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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The duty cycle is indeed above 50%. I tried modifying the compensation network of the error amp, increasing the value of the filter cap on pin 3, adding a small cap between pins 3 and 4 like its done in some power supplies, increasing the value of the current sense resistor, but none of those helped. What helped was decreasing the number of turns on the inductor, however it started running hot so this is not the solution.
I'll look into the slope compensation, what exactly is its purpose?
EDIT: Seems like I'll indeed have to use slope compensation. There is a circuit in the UC384x datasheet, but no values are given, so I will need to experiment.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Jan, it sounds like a control loop stability issue.
It's probably time to read up on the subject of control loop compensation, gain and phase margins and the dreaded right-half-plane zero. The latter can rear it's ugly head in certain types of boost and flyback converter that operate in the continuous current mode.
In general I'd recommend discontinuous current for low-power flyback or boost converters where losses are easy enough to manage and magnetics are generally not too bulky. This is because the lack of RHP zero makes the control loop compensation a lot easier. Once you get above about 70W or so, you really want to move to a continuous current design to minimise losses in the semiconductors, and keep the magnetic bits compact. Then you have to bite the bullet and try to stabilise the control loop in the presence of the right-half-plane zero for all likely line and load conditions.
Some of the Unitrode power application notes, and control theory notes by Lloyd Dixon give good summaries of these issues.
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Boost converter 12/48V, UC3843, hissing problem
