Breakdown of Thin-Film Dielectrics Under the Action of Video Pulses
Abstract Generated abstract
This paper reports experimental measurements of breakdown in thin-film polymer dielectrics subjected to repetitive rectangular video pulses. Using a capacitive-discharge pulse setup producing negative pulses up to 15 kV with a 0.3 microsecond front and 2 microsecond duration, the authors tested Styroflex, polyethylene, and Fluoroplast-4 films in air with standard flat electrodes, comparing results with constant voltage and 50 Hz alternating voltage measurements. At 1000 pulses per second, the electric strength of the films was found to be lower than under constant voltage but higher than under 50 Hz voltage, without the severe reduction associated with high-frequency breakdown. The observations suggest that corona near electrode edges contributes to reduced breakdown voltage, with increased pulse repetition frequency further lowering the breakdown voltage in polyethylene.
Full Text
BREAKDOWN OF FILM DIELECTRICS UNDER THE ACTION OF VIDEO PULSES
N. S. Geints, V. G. Golubchenko, A. M. Trubitsyn
(Presented by the Scientific Seminar on Dielectrics)
In modern radio equipment, polystyrene, polyethylene, and other organic high polymers are widely used. They possess high and stable dielectric properties. However, in high-frequency fields such dielectrics break down at relatively low field strengths [1, 2, 3]. It is also of practical importance to know the behavior of dielectrics under the action of video pulses of various shapes and repetition frequencies. Our investigations are being carried out in this direction.
Fig. 1.
Figure 1 shows the schematic circuit of a test setup operating on the principle of a modulator with a capacitive storage element. The GMI-89 tube is normally closed by a negative bias of \(-600\ \mathrm{V}\) on the grid, and capacitor \(C\) is charged from a high-voltage single-half-period rectifier. Trigger pulses from a 26I generator, through a power amplifier, are applied to the grid of the GMI-89, causing capacitor \(C\) to discharge through resistance \(R_2\) and the test specimen.
The setup makes it possible to obtain pulses of negative polarity with an amplitude from 0 to \(15\ \mathrm{kV}\). The pulses have a shape close to rectangular (front duration \(0.3\ \mu\mathrm{s}\), pulse duration \(2\ \mu\mathrm{s}\)). The pulse repetition frequency can be varied from 200 to 3000 pulses per second. The pulse shape was monitored
with the aid of an IO-4 oscilloscope with a DNE-9 voltage divider. The voltage on the specimen was raised at a rate of 0.5 kV/sec and measured with a VLI-2 pulse voltmeter with a DNE-9 divider.
All experiments were carried out in air on film dielectrics 15–65 microns thick (error of thickness measurement—1 micron), using flat standard electrodes 25 mm in diameter. Under identical conditions, no fewer than 10 specimens of each material were tested. The breakdown field strength was determined as the ratio of the breakdown voltage to the thickness of the specimen.
Table 1
Mean measured values of breakdown voltage and electric strength
| Test material | Thickness, μm | Constant voltage, kV | Constant voltage, kV/mm | Voltage 50 Hz, kV max | Voltage 50 Hz, kV max/mm | 1000 pulses/sec, kV max | 1000 pulses/sec, kV max/mm |
|---|---|---|---|---|---|---|---|
| Styroflex | 14–20 | 8.8 | 540 | 7.4 | 420 | 7.1 | 430 |
| Polyethylene | 30–36 | 11.3 | 310 | 6.4 | 170 | 8.6 | 300 |
| Fluoroplast-4 | 38–46 | 9.1 | 210 | 4.5 | 100 | 7.1 | 175 |
The mean values of the measured breakdown voltage and electric strength of Styroflex, polyethylene, and Fluoroplast-4 at a repetition frequency of 1000 pulses per second are presented in Table 1.
The table also gives the results of measurements under constant voltage and at a voltage of 50 Hz. The experimental procedure under constant and alternating voltages was the same as in tests with repetitive pulses. The only distinguishing feature was the increased rate of voltage rise on the specimen (1 kV/sec). The voltage was measured with an electrostatic kilovoltmeter of type S-96.
It follows from the table that, under the short-term action of video pulses repeated 1000 times per second, the electric strength of the films studied is lower than their electric strength under constant voltage, but exceeds the value of the electric strength at a voltage of 50 Hz. Such a large decrease in electric strength as occurs in breakdown at high frequency was not observed in our experiments with repetitive video pulses having a steep front.
In all experiments, breakdown of the specimen was preceded by intense corona at the edges of the electrodes. Under constant voltage, most specimens broke down under the electrode, whereas under alternating and pulse voltage they broke down in the region affected by the corona. Since the corona intensity is lowest under constant voltage and increases with increasing frequency of voltage variation, it should be assumed that even a short-term action of the corona lowers the breakdown voltage of the high-polymer film dielectrics investigated. This conclusion is confirmed by our measurements, which show that increasing the pulse repetition frequency from 1000 to 2000 per second leads to a decrease in the breakdown voltage of polyethylene.
REFERENCES
- Bragin S. M., Electrical strength of polyethylene at high frequencies, Izvestiya AN SSSR, vol. 22, No. 4, 1958.
- Whittchead S., Dielectric Breakdown of Solids, Oxford, 1951.
- Mason J. H., Deterioration and Breakdown of Dielectrics Resulting from Internal Discharges, Proc. I.R.E., vol. 98, p. 1, No. 109, 1951.