuit MC for the integrated controller circuit IC. With a value of 100 kOhm for the resistor RM, the modulation range is +/- 4,3 kHz.
The peak values of the harmonics of the voltage as seen at the mains terminal are reduced above the switching frequency of circa 70 kHz, as used in this embodiment. The improvement of the peak values is e.g. circa 6 dB at 210 kHz, on average measurement, as can be seen by comparing figures 5b and 5a.
The invention provides therefore an improved margin with regard to the EMC standard.
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