GB2179793A

GB2179793A - Pulsing capacitors in manufacture

Info

Publication number: GB2179793A
Application number: GB08521435A
Authority: GB
Inventors: Robert Carnegie Chittick
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-08-28
Filing date: 1985-08-28
Publication date: 1987-03-11
Also published as: GB8521435D0; GB2179793B

Abstract

Dielectric faults in a solid electrolytic, e.g. tantalum, capacitor are healed by applying to the capacitor from a low impedance supply a series of pulses of increasing voltage; the maximum voltage can correspond to the working voltage of the capacitor. An apparatus for effecting treatment includes a pulse and a ramp generator. <IMAGE>

Description

SPECIFICATION Improvements in capacitors This invention relates to electrolytic capacitors, and in particular to an apparatus and process for improving the reliability of electrolytic capacitors.

Electrolytic capacitors, and solid tantalum capacitors in particular, can fail catastrophically when subjected to a fast switch-on surge from a low impedance supply. This phenomenon, known as surge failure, is thought to arise from the breakdown of defective regions in the dielectric. This can then result in thermal runaway and ignition.

To reduce or prevent in-service failure it is common practice to test capacitors during manufacture by a surge test technique. In this test a large electrolytic capacitor is discharged through the capacitors under test, the current being switched by a mercury wetted relay.

Defective capacitors are detected either by observing the current waveform on an oscilloscope or visually and audibly as catastrophic failure occurs. Apart from the obvious disadvantage of capacitors burning up in the test apparatus, an important failure of this technique is the lack of reproducibility of the current surge due to the variability of the relay contact resistance. It will also be appreciated that the test is destructive in the sense that capacitors with a dielectric defect are destroyed whereas it is known that, under certain conditions, capacitor dielectrics can be healed or reformed.

The object of the invention is to minimise or to overcome these disadvantages.

According to one aspect of the invention there is provided a method of treating a solid electrolytic capacitor during manufacture, the method including applying to the capacitor a series of voltage pulses of increasing amplitude, the maximum amplitude corresponding to the working voltage of the capacitor.

According to another aspect of the invention there is provided an apparatus for treatment of a solid electrolytic capacitor, the apparatus including means for applying to the capacitor a series of pulses of increasing voltage, the maximum voltage corresponding to the working voltage of the capacitor.

We have found that, by quenching the surge current before thermal runaway can occur, dielectric defects are either isolated or burned out. The yield of the manufacturing process is thus increased.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a schematic diagram of a capacitor treatment apparatus, and Figure 2 illustrates the output waveform generated by the apparatus of Fig. 1.

Referring to Fig. 1, the apparatus includes a pulse generator 11 the putput of which is coupled via a trigger circuit 12 to a pulse output stage 13. A capacitor C1 under test is connected across this output stage. The output stage 13 is also fed with a ramp voltage derived from a ramp generator 14 via an analogue programmable power supply 15. The output of the output stage 13 is thus a series of voltage pulses of steadily increasing amplitude. Fig. 2 shows a typical output pulse sequence. Typically the pulse sequence is terminated by a number of pulses of a voltage equal to or greater than the working voltage of the capacitor.

The output stage 13 may comprise a power FET switching circuit. In one application this circuit comprises four parallel FET's to discharge a pair of 10,000 ,uF electrolytic capacitors through the capacitor C1 under test. The capacitor under test thus 'sees' a low impedance when each voltage pulse is applied. The pair of capacitors is charged, prior to each pulse, to the ramp voltage by the power supply 15. After each surge pulse the capacitor und,ergoing treatment is discharged through a lo value resistor, e.g. 100Q.

We have successfully treated 47,us, 25 volt solid tantalum capacitors by this technique using a ramp duration of 0.7 secs followed by 3 pulses at 25 volts. The pulse duration was 0.5 m sec and the pulse repetition rate was 15 sec The effect of the treatment is to increase the yield of fully functional capacitors. It is thought the dielectric faults in initially defective capacitors are burned out or isolated by the applied surge current. As each pulse is of short duration there is insufficient time for thermal runaway of the capacitor and dielectric healing can thus take place.

Whilst the technique has been described with particular reference to solid tantalum capacitors, it will be appreciated that it can also be employed on other types of solid dielectric.

The following example illustrates the invention.

Two samples each comprising 100 capacitors were drawn at random from a stock of resin dipped 150*F 1 6V solid tantalum devices. The stock was a known faulty batch having a relatively high filure rate. One sample was treated using the techniques described herein using a pulse repetition rate of 15 Hz and a pulse duration of 0.6 msec together with a range voltage rising from zero to 16 volts in 1 second. A further 15 pulses were applied at the working voltage of 16 volts.

Both groups were then subjected to a standard 16 volt surge test. Under these conditions the treated group of capacitors showed no failure whereas the untreated group had a 12% failure rate.

This illustrates the efficiency of the technique in increasing the yield of functional de vices.

Claims

1. A method of treating a solid electrolytic capacitor during manufacture, the method including applying to the capacitor a series of voltage pulses of increasing amplitude, the maximum amplitude corresponding to the working voltage of the capacitor.

2. A method as claimed in claim 1, wherein the series of pulses is followed by further pulses of constant amplitude.

3. A method as claimed in claim 1 or 2, wherein said capacitor is a tantalum capacitor.

4. A method as claimed in claim 1, 2 or 3 wherein the maximum voltage is equal to the capacitor working voltage.

5. A method of treating solid electrolytic capacitors substantially as described herein with reference to the accompanying drawings.

6. A capacitor treated by a method as claimed in any one of claims 1 to 5.

7. An apparatus for treatment of a solid electrolytic capacitor, the apparatus including means for applying to the capacitor a series of pulses of increasing voltage, the maximum voltage corresponding to the working voltage of the capacitor.

8. An apparatus for treatment of a solid electrolytic capacitor substantially as described herein with reference to the accompanying drawing.