JPH0193092A - Lighting device for electric discharge lamp - Google Patents

Abstract

PURPOSE:To make a normal oscillation in an invertor device by making no oscillation from, a zero cross point of an output voltage of a rectifier where the oscillation in a transistor invertor device becomes unstable, to a given delayed phase angle when lighting an electric discharge lamp using the transistor invertor device. CONSTITUTION:When the voltage from an AC power source 1 is made full-wave rectification in a rectifier 2, a semiconductor switch 23 becomes an OFF state at a zero cross point of output voltage, when an output voltage rises, the electric potential of a condenser 22 rises by a time constant determined with a resistance 21 and the condenser 22. Consequently, the switch 23 becomes an ON state and a transistor 5 is turned ON and a transistor 6 turned OFF, and a high frequency AC flows to an electric discharge lamp 14 connected to a secondary coil NS of an oscillation transformer 4 connected to the switch 23. Thus oscillation is made not to occur at a zero cross point of the device 2 with the switch 23 made to the ON or OFF state according to the variation of the electric potential of the condenser 22.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a discharge lamp lighting device that lights a discharge lamp using a transistor inverter device.

[Conventional technology]

FIG. 4 is a circuit diagram of a conventional discharge lamp lighting device.

In FIG. 4, 1 is an AC power supply, 2 is a rectifier for full-wave rectification of the AC power supply, and 3 is an inductor.

The transistor inverter device includes an oscillation transformer 4, oscillation transistors 5 and 6, a resonant capacitor 7, a resistor 8-9-1213, a rectifier 10, and a capacitor 11. In addition, the oscillation transformer 4 has a resonance transformer 4.
Next volume 'A N P + ・NF2 and second volume km
NS, a first feedback winding NP that supplies an oscillating voltage that maintains the ON state of the transistors 5 and 6 and alternately turns them both on, and a second feedback winding that supplies a DC bias to the transistors 5 and 6. It has a winding NP.

The starting circuit for transistors 5 and 6 consists of resistor 13 and resistor 8.
and a resistor 9, and a DC bias source includes a second feedback winding NP, a rectifier 10, and a capacitor 11. 14 is a discharge lamp as a load.

Next, the operation of the discharge lamp lighting device configured as described above will be explained. When DC voltage is supplied, first the resistor 1
3. Current flows to the bases of the transistors 5 and 6 via the resistors 8 and 9. At this time, due to the difference in the amplification factors of the transistors, for example, the transistor 5
Assuming that it is in the N state, a current flows through the primary winding ti N P + , and this generates a voltage in the first feedback winding vANP that increases the base current of the transistor 5.
Transistor 5 maintains the ON state. On the other hand, since no current flows through the base of transistor 6, it is in an OFF state. Further, the voltage generated in each winding of the oscillation transformer 4 oscillates due to the inductance of the oscillation transformer 4 and the resonance capacitor 7. Therefore, after a certain period of time has passed, the polarity of the first feedback winding NP3 is reversed, and the transistor 6
A current flows through the base of transistor 6, turning on transistor 6, and transistor 5 is reverse biased and turning off.

Thereafter, transistor 5 and transistor 6 are alternately turned on and oscillate. When oscillation starts, the voltage generated in the second feedback winding NP is simultaneously applied to the rectifier 10 and the capacitor 11.
The capacitor 1 is rectified by a rectifier circuit consisting of
1, a DC voltage is generated. This DC voltage is resistor 12,
A drive current is supplied to the base of the transistor 5 or transistor 6 which is in the ON state via the resistor 8 and the resistor 9. .

This drive current is for completely saturating one of the transistors which is in the ON state and causing that transistor to perform a switching operation.

In this way, the transistors 5 and 6 alternately repeat the ON state and OFF state, and a current made constant by the inductor 3 flows through the transistor that is in the ON state, which causes the secondary winding NS High frequency alternating current can be supplied to the discharge lamp 14 connected to the discharge lamp 14 .

[Problem that the invention seeks to solve]

By the way, when a non-smooth rectified power supply as shown in FIG. 4 is used, the output voltage waveform of the rectifier 2 becomes as shown in FIG. 5(a), and the transistors 5 and 6 have collector currents that follow the voltage. flows. Therefore, transistor 5.
It is desirable that the base current supplied to the capacitor 6 also follow the collector current. Therefore, it is desirable that the voltage waveform at the end of the capacitor 11 be similar to the output voltage waveform of the rectifier 2, with a slight voltage increase at the zero cross.

