This application claims priority under 35 U.S.C. §119(a) on Japanese Patent Application No. 2008-244808 filed on Sep. 24, 2008, the entire contents of which are hereby incorporated by reference.
The present disclosure relates to a PWM (pulse width modulation) switching power supply, and more particularly relates to a switching control circuit of the PWM switching power supply.
Switching power supplies have been widely used for power converters, such as AC-DC converters, DC-DC converters and the like, for converting input power into direct current power. In general, a switching power supply performs PWM control of a switching element to repeat a supply of a primary current to a transformer and a halt of the supply, thereby converting input power to desired direct current power.
As described above, in a switching power supply, a switching element repeats switching ON/OFF without interruption. Since a switching frequency of the switching element is relatively high, the switching power supply radiates switching noise. The switching noise causes malfunction of peripheral electronic devices, and therefore, it is desired to suppress the switching noise as much as possible. To solve this EMI (electro-magnetic interference) problem, peaks of the switching noise are suppressed by causing a PWM basic frequency to fluctuate to spread the spectrum of the switching noise (see, for example, Patent Document 1).
There are cases where a PFC circuit is not used and an input smoothing capacitor for smoothing a rectified direct current voltage is provided at an input side of a switching power supply. In such a switching power supply, in order to cause the PWM basic frequency to fluctuate using an input ripple, a capacity value of an input smoothing capacitor has to be small. That is, the input ripple has to be large. However, when the input ripple is large, a power supply capacity of the switching power supply fluctuates at all times. In general, in the switching power supply, overcurrent protection of the switching element is performed based on feedback of an output voltage, and thus, if the power supply capacity fluctuates at all times, the overcurrent protection becomes unstable. On the other hand, if a ripple is artificially generated without using an input ripple, a ripple generator circuit is needed and thus a circuit size is increased.
In view of the above-described problems, an example switching control circuit may allow a PWM basic frequency to fluctuate without specifically manipulating an input ripple of a switching power supply.
The detailed description describes a switching control circuit for performing PWM control of a switching element which controls supply of a primary current to a transformer in a switching power supply for converting input power to direct current power, the switching control circuit including: an amplifier circuit for amplifying an output ripple of an auxiliary winding of the transformer; a fluctuation generator circuit for generating a fluctuating signal, based on an output of the amplifier circuit; a basic signal generator circuit for generating a PWM basic signal whose frequency fluctuates according to the fluctuating signal; and a control circuit for ON controlling the switching element when receiving the PWM basic signal, and OFF controlling the switching element when receiving an OFF signal based on output feedback of the switching power supply.
Moreover, the detailed description describes a switching control circuit for performing PWM control of a switching element which controls supply of a primary current to a transformer in a switching power supply for converting input power to direct current power, the switching control circuit including: a feedback circuit for receiving a detection of an output of an auxiliary winding of the transformer to generate a feedback signal; a fluctuation generator circuit for generating a fluctuating signal, based on an output ripple of the auxiliary winding amplified by the feedback circuit; a basic signal generator circuit for generating a PWM basic signal whose frequency fluctuates according to the fluctuating signal; and a control circuit for ON controlling the switching element when receiving the PWM basic signal, and OFF controlling the switching element when receiving an OFF signal generated based on the feedback signal.
Furthermore, the detailed description describes a switching power supply for converting input power to direct current power, the switching power supply including: either one of the above-described switching control circuits; a transformer; a switching element which is connected to a primary winding of the transformer and is PWM controlled by the switching control circuit; a rectifier element, connected to the auxiliary winding of the transformer, for rectifying a secondary current of the auxiliary winding; and a smoothing element for smoothing a current rectified by the rectifier element to generate a direct current voltage.
With the above-described means, it is possible to cause the PWM basic frequency to fluctuate using the output ripple of the auxiliary winding of the transformer.
