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Old 25th December 2005, 03:52 PM   #1
Danko is offline Danko  Hungary
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Default UCC2808 slope compensation

Hi everyone!


Do I need slope compensation, if I build a circuit, which's specifications are on the attached picture? The PWM-controller IC is UCC2808, which is a current mode IC, so this IC requires slope compensation, if the duty cycle is more than 50%.

I'm confused about this ...
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Old 26th December 2005, 12:03 PM   #2
Eva is offline Eva  Spain
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Duty cycle is nearly 100% with these specifications so you are going to need slope compensation. Note that 290V DC is too high a low-line limit for 230V AC mains applications, something between 200V and 240V DC would fit better to actual working conditions (taking into account input capacitor ripple voltage).
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Old 26th December 2005, 12:46 PM   #3
Danko is offline Danko  Hungary
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Thanks, Eva! It looks like, I have to learn this slope-compensation trick :-)


For how much voltage shall I design the transformer's primary winding? 200-240, or 290-320?

A littlet bit offtopic: Under what circumstances do I have to solder paralell the MOSFET's source and drain a fast recovery high voltage diode? There's already one diode in the MOSFET. Isn't that enough?
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Old 26th December 2005, 05:09 PM   #4
Eva is offline Eva  Spain
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I would take 220V to 350V DC as an input range. Obviously, transformer turns ratio should be chosen for the lowest input voltage, since the regulation loop will reduce the duty cycle for high input voltages.

As you have mentioned, MOSFETs include an internal body diode, but its recovery characteristic may be slow or very slow in some devices, even 600ns Trr. Depending on the topology and the switching frequency, this slowness may be irrelevant or may have to be solved by adding external diodes.

In standard half bridge, push pull and full bridge topologies operating at moderate frequencies there is no need to worry about the recovery time of body diodes.
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Old 26th December 2005, 06:13 PM   #5
Danko is offline Danko  Hungary
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MOSFET's internal diode...
I plan to use about 100kHz switching frequency. I haven'y really decided, to use IRF740 (a cheap device), or APT5015BVR (I obtained a few of theese, for almost free) ).

About the slope compensation ....
In UCC2808's datasheet, there's a schematic, which includes a slope compensation. But I don't really see, if that's works at all. But, it's in an official datasheet, so that have to work :-)

May the attached, schematic work?
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Old 27th December 2005, 06:54 AM   #6
mflorin is offline mflorin  Romania
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search for :

slua101.pdf
slup080.pdf
slup122.pdf
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Old 27th December 2005, 07:35 AM   #7
Danko is offline Danko  Hungary
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Thanks mflorin!

I'm going to read theese seminars.
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