However, in the conventional discharge lamp lighting device, it is difficult to set the value of the capacitor 11 to obtain such a voltage waveform.In other words, in order to obtain such a voltage waveform in the conventional discharge lamp lighting device, it is difficult. , Normally, the capacitance of the capacitor 11 is about several to several tens of μF, and a small electrolytic capacitor is used for this capacitor due to the capacitance and the applied voltage.In this case, since the ripple current of the capacitor 11 is large, the capacitor 11 If the ripple current exceeds the allowable value, self-heating will occur, and the life of the capacitor 11 will be significantly shortened.If an attempt is made to suppress the ripple current below the allowable value, the capacitance of the electrolytic capacitor will increase, and the output voltage at the end of the capacitor 11 will increase. The waveform becomes as shown in Fig. 5(b).When the output voltage waveform at the end of the capacitor 11 becomes such a waveform, it becomes completely overdriven near the zero cross of the output voltage of the rectifier 2, and the transistors 5 and 6 become overdriven. As shown in FIG. 5(C), the collector voltage waveform jumps near the zero cross, causing abnormal oscillation at that portion.

Also, if you use a small capacity capacitor (for example, a film capacitor of 1 μF or less) with good high frequency characteristics, the voltage will drop near the atmosphere cross due to the lack of capacity, the base current will be insufficient, and the voltage will rise to some extent. There is a drawback that normal oscillation cannot occur until the output voltage at the end of the capacitor 11 reaches a certain value. The collector voltage waveform in this case is shown in Figure 5(d).
The voltage waveform at one end is shown in the same figure (e).

The present invention has been made based on the above circumstances, and even if the power supply supplied to the transistor inverter device is a non-smooth rectified power supply and the capacitor of the DC bias source of the oscillation transistor has a small capacity, It is an object of the present invention to provide a discharge lamp lighting device in which a transistor inverter device normally performs an oscillation operation.

[Means for solving problems]

To achieve the above object, the present invention includes an AC power supply, a rectifier for full-wave rectification of the AC power supply, a pair of oscillation transistors, an oscillation transformer, and a second feedback winding of the oscillation transformer. a transistor inverter device that operates with the output of the rectifier, including a DC bias source that saturates the oscillation transistor that is turned on by smoothing it with a capacitor after rectifying the voltage; In a discharge lamp lighting device that includes an inductor interposed between the transistor inverter device and the discharge lamp lighting device that lights the discharge lamp using the high frequency output of the transistor inverter device, a zero cross of the output voltage of the rectifier device is set for each cycle of the output voltage of the rectifier device. A starting circuit is provided which supplies a starting base current to the pair of oscillation transistors with a delay of a certain phase angle from the point to turn on the pair of oscillation transistors.

[Effect]

The present invention uses the above means to supply the starting base current to the oscillation transistor with a delay of a certain phase angle from the zero-crossing point of the output voltage of the rectifier for each cycle of the output voltage of the rectifier. Even when the output voltage of the device is applied to the transistor inverter device, the transistor inverter device does not oscillate near the zero cross point of the output voltage, but oscillates after a certain time delay from the zero cross point. This results in
Even if the power supplied to the transistor inverter device is a non-smooth rectified power source, abnormal oscillations caused by the capacitor of the DC bias source are prevented, and the transistor inverter device always performs only normal oscillation operations.

〔Example〕

A first embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is a circuit diagram of a discharge lamp lighting device according to a first embodiment of the present invention. In FIG. 1, components having the same functions as those of the conventional device shown in FIG. 4 are denoted by the same reference numerals, and detailed explanation thereof will be omitted.

The starting circuit of the transistors 5 and 6 consists of a series circuit of a resistor 21 and a capacitor 22 connected to the output terminal of the rectifier 2, and a resistor 8 and a resistor 9 whose both ends are connected to the bases of the transistors 5 and 6. Series circuit and one end with 2 resistors
Connect the other end to the connection point between 1 and capacitor 22 and resistor 9.
and a semiconductor switch 23 connected to the connection point.

