KR100990158B1 - High frequency dc-dc converter - Google Patents

Abstract

The DC-DC converter according to the present invention receives an external DC voltage and converts it into an AC voltage, and a primary side is connected to the DC-AC converter to drive the light-emitting diode with the converted AC voltage. In the DC-DC converter including a piezoelectric transformer for boosting or stepping down to a voltage level to be rectified, and a rectifier connected to the secondary side of the piezoelectric transformer to rectify the boosted or stepped AC voltage and apply it to the light emitting diode, The DC-AC converter is a PWM converter that oscillates the driving frequency of the piezoelectric transformer, and a half connected to the PWM converter to convert the external DC voltage into an AC voltage according to the driving frequency therefrom and apply the applied voltage to the piezoelectric transformer. And a bridge inverter, wherein the DC-DC converter is connected to an output terminal of the rectifier and an input terminal of the PWM converter and applied to the light emitting diode. Further comprising: a photo-coupler, the piezoelectric transformer can be fed back to the laminated type of piezoelectric ceramic represented by the following formula:

_{(Pb 0 .76 Ca 0 .23 Sr} 0 .1) Ti 0 .96 (Mn 1/3 Sb 2/3) 0.04 O 3 + 0.2wt% Na 2 CO 3 + 0.2wt% Li 2 CO 3.

DC / DC Converters, Piezoelectric Transformers

Description

High Frequency DC-DC Converters {HIGH FREQUENCY DC-DC CONVERTER}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter, and more particularly, to a DC-DC converter, which is miniaturized, lightweight, and operated at a high frequency by using a stacked piezoelectric transformer as a driving power source for a high brightness light emitting diode (LED).

The energy-saving light emitting diode (LED), a next-generation light source that is more energy-saving and can be used semi-permanently compared to the existing light source, consumes only 10 minutes to 1 times the life of an existing incandescent bulb and has a lifespan of more than 10 to 50 times. It is reported to be high.

In addition, LEDs that are free of harmful substances such as mercury used in fluorescent lamps have excellent environmentally friendly characteristics, and have been attracting more attention as next-generation light sources as the luminance problem, which has become a conventional problem, is greatly improved. Accordingly, as the application of lighting as an alternative to the conventional fluorescent lamp is spreading, the development of a DC-DC converter for driving such LEDs is accelerated.

In order to reduce the size of the DC-DC converter, an increase in operating frequency is inevitable. However, the increase in the operating frequency has been limited due to problems such as an increase in leakage inductance and core loss.

However, piezoelectric transformers that can be made smaller and lighter have advantages in that high efficiency can be obtained even at high frequencies because they do not have increased leakage inductance and core loss problems. There is no worry about it. Therefore, the DC-DC converter using the piezoelectric transformer can be miniaturized and light in weight, and is safe from fire. However, in order to drive the piezoelectric transformer, supply of a high frequency suitable for it is essential.

Accordingly, it is an object of the present invention to provide a DC-DC converter using a stacked piezoelectric transformer with excellent miniaturization, weight reduction, and fire stability by supplying an operating frequency in an appropriate high frequency band.

In order to achieve the above object, the DC-DC converter according to an aspect of the present invention is a DC-AC converter for receiving an external DC voltage and converting it to an AC voltage, and the primary side is connected to the DC-AC converter A piezoelectric transformer for boosting or stepping down the converted AC voltage to a voltage level for driving the light emitting diode, and a rectifier connected to the secondary side of the piezoelectric transformer to rectify the boosted or stepped AC voltage and apply the same to the light emitting diode. In the DC-DC converter comprising a PWM converter for oscillating the drive frequency of the piezoelectric transformer, the DC-DC converter is connected to the PWM converter and converts the external DC voltage according to the drive frequency therefrom AC voltage And a half bridge inverter applied to the piezoelectric transformer, wherein the DC-DC converter is connected to an output terminal of the rectifier and an input terminal of the PWM converter. Is the piezoelectric transformer further comprising a photo-coupler for feeding back a voltage applied to the LED may be a laminate of piezoelectric ceramic represented by the following formula:

_{(Pb 0 .76 Ca 0 .23 Sr} 0 .1) Ti 0 .96 (Mn 1/3 Sb 2/3) 0.04 O 3 + 0.2wt% Na 2 CO 3 + 0.2wt% Li 2 CO 3.

