UCC29002 Load sharing controling mosfets instead of sense lines

Hi Wesley,

Can you explain in more detail how the mosfets control the output current?

Thanks

Peter

The mosfets are used in linear mode and are used as a resister to allow the output voltage equal on both rails. A current sense resister can be used to detect the true current going through the mosfet as it dynamically changes to allow for 50% loading on all associated rails. The module will need to control how on/off the mosfet is dynamically to allow for current sharing. The problem encountered using UCC29002 is it controls the sense lines to allow for the current to balance. These COTS (Commercial Off The Shelf) power supplies don't have voltage sense lines to the outside world.

Example: if power supply 1 is at 12.5 VDC and power supply 2 is at 11.9 VDC the current sharing scheme will reduce the 12.5 VDC power supply to 11.9 VDC. The mosfet will then dissipate (0.6 VDC x (system total current / 2)) the difference allowing for 50% loading.

Hello Wesley

These file shows how the UCC29002 can be used to control a 'trim' input on a module - that is similar to what you have in mind and you may be able to use it as a starting point. BTW - I'm not at all sure that these files have loaded correctly - if they have that's great if not then send an email to me at colingillmor@ti.com and I'll email the files to you.

Regards

Colin

No remote Sense.ppt

No Sense Ref Design PMP10099 0820.sch.pdf

Colin,

Thanks for providing the documents listed in your reply on Nov 25, 2015 the links work great. On the no_remote _sense.ppt is the Trim Mod. junction being attached to a mosfet 's gate while the drain is attached to +Vout Mod. line and the source is attached to R12/R10? What transistors are recommended from TI for Q7, Q8, Q9, and Q10?

Thanks,

Hello Wesley

The Trim mod line on the 'No Remote sense' schematic was intended to feed into a trim pin at the output of the module being controlled - ie a pin on the module giving an output adjustment function. In your case, I guess the Trim Mod could connect to a MOSFET gate (somehow). I'm afraid we don't have any direct experience of doing this - I could suggest that you start with the basic MOSFET circuit you have in mind then run some sims to understand how you can drive them - BTW, TI has a free spice simulator - TI-TINA that you could use. Unfortunately we don't have a spice model of the UCC29002 - the only suggestion I could make is that you could 'construct your own' model using the block diagram in the data sheet as a guide.

As for Q7, Q8, Q9, Q10 - these are standard NPN / PNP bipolar signal transistors - something like 2N2904 / 2N2906. they are being used as current mirrors and an alternative would be to buy off-the shelf current mirrors. they are needed to allow the ADJ pin output to pull down the Trim Mod pin - I think this application was based on a 12V output. It depends on your own exact circumstances, the current mirrors aren't inherent in the design -  the use of the ADJ pin to control the Trim Mod pin is the central idea here.

Be careful in your selection of the series MOSFETs. Many of the newer switching MOSFETs come with a 'not suitable for use in linear applications' warnings. This is usually because the transconductance is a positive function of temperature and you can get current focussing if one part of the die gets hotter than the rest. Really old technology MOSFETs were better in this regard.

Have you considered degrading the modules load regulation so that they share naturally ? A resistor in series with the output is a basic way to do this.

Regards

Colin