Other features are similar to conventional discharge lamp lighting devices.

Next, the operation of the discharge lamp lighting device configured as described above will be explained. When AC power supply 1 is supplied, AC type a1
is full-wave rectified by the rectifier 2. At the zero-crossing point of the output voltage of the full-wave rectified rectifier 2, the semiconductor switch 23 is in an OFF state. When the output voltage of the rectifier 2 increases, the potential of the capacitor 22 increases due to a time constant determined by the resistor 21 and the capacitor 22. and,
When the potential of the capacitor 22 becomes equal to or higher than the breakover voltage of the semiconductor switch 23, the semiconductor switch 23 is turned on.
current flows through the resistors 8 and 9 to the bases of the transistors 5 and 6. Then, due to the difference in amplification factors of the transistors, for example, transistor 5
Assuming that becomes the ON state first, a current flows through the primary winding NP, which generates a voltage in the first feedback 5vANpz in the direction of amplifying the base TL flow of the transistor 5,
Transistor 5 maintains the ON state. On the other hand, since no current flows through the base of the transistor 6, the transistor 6 is in an OFF state. Next, the voltage generated in each winding of the oscillation transformer 4 oscillates due to the leakage inductance of the oscillation transformer 4 and the resonance capacitor 7. Therefore, after a certain period of time, the first feedback winding N P s
The polarity of the voltage generated at the transistor is reversed, and the transistor 5 is reverse biased and turns off. At the same time, current flows to the base of the transistor 6, turning the transistor 6 on.
It becomes a'6. Next, due to the vibration of the leakage inductance of the oscillation transformer 4 and the resonant capacitor 7 again, the polarity of the voltage generated in the first feedback winding NPx is reversed again, and the transistor 6 is reverse biased and turned off, and at the same time Transistor 5 is turned on. In this way, when vibration starts, the voltage generated in the second feedback winding NP is rectified by the rectifier circuit composed of the rectifier 10 and the capacitor 11, and a DC voltage is generated in the capacitor 11. This DC voltage is used to completely saturate the transistor 5 or 6 which is in the ON state through the resistor 12, the resistor 8, and the resistor 9, and cause the transistor to perform a switching operation. Thereafter, the transistors 5 and 6 are alternately turned on to supply high-frequency alternating current to the discharge lamp 14 connected to the secondary winding NS.

FIG. 2 is a diagram showing waveforms of various parts of the discharge lamp lighting device according to the first embodiment. FIG. 2(a) is a diagram showing the output voltage waveform of the rectifier 2, and FIG. 2(b) is a diagram showing the waveform of the charging voltage of the capacitor 22. The capacitor 22 is charged by the output voltage of the rectifier 2 and its potential increases,
When the voltage reaches the breakover voltage of the semiconductor switch 23, it instantly discharges and becomes zero. Further, the point in time when a voltage higher than the breakover voltage is applied to the semiconductor switch 23 and the semiconductor switch 23 turns ON is a constant value determined by the relationship between the breakover voltage of the semiconductor switch 23 and the time constant of the resistor 21 and the capacitor 22. lags behind the cross point of the output voltage of the rectifier 2 by a time of . The collector voltages of transistors 5 and 6 at this time are shown in Figure 4 (C).
The waveform is shown in . Furthermore, when the output voltage of the rectifier 2 decreases and passes through the zero cross to move on to the next cycle, the charging voltage of the capacitor 11 also increases as shown in FIG. 4(d).
- Since the temperature drops to near zero, no base current is supplied to the transistors 5 and 6, and oscillation temporarily stops. and,
In the next cycle, when the output voltage of the rectifier 2 rises and the charging voltage of the capacitor 22 becomes higher than the breakover voltage of the semiconductor switch 23, the semiconductor switch 23 turns OFF.
The N state is entered, base current is supplied to the transistors 5 and 6, the transistors 5 and 6 oscillate, and the above-described operation is repeated. Hereinafter, for each cycle of the output voltage of the rectifier 2,
The base current is supplied to the transistors 5 and 6 at a phase angle delayed from the zero cross point of the cough output voltage, and the transistor inverter device oscillates. Therefore, by selecting the values of the breakover voltage and the time constant, oscillation can be prevented near the zero cross of the output voltage of the rectifier power supply 2, where oscillation becomes unstable when a small capacitance capacitor is used as a DC bias source, for example. In order to prevent this, the phase angle at which the semiconductor switch is turned on, that is, the oscillation start position of the transistor inverter device, can be adjusted for each cycle of the output voltage.