In this case, the DC-DC converter may further include a resonant inductor connected in series between the output terminal of the half-bridge inverter and the primary side of the piezoelectric transformer to make the AC voltage applied to the piezoelectric transformer into a sine wave. The DC-DC converter may further include a sensing resistor connected between the output terminal of the PWM converter and the primary side of the piezoelectric transformer to prevent overcurrent applied to the piezoelectric transformer.

As described above, the present invention provides a DC-DC converter excellent in miniaturization, light weight, and fire stability by implementing a supply of an operating frequency of a high frequency band suitable for the piezoelectric transformer in a DC-DC converter using a stacked piezoelectric transformer. . The DC-DC converter of the present invention is suitable for driving high brightness light emitting diodes.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. In particular, the following examples are provided to aid the overall understanding of the present invention, and the present invention is not limited to the following examples.

Thickness vibration mode Stacked  Manufacture of Piezoelectric Transformer

First, the DC-DC converter according to the present invention includes a thick vibration mode multilayer piezoelectric transformer having the following formula (1):

_{(Pb 0 .76 Ca 0 .23 Sr} 0 .1) Ti 0 .96 (Mn 1/3 Sb 2/3) 0.04 O 3 + 0.2wt% Na 2 CO 3 + 0.2wt% Li 2 CO 3 ( formula 1 )

In the present invention, the piezoelectric transformer may be manufactured by a method known in the art, including an oxide mixing method and a sol gel method. In particular, according to one embodiment of the invention using the oxide mixing method, the thickness vibration mode multilayer piezoelectric transformer is preferably manufactured as follows: (Pb _0.76 Ca _0.23 Sr _0.1 ) Ti _0.96 (Mn _1/3 Sb _2/3 ) The composition of _0.04 O ₃ is weighed to 10 ^-4 g with a powder having a purity of 99% or more, ball milled for 24 hours with acetone as a dispersion medium, and the ball milled powder is dried for 12 hours or more. Calcining at 850 ° C. for 2 hours; Then, 0.2 wt% Na ₂ CO ₃ +0.2 wt% Li ₂ CO ₃ of the formula 1 was added to the calcined powder, followed by remixing and grinding for 24 hours, followed by drying; Then, after mixing the dry powder and PVB (Ferro B73305) in a ratio of 72:28, tape casting is performed by a doctor blade to prepare a sheet having a thickness of 75 μm; And laminating at least one of the sheets at a temperature of 3200 psi at 70 ° C., then burn-out at 340 ° C. for 3 hours and sintering at 900 ° C. for 90 minutes; Then, after sintering, the electrode was polished to a thickness of 3.5 mm to apply an electrode to the surface, followed by heat treatment at 600 ° C. for 10 minutes to form an electrode, which was polarized at 45 ° C./cm in 50 ° C. silicon oil; Each of the laminated piezoelectric ceramic sheets is operated in a thickness vibration mode when an electric field is applied.

Fig. 1A shows a stacked piezoelectric transformer manufactured by the present embodiment, which is a step-down type with three layers of outputs and one layer of inputs.

In addition, Figure 1b is a graph of the resonance and anti-resonance frequency characteristics of the stacked piezoelectric transformer manufactured by the present embodiment, based on the piezoelectric and dielectric properties, that is, the effective electromechanical coupling coefficient k _eff , output impedance Z of the piezoelectric transformer _out , the mechanical quality factor Q _mt , and the electromechanical coupling factor k _t can be obtained from the following Equations 2-5 according to the IEEE:

식 2

Equation 2

식 3

Expression 3

식 4

Equation 4

식 5

Equation 5

Accordingly, the values of the various components on the equivalent circuit of the piezoelectric transformer are summarized as in Table 1 below:

TABLE 1

f _r [MHz] f _a [MHz] R [Ω] C ₀ [nF] Input 1.225 1.3175 23.16 0.59 Output 1.1025 1.3175 2.9 7.74 Z _out [Ω] K _eff Q _mt k _t Input - 0.31 635 33.42 Output 17.29 0.55 226 58.7

DC - DC  Converter design

 The DC-DC converter using the stacked piezoelectric transformer according to the present invention generally requires the following requirements. First, in order to drive a stacked piezoelectric transformer, a high frequency DC-AC converter must be provided. That is, unlike the general winding type transformer, the multilayer piezoelectric transformer needs to apply a linear AC voltage of the same driving frequency as the resonance frequency of the piezoelectric transformer to obtain a high step-down ratio, and the resonance frequency region has a narrow region due to the characteristics of the piezoelectric transformer. Since the matching of the driving frequency is essential. Second, overvoltage protection, undervoltage input protection, and overcurrent protection circuits are required in case of load damage.

