LNK306 for high output voltage (310V/75W) buck converter

Posted by: graham on 01/11/2009

I am trying to generate a 310V output from the DC bus (around 380V) of a power factor corrected supply. I can't simply lower the DC bus voltage as the bus voltage needs to be above the maximum AC input voltage in order for the boost PFC circuit to work (unless I go for some complicated buck/boost arrangement) and there are some other parts of the system which need 380V.

So I am thinking to use a LNK306 to generate the 310VDC from the 380VDC bus. I need around 75W at 310V (around 240mA), so this is within the indicated limits (360mA) for the LNK306. However, I suppose those limits assume a low output voltage (say below 30V), where the switch will only be conducting for <10% of the time? Can I use the the LNK306 at high duty cycles (say 80%)? The FET conducted losses with 240mA and 80% duty cycle will be around 500mW for the LNK306 - what other losses to I need to consider when calculating the total power losses for the LNK306?

If the LNK306 can't handle this much power, can anyone suggest another suitable device, or perhaps a method of driving an external FET to increase the power handling capacity?

Thanks

hello, its an interetsing post, but did you calculate your switching losses too? ....and when the freewheel diode conducts...the on chip FET has its source to ground, and its drain to 380V.....possibly more than 380v cause pfc's are prone to voltage over/undersoot...can the on chip fet handle this voltage?... and the diodes have to be fast trr dont they?......can high voltage diodes like that have the low enough trr that you need?

The maximum duty cycle for LNK306 is only 69%. So you should consider other solutions. Regards, PI-Scotty

Okay, thanks - I missed this point in the datasheet. Would any other PI products be suitable for this application?

treez - thanks for your comments. yes, switching losses do need to be considered too, as well as any other internal losses such as for generating the internal supply to the IC. The switching stresses are not that different to a typical continuous conduction application with low output voltage - the link switch FET would in any case see the full input voltage (perhaps 330V DC without PFC) when the freewheel diode conducts and the freewheel diode needs to be rated at more than the maximum input voltage (so typically a 600V diode would be needed anyway). The LNK306 FET is rated at 700V, so this should be okay. However, as Scotty has pointed out, the LNK306 is limited to a maximum duty cycle of 69% (typ), so this would limit the output voltage I could achieve with an input voltage of 380V to around 262V, with is too low for me. So, back to the drawing board...

LNK306 for high output voltage (310V/75W) buck converter

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