As described above, according to the first embodiment, since the power source of the transistor inverter device is a non-smooth rectified power source, the oscillation of the transistor inverter device becomes unstable due to a certain delay from the zero cross point of the output voltage of the rectifier device. Since the semiconductor switch prevents oscillation until the phase angle is reached, only stable oscillation can be obtained. Furthermore, according to this embodiment, good oscillation can be obtained even if a small capacitor with excellent high frequency characteristics, such as a film capacitor, is used as the DC bias source capacitor. Furthermore, the starting phase angle of oscillation can be controlled by appropriately selecting the breakover voltage of the semiconductor switch and the time constant of the starting resistor and capacitor.

FIG. 3 is a circuit diagram of a discharge lamp lighting device according to a second embodiment of the present invention. Components in FIG. 3 having the same functions as those shown in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

The starting circuit for the transistors 5 and 6 in the second embodiment includes a resistor 2
1, a capacitor 22, a semiconductor switch 23, a transistor 31, a thyristor 32, a constant voltage diode 33, resistors 34 and 35, and resistors 8 and 9. Other aspects are the same as in the first embodiment.

Next, the operation of the second embodiment shown in FIG. 3 will be explained. When the output voltage of the rectifier 2 rises from zero cross,
The transistor 31 includes a resistor 34 and a constant voltage diode 33.
A base current is supplied through the transistor 31, and the transistor 31 becomes saturated, that is, conductive. For this reason, the resistance 35
Since the current flowing through the transistor 31 flows through the transistor 31, it is not supplied to the transistors 5 and 6 through the resistors 8 and 9, and the inverter device cannot be started.

Then, with a time constant determined by the values of the resistor 21 and the capacitor 22, the potential of the capacitor 22 increases, and the semiconductor switch 2
3, the semiconductor switch 23 is turned on, the thyristor 32 is triggered and turned on, and the current flowing through the resistor 34 flows through the thyristor 32, so it is not supplied to the base of the transistor 31, and the transistor 31 is in the OFF state. As a result, the current flowing through the resistor 35 is supplied to the bases of the transistors 5 and 6 through the resistors 8 and 9, and the inverter device is activated. That is, even if the output voltage is applied to the transistor inverter device when the output voltage of the rectifier 2 reaches the zero-crossing point, the transistor inverter device is more sensitive to the semiconductor switch 23 than the zero-crossing point of the output voltage of the rectifying device 2. The oscillation operation is performed with a certain time delay determined by the breakover voltage of , and the time constants of the resistor 21 and capacitor 22. Other functions and effects of the second embodiment are similar to those of the first embodiment.

