- UCC28061 bodge - 1 Update
- Distiibution amplifier vs. ???? - 1 Update
| legg <legg@nospam.magma.ca>: Feb 18 04:47PM -0500 An HPS lamp ballast on the bench came in with a shorted fet. This is in a UCC28061 two phase PFC that is designed for critical conduction. The design seems to follow the typical guidelines for the application, except that the drains of the two fets are shorted - completely bolloxing the concept of dual phase interleaving. Two fets, two boost inductors, two boost rectifiers, two independent zero-current-detection windings and drive circuits - but only one fet is driven at a time, with the other slogging the dv/dt and loading the node, with both chokes 'sharing' the current. The boost diodes can only share as well as such diodes might, when connected in parallel. Not a wide-range input circuit (240VAC only), but still using the book 300uH compromise inductor value. Concievably it still benefits from zero current switching, and at this high voltage, it's unlikely that much benefit would result from interleaving, but fet conduction losses have got to be 4x that of a single phase controller, with the same fets paralleled. Are there any other things to look out for in this kind of misapplication? It ran for about an hour with both fets replaced before the same fet position failed (phase B). No evidence of gross overheating. I see nothing wrong in the drive circuitry. The load is a conventional lamp inverter. I assume that a sudden inverter limit would produce standard overvoltage response in the 400V PFC circuit - fets are 600V. RL |
| "Ron D." <ron.dozier@gmail.com>: Feb 18 11:46AM -0800 You need to post what you have. If it's from yesteryear, I suppose it;s a channel 3/4 combiner possibly with an RF amplifier. It used to be really easy to take the output of a VCR run it to one of these and combine ATSC RF channel 4 with the TV feed not containing channel 4 and just select it on your TV. It was like $20.00 USD or less. To handle other devices, you used an RF modulator to take composite video and convert to NTSC channel 3 or 4. Now it's way harder and way more expensive to convert to ATSC. levels will be lower, but there would be better immunity to dropouts. ATSC 3.0 which is coming soon will make everything that you do obsolete anyway. |
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