Suppression of Leakage Current and Distortion in Variable Capacitance Devices and their Application to AC Power Regulators

Abstract

The quantity of alternating current (AC) leakage and the value of distortion factor in capacitor currents are discussed with regard to a new power component called variable capacitance device (VCD). This component has terminals for controlling its capacitance. Nonlinear dielectric characteristics are utilized in this device to vary the capacitance. When VCD operates in an AC circuit, the AC leakage from this device through direct current (DC) control voltage source increases according to the conditions of DC control voltage and so on. To solve this problem, we propose techniques for suppressing AC leakage. Although VCD has strong nonlinear characteristics, the current through the capacitor is not distorted significantly. The relations between AC leakage and the distortion in current waveforms are investigated. An application example for an AC power regulator is also introduced to evaluate the distortion in waveforms.

I. INTRODUCTION

If power devices of variable capacitance can be used, then many applications will be possible in the field of power electronics. In alternating current (AC) voltage regulators, such as ferroresonant circuit, inductances are usually varied to control output voltage. A simulated saturating reactor [1] is assembled by a small inductor and semiconductor switches. A voltage regulator with this circuit has a reduced size and weight in comparison with a ferroresonant transformer [2]. However, considerable magnetic fields are generated, thereby causing electronic equipment to malfunction. This condition is not desirable for a clean and safe environment.

Varactors are passive components that can vary their capacitance. However, they have a small operating current (e.g., less than approximately 10 μA) because they are basically reverse-biased diodes. Therefore, their application is limited to small signal use.

Variable capacitance device (VCD) [3], [4] is proposed on the supposition that the value of an operating current is several amperes or more. This device is realized by using the capacitance varying characteristics of direct current (DC) bias voltage across a ceramic capacitor [5]–[7].

A ceramic capacitor has nonlinearity in the characteristics between dielectric constant and electric field, that is, those between capacitance and DC bias voltage. Ceramic capacitors are usually used in the circuit with a fixed DC bias voltage to avoid this influence. However, the influence of nonlinearity on the use of such capacitors in AC circuits cannot be neglected.

In this study, the major problems on the use of ceramic capacitors in AC circuits, that is, AC leakage from VCD to DC controlling voltage source, and waveform distortion are investigated in detail. Leakage current suppression techniques can be applied to AC power regulators. With respect to an AC power regulator with constant voltage/constant current (CV/CC), the relationship between AC leakage suppression technique and waveform distortion is reported, including the observed waveforms and measured data on efficiency.

 

II. VCD

We introduce the use of VCD in an AC circuit. The circuit symbol of VCD is depicted in Fig. 1. Fig. 2 presents an example of the measured data between capacitance C1 and DC control voltage VC1, in which the AC current for measurement is sufficiently small. Fig. 2 shows that the capacitances can be adjusted by changing DC control voltage.

Fig. 1.VCD.

Fig. 2.Example of measured control characteristics in VCDs.

The structure of a four-terminal VCD is depicted in Fig. 3. Four capacitors CA, CB, CC, and CD have the same characteristics. With the use of two resistors RB, the same DC voltages are imposed on the four capacitors.

Fig. 3.Structure of a VCD.

 

III. LEAKAGE CURRENT SUPPRESSION

In Fig. 3, the voltages vA, vB, vC, and vD across the capacitors CA, CB, CC, and CD can be expressed by VC1 and the AC voltage vac as follows:

The DC control voltage source VC1 has low impedance against AC. In this study, its impedance is assumed to be zero.

The bridge connection of the four well-balanced capacitors reduces the AC leakage ic considerably through DC control loop when VC1 is higher than the AC voltage vac. However, an impedance device Zs needs to be inserted to suppress AC leakage, as shown in Fig. 4, when the AC voltage vac becomes high.

Fig. 4.Leakage current suppression in the VCD.

Figs. 5 and 6 show data on the AC leakage ic from the VCD C1 to the DC control voltage source VC1. The current iac is the total AC current. These graphs depict the ratio ic /iac. In this study, vac is sinusoidal. Its frequency fi and amplitude equal 60 Hz and 10 V, respectively.

Fig. 5.Measured data on the AC leakage ratio ic/iac vs. the DC control voltage VC1 characteristics having the inductor LS as series impedance device ZS.

Fig. 6.Measured data on the AC leakage ratio ic/iac vs. the DC control voltage VC1 characteristics having the resistor RS as series impedance device ZS.

As reference, the graph LS = 0 in Fig. 5 or the graph RS = 0 in Fig. 6 presents the data without ZS. When VC1 is approximately 30 V, the ratio ic/iac has a maximum value. This condition corresponds to the maximum declivity in Fig. 2.

In Fig. 5, the inductor LS is used as a series impedance device. Under a low VC1, an inductor with relatively small inductance will be applicable.

The series component in Fig. 6 is the resistor RS. Even if RS is low, usage with a relatively high VC1 leads to good reduction characteristics on ic/iac. A 51 Ω resistor is used below for the impedance device ZS in consideration of cost, size, and weight.

 

IV. SUPPRESSION OF DISTORTION

For example, the capacitance CA is a function of the voltage vA. Given that vA varies with the time t, CA becomes a time-variant parameter. In this case, with respect to the capacitor CA, the following relations are valid:

The second term in the right part of Equ. (4) provides the information on distortion.

