JP2008161031A - Technology for enhancing efficiency in flyback pfc (power factor correction) converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive switch-mode power supply with a simple structure, high stability, high performance and high reliability by improving the circuit of a flyback PFC converter with extremely high power-factor and high efficiency, which has been rarely used so far owing to its inherent problem. <P>SOLUTION: The insulating transformer of the flyback PFC converter includes an auxiliary winding and a reduced-capacity capacitor is connected in series with the auxiliary winding to constitute a series resonant circuit. The series resonant circuit is designed matching a ringing frequency when the flyback PFC converter is at full load, thereby reducing the peak value of ringing to enhance reduction in the virtual withstand pressure of an FET, noise terminal voltage, load regulation and line regulation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to power devices, and more particularly to techniques and methods for improving efficiency in switch mode power supplies.

The switch mode power supply has a power supply efficiency higher than that of a conventional transformer type power supply, and a maximum efficiency of 60% or more is obtained even in a switch mode power supply which is said to be low efficiency.

Switch mode power supplies generally have a maximum efficiency of about 70%, although the maximum efficiency is different depending on the system. There are various factors that degrade the efficiency of switch mode power supplies, but the main cause is a decrease in power factor.

The reason why the power factor decreases in the switch mode power supply is that the active power, which is the product of the voltage and the current, decreases due to the input current waveform being differentiated by the storage capacitor connected to the rectifier diode (FIG. 1). FIG. 2). Lowering the power factor causes power harmonics to flow out to the primary AC power supply, which may lead to burnout of phase-advanced capacitors and transformers connected to the same power supply. In addition, since there is a possibility that high-speed power communication (power line communication) may be hindered, suppression of outflow harmonics is an important issue in future switch-mode power supply development.

As a method of improving the power factor, a passive PFC method that reduces outflow harmonics by inserting a high-inductance choke coil called a reactor on the primary side of the switch-mode power supply and canceling out the differential characteristics of the circuit, and a low-inductance An active PFC method that improves the power factor by changing the input current conduction angle by a boost chopper circuit using a choke coil (commonly called boost choke) and a capacitorless converter (see Fig. 3) that eliminates the storage capacitor are practical. It has become. FIG. 4 is a circuit example of a capacitorless converter that uses an industrial standard PFC controller L6562 as a PWM controller in order to facilitate circuit comparison with the present invention.

The passive PFC method is superior in characteristics to the active PFC method, but because it uses a large choke coil, it is difficult to reduce the size and weight, which are the advantages of the switch mode power supply. It is only used for switch mode power supplies.

In the active PFC system, since the output to the primary side AC power supply is non-insulated, usually a two-stage converter in which a switch mode power supply and an active PFC circuit are connected in series is used. The flyback PFC converter of nonpatent literature 1 in which the insulation output of composition and PFC single substance is possible is also devised.

The flyback PFC converter (see Fig. 5) has a simple configuration despite its extremely high power factor and efficiency, and the basic operation of the flyback converter is such that the secondary side DC voltage can be set almost arbitrarily. Is similar. However, since the primary side rectified power supply is a pulsating current, the peak current value flowing through the switch element may be two to three times that of the flyback converter, and the ringing value generated in the switch waveform is isolated. Since the effects of the reflected voltage of the transformer and the transient saturation of the core material overlap, there is a risk of an increase from 3 to 4 times that of a flyback converter.

An increase in the ringing value will lead to an increase in noise terminal voltage and unnecessary radiation, and a large heat sink may be required for a snubber resistor that normally does not interfere with the use of a small heat sink. Since the apparent withstand voltage of the FET used as a low voltage is reduced, the FET with a drain withstand voltage of 600V, which is generally used in a design that supports the world-wide primary power supply voltage, may have insufficient withstand voltage. It has become.

For these reasons, it is extremely rare for flyback PFC converters to be used in general appliances. However, if it is possible to reduce the ringing generated in the relatively large noise terminal voltage and switch waveform, an isolated switch mode power supply with PFC characteristics can be realized at a very low cost. It seems to be the most suitable method for power supply.

Origin electric technology (see Patent Document 2) is disclosed as similar prior art.

As a similar prior art, there is an Active Clamp circuit (see Non-Patent Document 2) of Vicor, USA.

The basic technique of the present invention is described as a magnetic snubber in application technical data and the like and is widely known (see Non-Patent Document 3).
JP 2002-101660 A JP-A-5-260750 STMicroelectronics Application Technical Data AN1059 ON Semiconductor NCP1280 Data Sheet STMicroelectronics Applied Technical Materials MAGNETIC SNUBBER FOR 200W PFC WITH UNIVERSAL MAINS

A switch mode power supply with a simple configuration and a high power factor and low outflow of power supply harmonics is expected to become more important with the spread of high-speed power communication. However, since the power factor improvement by the passive PFC method uses a heavy and expensive choke coil, it is difficult to realize the light weight and small size that are the advantages of the switch mode power supply.

