Will parallel diodes be well thermally coupled?

cupoftea

Hi,
We have two MBR6045 diodes in parallel at the output of a 24V, 50A PSU.
They are 7cm apart, and both screwed in to the same 11cm x 11cm aluminium plate which is 1.5mm thick.
Do you think it would help to have them closer together? or is 7cm close enough that there temperatures will be the same? (we want to avoid one gettign much hotter than the other and shipping the majority of the current)

MBR6045

Super Moderator
Staff member
To balance load through two parallel diodes, the ARRL handbook advises to place resistors inline with each diode.

Say diode thresholds differ by 0.1 V. (Perhaps due to mismatch or to thermal runaway.) One diode hogs the bulk of current and is liable to burn up.

Balancing resistors establish a safe Ampere level, in case either diode turns rogue. Ohm value does not necessarily need to be great. Sample simulation yields tentative values of .01 ohm, rated 8W each.

FvM

Super Moderator
Staff member
You are asking if the distance on the heatsink matters. To answer the question yourself, you can estimate the differential thermal resistance and relate it to diode Rth,jc of 1 K/W. You'll see that the differential resistance is a very small fraction of the latter, e.g. < 0.02 K/W, respectively negligible.

Regarding thermal stability, you'll compare negative differential resistance in operation point due to forward voltage thermal coefficient, forward voltage and total differential thermal resistance (about -0.6 mOhm) with diode bulk resistance (about 2 mOhm) -> unconditionally stable. In contrast it won't be stable when using separate heat sinks.

tonigau

Newbie level 4
Were the diodes checked for Forward Volt first ? to see how closely matched , often they can be not. Use a low constant current & measure VF & ensure temperature is same.

Sounds like the diodes were positioned to optimize thermal dispersion on the heatsink plate.
Could have mounted the 2 diodes close & a copper plate about ~50mm square between diodes & Al plate, especially if anticipating significant cross air flowing. & then test with DC current clamp

How hot does the plate get? (temperature rise above ambient) assuming full current [& considering high ambient temperature].

Is it part of a redundant power system ?
We had an issue of schottky diodes backfeeding when 1 power supply was turned off (diodes were hot) & card power good light back at control centre would stay on.
The fix was "wait for it to cool" & then call after 5 min.
.