Impulse wear of MBM capacitors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Impulse wear of MBM capacitors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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The most common reason for the failure of the output shaper units (BVF) of high-voltage converters PVT-2 is the melting and burnout of vitrified resistors R2 (PEV-25-330 Ohm). The assumption that the reason for this is the breakdown of the thyristor VT1 (TCh-40) is not confirmed in most cases, and often the units are restored to working capacity with a simple replacement of burned-out resistors. The reason for these damages to the BVF is the loss of capacitance of the capacitors C2, C3, C4 included in the forming circuit L1C2L2C3R4C4.

This circuit serves to obtain a powerful pulse of a given duration on the load (communication loop), and at the same time locks the thyristor VT1 when it ends. When the pulses are shortened, due to the loss of capacitance by the capacitors, the thyristor "does not have time" to lock up. A simplified fragment of the BVF scheme is shown in Fig.1.

(click to enlarge)

An autopsy of faulty capacitors C2-C4 showed that their steel plates "dissolved" (until the metal was completely transparent). Capacitors, of course, are in difficult conditions, because the boost pulses generated 50 times per second are hundreds (!) of amperes.

Impulse wear of MBM capacitors. Cover wear illustration

The nature of wear resembles electrocorrosion in a humid environment, but despite the presence of a polarizing voltage of 400V, both plates have the same wear. The fact that this wear and tear is not associated with “manufacturing defects” of the capacitors is proven by the fact that out of several dozen units with an operating time of ~ 80000 hours, ALL capacitors had, albeit different, loss of capacity. The insulating films in damaged capacitors are clean and transparent, the impression is that metal ions (at high shock currents) migrated to the soldered ends of the capacitors.

For specialists servicing such equipment - check if your capacitors have "floated" at least once a year. After all, it is easier and cheaper to replace them in advance than to fix the accident in a hurry.

For radio amateurs who design high-voltage (10-20 kV) controlled rectifiers and converters, pay attention to the method used in this design (Zaporozhye Converter Plant) for multiplying control pulses for power thyristors. Here, instead of complex and expensive high-voltage isolation transformers, a loop of wire with high-voltage insulation is used, on which toroidal coupling transformers are placed. The control pulse consists of a base with a high-frequency (tens of kilohertz) filling (generated by a separate circuit), to improve transformation due to the low inductance of the primary winding (one turn), in combination with a forcing pulse (to synchronize the moment when all power thyristors begin to unlock). The pulses taken from the toroids are rectified and fed to the control electrodes of power thyristors. In this way, it is possible to build very high-voltage, ground-isolated power converters. Or transform, in your designs, long pulses through transformers with a small number of turns, without bevels of their tops

The calculation of forming circuits, similar to those used in the BVF, can be carried out in program for radio calculations (menu "Pulse circuits").

Author: E.Shustikov, UO5OHX ex RO5OWG; Publication: shustikov.by.ru

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