Even in the active PFC system, the configuration is complicated by the two-stage converter and other systems, and it is difficult to realize light weight and small size. Since the capacitorless converter does not use a rectifying capacitor, the fluctuation of the input current appears in the primary AC power supply, and it is difficult to satisfy the characteristics such as the noise terminal voltage even if the power factor is improved.

The flyback PFC converter, which is considered to be an advanced type of capacitorless converter, can approximate the input current waveform as a sine wave, and has a great advantage such as switching the transformer while detecting the zero cross of the primary AC voltage. Depending on the situation, it is possible to improve various noise characteristics compared to capacitorless converters.

In the present invention, by making the inductor of the magnetic snubber a part of the insulation transformer winding, the ringing component generated in the switch waveform, which is considered to be the heat generation of the snubber resistor, is effectively used, and the noise terminal voltage is reduced while improving the efficiency. It is intended to improve the deterioration.

Since the flyback PFC converter uses the mutual induction of the boost and choke currents to obtain the secondary side isolated output, other types of PFC devices such as continuous mode PFC can be used.

In the circuit examples shown in the examples, an efficiency improvement of 10% or more was confirmed in comparison with the conventional flyback PFC converter.

The flyback PFC converter can achieve a power factor of 0.9 and an efficiency of 90% at the best design, but in actual design, it is limited by the oscillation frequency, duty ratio, maximum output, primary AC voltage, etc. Maximum efficiency is reduced to about 80%.

By using the present invention, it was confirmed that the design freedom can be expanded while maintaining the original high efficiency of the flyback PFC converter. In the experimental circuit, using a small heatsink with a thermal resistance of about 10K / W (35mm x 30mm x 20mm) for the snubber resistor R1, the switching element Tr1, and the secondary side rectifier D3 would hinder continuous output of 150W. It is confirmed that there is no.

A flyback PFC converter using a discontinuous mode PFC is considered to be optimal because the loss due to ringing is particularly large and the efficiency improvement effect by using the present invention is remarkable.

FIG. 6 is a circuit example of FIG. In this example, an upward compatible product L6562 (non-continuous mode PFC) of STMicroelectronics L6561 is used as the PFC / PWM controller.

When a full load operation of 200 W is performed with the circuit of FIG. 6, the waveform at the point A generates extremely large ringing as shown in FIG. In an actual flyback PFC converter application, such a large ringing is avoided by increasing the loss of the snubber circuit. In FIG. 7, the snubber circuit is not taken countermeasures for the purpose of comparing with the result of using the present invention. In the experimental circuit of FIG. 6, at full load, the frequency at point A is about 20 kHz and the ringing frequency is about 200 kHz.

FIG. 8 is a block diagram of a flyback PFC converter using the present invention, and FIG. 9 is a circuit example. This circuit is obtained by adding a circuit 11 to FIG. 6, and the circuit 11 constitutes a series resonance circuit. Ct1 and Lt1 are designed resonance frequencies and the combined reactance approximates 0Ω. D1 is a trap that prevents the reflected voltage from Lt1 from flowing into L1.

In FIG. 9, the operation was confirmed with L1 and Lt1 being bifilar widing and the secondary side open inductance being about 500 μH. Since the actual winding inductance of the isolation transformer varies depending on the design and structure of the transformer and the configuration of the power supply, the resonance capacitor Ct1 selects a value that minimizes the ringing at the point D while checking the circuit operation. When a full load operation of 200 W is performed with this circuit, it can be confirmed that the waveform at point D is a waveform with little ringing as shown in FIG.

In the flyback PFC converter, the PWM frequency increases in inverse proportion to the negative overcurrent, and the ringing value increases in proportion to the negative overcurrent. Therefore, the ringing can be reduced with a snubber circuit having a general value at no load or light load. In the experimental circuit using FIG. 9, the ringing frequency at no load is 1 MHz or more, and the ringing is sufficiently attenuated as shown in FIG. 11 in the snubber circuit composed of C1, R1, and D2.

FIG. 12 is a typical waveform when the full load operation is performed by a general flyback PFC converter with the voltage waveform between the points B and C in FIG. The noise component with a low peak value is peculiar to the discontinuous mode PFC, and the integrated result corresponds to a power factor improving component. . However, since the noise component with a high peak value is a noise component due to ringing of the switch waveform, it is uncorrelated with the power factor improving component and has a high frequency, and this may cause a problem that may hinder high-speed power communication It is.

FIG. 13 is a voltage waveform between point E and point F in FIG. In FIG. 12, in order to facilitate the waveform comparison with FIG. 13, the noise component due to ringing is expressed less than the actually observed noise waveform.

