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Registered Member #3253
Joined: Wed Sept 29 2010, 12:21AM
Location: New Zealand
Posts: 13
I've recently done a little rebuild of my capacitor charging TL494 boost converter to try and fix the heating problems I used to have with some HV FET's (which had an Rds(on) of 0.27 Ohms, which I thought was the problem). I replaced them with IRFZ44N's which have an Rds of 17.5 mOhms hoping it would solve the problem. The heat generated with either type of FET was indistinguishable, in both cases they were/are heatsinked.
From the circuit diagram I build the circuit off, there were 2 main changes I made. Before my rebuild I was using 10k gate discharge resistors for the FET's, which is unacceptable at 150kHz so I replaced them with 680R 1/4 resistors (running a little under rated power) although the circuit diagram used 220R 1W resistors, I do not have any available but will probably buy some if required. In both cases, the heat is extremely excessive and there is no noticeable difference. I believe this may be the most significant part of the problem but am still doubtful after seeing the effects of changing from 10k to 680R. The heatsinks hit around 60C in 5 seconds when trying to charge a 450V 4700uF capacitor to 50J or so. I originally designed the circuit to run at 12V, but it will happily draw 6A while charging and cause massive heat dissapation from my FET's so I usually run it at 9V @ 4A to try and keep it cooler. The only way to do an extended charge is using air cooling, which I currently run a fan over the heatsinks which seems to help alot. Considering the input power is changing throughout the charging, I can charge the capacitor to 270J in 16s while using a fan to cool the heatsinks any they will reach ~40C
I understand that the efficiency is going to change right throughout the charge cycle, since it is trying to produce a constant output voltage where it is set, but this seems more then just an efficiency issue.
The circuit i'm using uses the TL494 configured to use 2 anti-phase outputs for a centre tapped primary on the transformer. The transformer is wound with 6 +6 primary and a 40 turn secondary (on a toriod core), although the circuit stated a 275T secondary. I stopped at 40T because it was already producing 500V with ease, anything more would be unesessary.
You need to have some kind of buffer between the tl494 and the fet's gates, or the fets will stay too long in the linear region (not fully on, or off).
It is also possible that the flux in the transformer will walk in one direction and saturate the core.
In push pull converters you need to have some way to balance the flux.
For small power I would use a flyback or a forward converter, the flux walking is not a issue with this topologies.
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TL494 Boost converter heating and efficiency issues