Fig. 7 shows the measured characteristics on total harmonic distortion (THD) with respect to the capacitor currents iac and iA. From Fig. 4, iA means the current that flows through the capacitor CA. The amplitude of vac is set to 10 V. The THD of vac is approximately 0.17%, which is smaller than that of iac or iA. In both cases of RS = 0 and RS = 51 Ω, the distortion in iac becomes smaller than that in iA. The distortion in iac can be negligibly small particularly under the condition of RS = 0. In this case, CA and CB and CC and CD are paralleled. The influence of nonlinearity can be compensated effectively. Even if RS = 51 Ω, the total current iac becomes near sinusoidal.

Fig. 7.Measured characteristics on the THD of the currents iA and iac with the resistance RS as parameter.

 

V. EVALUATION OF THE DISTORTION IN AN AC POWER REGULATOR

Fig. 8 presents a CV/CC AC power regulator with input power factor correction (PFC) [8] as example of the application of VCDs. In this study, the capacitances C1 and C3 are variable. Load is assumed to be resistive.

Fig. 8.PFC CV/CC AC power regulator by the use of VCDs.

The simplified block diagram for control circuit is also included in Fig. 8. Two boost-type DC–DC converters are utilized in this study for the high-speed drivers of two VCDs. In the experimental circuit, the constant output voltage Vo = 28 V or the constant output current Io = 0.81 A can be obtained against the input voltage Vi = 18–24 V. For the VCDs C1 and C3, the measured data on capacitance against DC control voltage are shown in Fig. 2.

The conditions are as follows: inductance of L1, L1 = 58 mH, loss resistance of L1, r1 = 0.6 Ω, inductance of L2, L2 = 35 mH, loss resistance of L2, r2 = 0.2 Ω, and capacitance of C2, C2 = 290 μF.

Figs. 9–11 show the observed waveforms, where Vi, Vo, Ii, and Io are the input voltage, the output voltage, the input current, and the output current, respectively. With respect to Vi, amplified sinusoidal voltage is used in Fig. 9. By contrast, Vi is furnished by a commercial AC in our laboratory for Figs. 10 and 11. In Figs. 9 and 10, RS is equal to 51 Ω.

Fig. 9 presents the distortions generated when the input is a sinusoidal wave. A comparison between Figs. 9 and 10 implies that the THD of Vo or Io does not gain much even if Vi is distorted. The high harmonic components in input voltage are effectively reduced.

Fig. 9.Observed waveforms with the VCDs C1, C3, the sinusoidal input voltage Vi, and RS = 51 Ω, where Vi = 21 V; vertical: 20 V/div for Vi, V1, and Vo, 2 A/div for Ii, I1–I5, and Io; horizontal: 5 ms/div: (a) Io = 0 and Vo = 28 V (no load), (b) Io = 0.81 A and Vo = 28 V (full load), and (c) Io = 0.81 A and Vo = 0 (short-circuit load).

Fig. 10.Observed waveforms with the VCDs C1, C3, the commercial AC input voltage Vi, and RS = 51 Ω. The others are the same as in Fig. 9.

As reference, Fig. 11 shows the observed waveforms using film capacitors of the same capacitance with VCDs. In this case, the included distortion is not derived from the VCDs. We suppose that the distortion is caused by magnetic devices and the high harmonic components of the commercial AC.

Fig. 11.Observed waveforms with the film capacitors C1, C3 and the commercial AC input voltage Vi as reference. The others are the same as in Fig. 9.

The measured values on THD are as follows:

(1) VCDs, sinusoidal AC input, and RS = 51 Ω (see Fig. 9).

(2) VCDs, sinusoidal AC input, and RS = 0.

(3) Film capacitors and sinusoidal AC input.

(4) VCDs, commercial AC input, and RS = 51 Ω (see Fig. 10).

(5) VCDs, commercial AC input, and RS = 0.

(6) Film capacitors and commercial AC input (see Fig. 11).

The data indicate that the distortion caused by VCDs is insignificant at RS = 51 Ω and RS = 0.

Finally, the measured data on efficiency with VCDs and film capacitors are given in Figs. 12 and 13, respectively. In Fig. 13, VCDs are replaced with film capacitors of the same capacitance. The power loss in VCD is small.

Fig. 12.Measured data of efficiency η with the VCDs C1 and C3: (a) CV region and (b) CC region.

Fig. 13.Measured data of efficiency η with the film capacitors C1 and C3 as reference: (a) CV region and (b) CC region.

 

VI. CONCLUSION

Inserting a suitable small-resistance resistor between the VCD and the DC control voltage source is effective for leakage current suppression. This resistor has almost no influence on capacitor current distortion.

Although the proposed VCD is made by using strong nonlinear characteristics, the waveform of the total current is not distorted considerably. The low distortion is due to the effective canceling of the high harmonic components by the internal currents in VCD.

In the proposed AC regulator for application in VCD, the distortion in input voltage is reduced by the effect of circuit configuration. Even if the AC input is a distorted voltage, such as the commercial AC voltage, the output waveform becomes close to a sinusoidal wave.