According to the above requirements, the DC-DC converter using the stacked piezoelectric transformer according to the present invention is implemented as shown in FIGS. 2A-2C. 2A is a block diagram of a DC-DC converter according to the present invention, FIG. 2B is a block diagram of a DC-DC converter according to an embodiment of the present invention, and FIG. 2C is a schematic circuit diagram of FIG. 2B.

First, referring to FIG. 2A, the DC-DC converter 10 using the stacked piezoelectric transformer according to the present invention may convert a DC voltage (for example, DC 34V) input from the power supply unit 20 into an AC voltage. An AC converter 30, a piezoelectric transformer 40 for boosting or stepping down a voltage upon receiving the output thereof, and a rectifier 50 for driving the LED unit 70 by converting the output of the piezoelectric transformer into a DC voltage. It is configured to include. In addition, the DC-AC converter 30 converts the applied DC voltage into pulse width modulation (PWM) to oscillate the driving frequency of the piezoelectric transformer 40, and converts the DC voltage into an AC voltage. The half bridge inverter 34 is comprised.

In an embodiment of the present invention, referring to FIG. 2B, the PWM converter 32 may use a PWM IC (LM5035, National Semiconductor), which IC 105V / 2A Half-bridge Gate Drivers, Line Under-Voltage. Lockout (UVLO), Line Over-Voltage Protection (OVP) and Current Sense input for current limit (CS).

In addition, the two MOSFET switches of the half-bridge inverter 34 alternately operate in accordance with the driving frequency from the PWM converter 32 to convert the DC voltage input to the square wave AC voltage. 2B, the frequency oscillation of the PWM converter or PWM LM5035 (32) is applied between the R _T and AGND pins to apply the gate pulse signal voltages of the two MOSFETs of the half-bridge inverter 34. It can be set by external resistance R _T. This R _T is induced by Equation 6, whereby the pulse signal programmed by the R _T is alternately oscillated by the HO and LO pins of the PWM LM5035 (32) and applied to the two MOSFET switches of the half bridge inverter 34. do:

식 6

Equation 6

In addition, in the half-bridge topology as described above, the adjustable part is a resonant inductor L _r to produce a waveform closer to sinusoidal wave. This is because not only all constant values are determined when the piezoelectric transformer 40 is manufactured, but also the piezoelectric transformer is a device having a large capacitor value, so selection of a matching inductor is important. Since the LC resonant frequency must be greater than the switching frequency, the capacitance of the series resonant inductor L _r can be derived from Equations 7 and 8:

식 7

Equation 7

식 8

Equation 8

In addition, since the resonance frequency f _r measured in the piezoelectric transformer of the above-described embodiment (see Table 1) is about 1.2 MHz and the measured value of the input capacitor C ₀ is 0.59 nF, L _r is selected to be smaller than 30 μH. desirable. In addition, the capacitance of the series resonant inductor L _r is not sine wave due to harmonics, and the efficiency is low due to the increase of the input current, and in the large case, it is difficult to obtain sufficient output due to the decrease of the input current. The capacity of the series resonant inductor L _r selected in this example is 22 µH.

Also in this embodiment, referring to FIG. 2B, the protection circuit used in this drive circuit is automatically programmed at the overvoltage input and undervoltage input, programmed with a PWM converter or a resistor connected to the UVLO and OVP pins at the PWM LM5035 (32). It is desirable to configure a protection circuit for the input signal to have an off function.

In addition, in general, the piezoelectric transformer 40 exhibits a maximum boost ratio at no load, and thus, when the load is opened suddenly, the oscillation rises, causing over-vibration, resulting in the risk of damage. Accordingly, continuous sensing is required to prevent such breakage so that the entire converter 10 and the piezoelectric transformer 40 are not overwhelmed. In an embodiment of the present invention, as shown in FIG. 2B, the primary side current signal of the piezoelectric transformer 40 is sensed by the sensing resistor R _s to protect it from overcurrent, and a photo coupler (eg, NEC PS2811-1M) is used. It is preferable to configure the feedback circuit (60 of FIGS. 2A and 2B) to adjust the duty cycle of the pulse signal output from the PWM converter or the HO and LO of the PWM LM5035 32 to fix the output voltage.