Also, the detailed description describes a switching control circuit for performing PWM control of a switching element which controls supply of a primary current to a transformer in a switching power supply for converting input power to direct current power, the switching control circuit including: an amplifier circuit for amplifying an output ripple of the switching power supply; a fluctuation generator circuit for generating a fluctuating signal, based on an output of the amplifier circuit; a basic signal generator circuit for generating a PWM basic signal whose frequency fluctuates according to the fluctuating signal; and a control circuit for ON controlling the switching element when receiving the PWM basic signal, and OFF controlling the switching element when receiving an OFF signal based on output feedback of the switching power supply.
Moreover, the detailed description describes a switching control circuit for performing PWM control of a switching element which controls supply of a primary current to a transformer in a switching power supply for converting input power to direct current power, the switching control circuit including: a feedback circuit for receiving a detection of an output of the switching power supply to generate a feedback signal; a fluctuation generator circuit for generating a fluctuating signal, based on an output ripple of the switching power supply amplified by the feedback circuit; a basic signal generator circuit for generating a PWM basic signal whose frequency fluctuates according to the fluctuating signal; and a control circuit for ON controlling the switching element when receiving the PWM basic signal, and OFF controlling the switching element when receiving an OFF signal generated based on the feedback signal.
Furthermore, the detailed description describe a switching power supply for converting input power to direct current power, the switching power supply including: either one of the above-described switching control circuits; a transformer; a switching element which is connected to a primary winding of the transformer and is PWM controlled by the switching control circuit; a rectifier element, connected to a secondary winding of the transformer, for rectifying a secondary current of the transformer; and a smoothing element for smoothing a current rectified by the rectifier element to generate a direct current voltage.
With the above-described means, it is possible to cause the PWM basic frequency to fluctuate using the output ripple of the switching power supply.
Hereinafter, best modes of the present invention will be described with reference to the accompanying drawings.
In the switching control circuit 20, an amplifier circuit 202 amplifies an intermediate voltage of the output smoothing capacitor 15′, and outputs the signal S1. That is, the amplifier circuit 202 amplifies an output ripple of an auxiliary winding of the transformer 13. Specifically, the amplifier circuit 202 can be formed of an operational amplifier, a mirror circuit or the like. The fluctuation generator circuit 203 generates a fluctuating signal S2, based on the signal S1. Specifically, the signal S1 is input to a gate of a transistor 2030. The transistor 2030 is connected to an input side of a current mirror circuit 2031. Thus, a current which fluctuates according to the signal S1 is output from the current mirror circuit 2031. The current becomes the fluctuating signal S2. A basic signal generator circuit 204 generates a PWM basic signal S3 whose frequency fluctuates according to the fluctuating signal S2.
Note that the frequency of the fluctuating signal S2 is preferably about 10% of the PWM basic frequency even at highest setting. For example, when the PWM basic frequency is 100 kHz and the frequency of the fluctuating signal S2 is 10 kHz, the PWM basic signal S3 varies between 100 kHz and 110 kHz.
A current detector circuit 205 detects a current flowing through the switching element 201, and outputs a detection signal S4. The current detector circuit 205 may be provided at a source side of the switching element 201.
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As described above, according to this embodiment, it is possible to cause the PWM basic frequency to fluctuate using the output ripple of the auxiliary winding of the transformer 13. The auxiliary winding is provided solely for the purpose of feedback control of the switching power supply, and thus a capacity value of the output smoothing capacitor 15′ can be freely changed without affecting the direct current output Vout. Therefore, peaks of switching noise can be reduced by fluctuating the PWM basic frequency without specifically manipulating the input ripple.
Note that the alternate current input Vin may be full wave rectified using a diode bridge, instead of the input rectifier diode 11.
When the transformer 13 does not include an auxiliary winding, the output voltage detector circuit 16 may be configured to detect a direct current output Vout.
The feedback circuit 206 amplifies an output ripple in the course of generating the feedback signal S5. Therefore, an internal signal of the feedback circuit 206 can be supplied as the signal S1 to the fluctuation generator circuit 203.
When the transformer 13 does not include an auxiliary winding, the output voltage detector circuit 16 may be configured to detect a direct current output Vout.
Number | Date | Country | Kind |
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2008-244808 | Sep 2008 | JP | national |