〔Effect of the invention〕

As explained above, according to the present invention, the semiconductor switch does not oscillate until a certain delayed phase angle from the zero cross point of the output voltage of the rectifier at which the oscillation of the transistor inverter becomes unstable. The power supplied to the inverter device is a non-smooth rectified power supply,
And even if the DC bias source capacitor has a small capacity,
A discharge lamp lighting device in which a transistor inverter device normally performs oscillation operation can be provided.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a discharge lamp lighting device which is a first embodiment of the present invention, Fig. 2 is a diagram showing waveforms of each part of the discharge lamp lighting device which is a first embodiment of the present invention, and Fig. 3 is a diagram of the present invention. A circuit diagram of a discharge lamp lighting device according to a second embodiment of the invention, FIG. 4 is a circuit diagram of a conventional discharge lamp lighting device, and FIG. 5 is a diagram showing waveforms of various parts of the conventional discharge lamp lighting device. . DESCRIPTION OF SYMBOLS 1... AC power supply, 2... Rectifier, 3... Inductor, 4... Oscillation transformer, 5, 6... Transistor, 7... Resonant capacitor, 8, 9, 12, 13,
1. Resistor, 10... Rectifier, 11... Capacitor,
14... Discharge lamp, 21... Resistor, 22... Capacitor, 23... Semiconductor switch, 31... Transistor, 32... Cyrisk, 33... Constant voltage diode, 34, 35... ··resistance. Applicant: Iwasaki Electric Co., Ltd. (b) Procedural Amendment June 1988 / Date: Director General of the Patent Office Kunio Ogawa 1, Indication of the case 1988 Patent Application No. 97471 2, Name of the invention Discharge lamp Lighting device 3, relationship with the case of the person making the amendment Patent applicant address: 3-12-4 Shiba, Minato-ku, Tokyo (voluntary) 5. Subject of amendment (1) Column 6 of the name of the invention in the application, Contents (1) In the column of the title of the invention in the application, the text "discharge lamp lighting device" is corrected to "lighting device." (2) In the column of the title of the invention in the specification, "discharge lamp lighting device" should be corrected to "lighting device." (3) Correct the claims column as shown in the attached sheet. (4) The detailed description of the invention column in Book a is corrected as follows. ■ On page 2, line 8 of the specification letter, the phrase ``discharge lamp lighting device for lighting discharge lamps'' has been replaced with ``a so-called electronic device for lighting electric light loads such as incandescent light bulbs such as halogen bulbs, and sometimes high-pressure metal vapor discharge lamps.'' A lighting device called a transformer.'' (“Conventional discharge lamp lighting device” on page 2, line 11 of specification D)
The statement has been corrected to read, ``-A lighting device called an electronic transformer used for boat fishing.'' ■Ming! In lines 7 to 8 of page 3 of IIS, the phrase ``discharge lamp as a load'' is corrected to ``electric light load such as halogen light bulb.'' ■Page 3, line 9 of the specification, page 5, line 15, page 5, lines 17 to 18, page 7, line 9 of the specification. Page 8, line 3, page 9, line 6, page 9, line 17, page 9, line 19, page 12, line 3, page 14, lines 15 to 16, Page 17, line 2, page 17, lines 4 to 5, page 17, lines 5 to 6, page 17, line 7,
On page 17, line 8 and page 17, line 9, the words "discharge lamp lighting device" are corrected to "lighting device J." ■Page 3, line 10 of the specification states ``When DC voltage is supplied,
'' should be corrected to ``When DC voltage is supplied to the circuit through the rectifier 2.'' ■The words "discharge lamp" on page 5, line 3, page 8, line 2, page 12, line 1, and page 17, line 15 are replaced with "electric lamp load." correct. (2) In the 19th line of page 5 of the specification, the statement "about several to several tens of μF" is corrected to "about several μF to several tens of μF." ■“Figure 4 (C)” on page 12, lines 17 and 20 of the specification
J, rFigure 4 (d)'' to ``Figure 2 (c)'', r Figure 2 (
d)”. 2. Claims (1) An alternating current power source, a rectifier for full-wave rectification of the alternating current power source, a pair of oscillating transistors, an oscillating transformer, and a second feedback winding of the oscillating transformer that rectifies the oscillating voltage generated in the second feedback winding. a transistor inverter device that operates with the output of the rectifier, including a DC bias source that saturates the oscillation transistor that is smoothed by a rear capacitor and turned on; and between the rectifier and the transistor inverter device. and the inductor interposed in the inductor, and the high frequency output of the transistor inverter device