In addition, since the piezoelectric transformer 40 has a characteristic that the step-down rate varies according to the load, it is preferable to perform impedance matching between the load impedance and the piezoelectric transformer in order to obtain a high step-down ratio. For example, six 3.4 [V], 50 [mA] high-brightness LEDs can be connected in parallel to load such impedance matching.

characteristic

3A and 3C are graphs showing general characteristics of the DC-DC converter 10 according to the present embodiment, and FIG. 3A is a gate signal of two MOSFETs of the half-bridge inverter 34 and an input voltage of the piezoelectric transformer 40. 3b shows a waveform when the high brightness LED unit 70 is turned on with a DC waveform passed through the rectifier 50, and FIG. 3c shows a piezoelectric transformer (when the LED unit 70 is turned on twice for 40 minutes). 40) is a temperature characteristic graph.

Referring to FIG. 3A, the driving frequency of the piezoelectric transformer 40 in this embodiment receives the input of DC38 [V] at about 872 [KHz] and shows the highest step-down ratio of DC 3.4 [V]. The resonant frequency of the piezoelectric transformer measured in this embodiment is around 1.2 MHz (see Table 1), but the maximum step-up ratio when applied to the actually designed DC-DC converter 10 is as described above when the driving frequency is 872 [KHz]. It is considered that the highest driving frequency is reduced by the inductor L _r of the series-connected resonator and its own inductor value.

In addition, referring to FIG. 3C, the surface temperature of the piezoelectric transformer 40 continuously increases until 15 minutes of driving of the LED unit 70, decreases after 15 minutes, and the surface temperature of the piezoelectric transformer 40 is approximately 33 after 25 minutes. Saturated and stabilized at [° C.], the temperature rise of the piezoelectric transformer 40 is about 17 [° C.].

In the above-described embodiments of the present invention, the powder characteristics such as the average particle size, distribution and specific surface area of the composition powder, the purity of the raw material, the amount of impurity added and the sintering conditions may vary slightly within the usual error range. Is quite natural to those of ordinary skill in the art.

In addition, the preferred embodiment of the present invention is disclosed for the purpose of illustration, anyone of ordinary skill in the art will be possible to various modifications, changes, additions, etc. within the spirit and scope of the present invention, such modifications, changes, Additions and the like should be considered to be within the scope of the claims.

1A is a schematic structural diagram of a stacked piezoelectric transformer manufactured by the present embodiment.

Figure 1b is a graph of the resonance and anti-resonance frequency characteristics of the multilayer piezoelectric transformer manufactured by the present embodiment.

Figure 2a is a block diagram of a DC-DC converter according to the present invention.

Figure 2b is a block diagram of a DC-DC converter according to an embodiment of the present invention.

2C is a schematic circuit diagram of FIG. 2B.

<Code Description of Main Parts of Drawing>

10: DC-DC converter 20: power supply

30: DC-AC converter 32: PWM converter

34: half bridge inverter 40: piezoelectric transformer

50: rectifier 60: feedback circuit

70: LED unit

Claims (3)

A DC-AC converter which receives an external DC voltage and converts it into an AC voltage, and a primary side is connected to the DC-AC converter to boost or step down the converted AC voltage to a voltage level for driving a light emitting diode. A DC-DC converter comprising a piezoelectric transformer and a rectifier connected to a secondary side of the piezoelectric transformer to rectify the boosted or stepped AC voltage and apply the same to the light emitting diode. The DC-AC converter is a PWM converter that oscillates the driving frequency of the piezoelectric transformer, and a half connected to the PWM converter to convert the external DC voltage into an AC voltage according to the driving frequency therefrom and apply the applied voltage to the piezoelectric transformer. And a bridge inverter, wherein the DC-DC converter is connected between the output terminal of the rectifier and the output terminal of the PWM converter to control the duty cycle of the pulse signal output from the PWM converter and to fix the output voltage of the rectifier to the light emitting diode. And a photocoupler for feeding back a voltage applied to the piezoelectric transformer, wherein the piezoelectric transformer is a laminate of piezoelectric ceramics represented by the following compositional formula: (Pb _0.76 Ca _0.23 Sr _0.1 ) Ti _0.96 (Mn _1/3 Sb _2/3 ) _0.04 O ₃ + 0.2wt% Na ₂ CO ₃ + 0.2wt% Li ₂ CO ₃ The method of claim 1, The DC-DC converter further comprises a resonant inductor connected in series between the output terminal of the half-bridge inverter and the primary side of the piezoelectric transformer to make the AC voltage applied to the piezoelectric transformer into a sine wave. Converter. The method according to claim 1 or 2, The DC-DC converter further comprises a sensing resistor connected between the output terminal of the PWM converter and the primary side of the piezoelectric transformer to prevent overcurrent applied to the piezoelectric transformer.

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