The noise terminal voltage in FIG. 9 is confirmed to be able to obtain the characteristics of VCCI-B and FCC-B or higher with CL1 using a Panasonic HSC-1 which is a general-purpose power factor improving choke and the output is 150 W. ing. FIG. 14 is a comparison between the input current waveform indirectly observed by measuring the voltage G point and the H point at both ends of the inrush suppression thermistor Th1 (about 0.01Ω during actual operation) and the primary AC voltage waveform. . Even at 200 W output, which is the limit output of the experimental circuit, no problematic noise outflow was observed in the primary AC voltage waveform, and it was confirmed that the input current approximated a sine wave.

Although the efficiency improvement effect is reduced, it can be used for conventional flyback converters and forward converters.

FIG. 4 is a schematic diagram of the generation principle of harmonic current in a rectifier circuit. It is a comparison figure of a primary side alternating voltage waveform and an input current waveform. It is a block diagram notation of a capacitorless converter. It is an example of a circuit of a capacitorless converter. It is a block diagram notation of a flyback PFC converter. It is a circuit example of a flyback PFC converter. It is an example of ringing generated in a switch waveform of a flyback PFC converter. 1 is a block diagram representation of a flyback PFC converter utilizing the present invention. 1 is an embodiment of a flyback PFC converter utilizing the present invention. It is an improvement example of ringing according to the present invention. FIG. 9 is an example of occurrence of ringing at light load where the circuit 11 does not function. It is the noise outflow example of FIG. 6 observed in front of the primary side rectifier. FIG. 10 is an example of noise outflow of FIG. 9 observed in front of the primary side rectifier. It is an example of power factor improvement in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary side AC input voltage waveform in general switch mode power supply 2 Primary side AC input current waveform in general switch mode power supply 3 Block diagram notation of primary side rectifier circuit 4 Block diagram of PFC and PWM control circuit Notation 5 Block diagram notation of FET switch control circuit 6 Block diagram notation of isolated transformer / switching circuit 7 Block diagram notation of secondary side rectifier circuit 8 Block diagram notation of isolated negative feedback circuit 9 Block diagram notation of PWM control circuit 10 , A point full load waveform 11 D1, Ct1, and Lt1 ringing elimination circuit 12 FIG. 9, D point full load waveform 13 FIG. 9, D point no load, light load waveform 14 FIG. Input voltage waveform 15 Primary side AC input current waveform 16 in FIG. 9 Block diagram notation of ringing elimination circuit 17 Outflow noise waveform 18 in FIG. 6 Outflow noise waveform in FIG. Primary rectifier input voltage of the drain voltage B Figure 6 in FIG. 6 1
C Primary side rectifier input voltage 2 in FIG.
D Drain voltage E in FIG. 9 Primary side rectifier input voltage 1 in FIG.
F Primary side rectifier input voltage 2 in FIG.
G Primary side rectifier input voltage 1A in FIG.
H Primary side rectifier input voltage 1B in FIG.
C1 Snubber capacitor C2 in FIG. 9 Noise removing capacitor 1
C3 Noise removal capacitor 2
C4 Noise removal capacitor 3
Ct1 Ringing Rejection Resonant Capacitor L1 FIG. 9 Isolation Transformer Primary Winding Lt1 FIG. 9 Ringing Removal Resonance Auxiliary Winding D1 Reflected Voltage Trap Diode D2 FIG. 9 Snubber Diode D3 FIG. Secondary rectifier Tr1 Insulation transformer switch element of FIG.

Claims (6)

A series used for the purpose of reducing the ringing generated in the primary winding of the insulating transformer when the element for driving the insulating transformer is in a transient OFF state without reducing the power transmission efficiency when applied to an AC / DC converter. Insulation transformer auxiliary winding with resonance characteristics. Insulation transformer auxiliary winding with series resonance characteristic used to improve power transmission efficiency by outputting loss due to snubber circuit as positive feedback component to insulation transformer secondary side when applied to AC / DC converter . Insulation transformer with series resonance characteristics used for the purpose of reducing noise terminal voltage and radiation component by outputting loss due to snubber circuit as positive feedback component to insulation transformer secondary side when applying to AC / DC converter Auxiliary winding. Especially when applied to flyback PFC converters, a series resonant circuit and an isolation transformer auxiliary winding are used to improve power transmission efficiency by outputting the loss due to the snubber circuit as a positive feedback component to the isolation transformer secondary side. line. Especially when applied to flyback PFC converter, series resonance circuit and insulation used to reduce noise terminal voltage and radiation component by outputting loss due to snubber circuit as positive feedback component to secondary side of insulation transformer Transformer auxiliary winding. Series resonant circuit and insulation transformer auxiliary winding for the purpose of trapping reflected voltage in the series resonance circuit and insulation transformer auxiliary winding used for the purpose of outputting the loss due to the snubber circuit to the secondary side of the insulation transformer as a positive feedback component 6. The converter circuit according to claim 1, wherein a diode is connected in series to the line.

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