1, a starting base current is supplied to the pair of oscillation transistors with a delay of a certain phase angle from the zero cross point of the output voltage for each cycle of the output voltage of the rectifier. A Tachikawa device characterized in that a starting circuit is provided for turning on the pair of oscillation transistors. (2) The capacitor of the DC bias source is a film capacitor.2゜Procedural Amendment Written by the Commissioner of the Japan Patent Office, October ζ, 1986, 1) Article 1) Tsuyoshi Tono 1, Indication of the case Patent Application No. 97471 filed in 1988 2, Name of the invention Discharge lamp lighting device 3, Relationship with the person making the amendment Patent applicant address 3-12-4 Shiba, Minato-ku, Tokyo August 4, 1988 5, Subject of amendment (1) Procedural amendment 6 submitted on June 10, 1988, Contents of amendment (1) June 1988 with the column of subject of amendment accurately stated Submit a letter of return for the procedural amendment submitted on the 10th. (2) Submit a document that accurately describes the contents of the amendment. (3) The application attached to the procedural amendment submitted on June 10, 1986 will be deleted. Written amendment to the above procedure June 10, 1988 Kunio Ogawa, Commissioner of the Patent Office 1. Indication of the case, Patent Application No. 97471 filed in 1988. 2. Title of the invention: Discharge lamp lighting device 3. Person making the amendment. Related Patent Applicant Address: 3-12-4 Shiba, Minato-ku, Tokyo (voluntary) 5. Subject of amendment (1) Title of the invention in the specification (2) Claims in the specification (3) ) Column 6 of the detailed explanation of the invention in the specification, contents of the amendment (1) In the column of the title of the invention in the specification, the phrase "discharge lamp lighting device" is corrected to "lighting device." (2. The Scope of Claims column is corrected as shown in the attached sheet. (3) The Detailed Description of the Invention column in Book I is corrected as follows. The phrase ``discharge lamp lighting device that lights discharge lamps'' was corrected to ``a lighting device called an electronic transformer that lights electric light loads such as incandescent light bulbs such as halogen bulbs, and sometimes high-pressure metal vapor discharge lamps.'' ■In the 11th line of page 2 of the specification letter, the words ``conventional discharge lamp lighting device'' are corrected to read ``-a lighting device called an electronic transformer, which is used for boat fishing.'' ■Specification In boxes 3, lines 7 to 8, the words ``discharge lamps as loads'' are corrected to ``electric light loads such as halogen light bulbs.'' ■Specification letter, page 3, line 9, page 5, line 15. Lines, page 5, lines 16 to 17, page 7, line 9. Page 8, line 3, page 9, line 6, page 9, line 17, page 9, line 19. line, page 12, line 3, page 14, line 15-16, page 17, line 2, page 17, line 4-5, page 17, line 5- Line 6, page 17, line 7,
On page 17, line 8 and page 17, line 9, the words ``discharge lamp lighting device'' are corrected to ``lighting device.'' ■Page 3, line 10 of the specification states ``When DC voltage is supplied,
'' should be corrected to ``When DC voltage is supplied to the circuit through the rectifier 2.'' ■The words "discharge lamp" on page 5, line 3, page 8, line 2, page 12, line 1, and page 17, line 15 of the specification letter are corrected to "electric lamp load." . ■In the 19th line of page 5 of the specification box, correct the statement ``several to several tens of μF'' to read ``several μF to several tens of μF''. ■“Figure 4 (C)” on page 12, lines 17 and 20 of the specification
J, rFig. 4 (d)'' to ``Fig. 2 (C) J, r Fig. 2 (
d)”. Attachment 2, Claims (1) An AC power source, a rectifier that performs full-wave rectification of the AC power source, a pair of oscillation transistors, an oscillation transformer, and a second feedback winding of the oscillation transformer that rectifies the oscillating voltage generated in the second feedback winding. a transistor inverter device that operates with the output of the rectifier, including a DC bias source that saturates the oscillation transistor that is turned on by smoothing it with a capacitor; and an inductor interposed between the transistor inverter device, and in which the voltage is constant from the zero cross point of the output voltage of the rectifying device every half cycle of the output voltage of the rectifying device. A starting base current is supplied to the pair of oscillation transistors with a phase angle delay of
(2) The lighting according to claim 1, characterized in that the capacitor of the DC bias source is a film capacitor. Device.

Claims (2)

[Claims] (1) An AC power source, a rectifier that full-wave rectifies the AC power source, a pair of oscillating transistors, an oscillating transformer, and a second feedback winding of the oscillating transformer, which rectifies the oscillating voltage and then smoothing it with a capacitor. a transistor inverter device that operates with the output of the rectifier and includes a DC bias source that saturates the oscillation transistor that is in an ON state; and an inductor that is interposed between the rectifier and the transistor inverter device. In a discharge lamp lighting device for lighting a discharge lamp using the high frequency output of the transistor inverter device, the above-mentioned discharge lamp is delayed by a certain phase angle from the zero cross point of the output voltage for each cycle of the output voltage of the rectifier. A discharge lamp lighting device comprising a starting circuit that supplies a starting base current to a pair of oscillating transistors to turn on the pair of oscillating transistors. (2) The discharge lamp lighting device according to claim 1, wherein the capacitor of the DC bias source is a film capacitor.