cupoftea
Advanced Member level 5
Hi,
In the attached schem of an LLC converter (400vin, 2000W, 180Vout), there are 8 x TO220 power diodes. All have the cathode pin connected to their metal tab.
Where the cathode is connected to a “quiet node” (non switching node)…..we will simply directly screw the TO220 tab to the metal heatsink, and connect the heatsink “leg” to the same “quiet node”….and use no insulating spacer.
However, some of the diodes have the cathode connected to a switching node………..so we will have to use an insulating spacer between diode & heatsink, & then connect the heatsink to the nearest “quiet node”, eg the local ground………
However……..the insulating tab is going to mean an increase of the Rth(JA) for the diode….as such, I was wondering, since this version is just a cheap prototype (not going for approvals testing)…..can we get away with just directly screwing the diode to the heatsink, and just leave the heatsink floating? (so the heatsink will be connected directly to a switching node). I appreciate this is bad……but isn’t it only bad for approvals?….and regarding noise which could interfere with circuit operation, well, it wont make any difference?
In the attached schem of an LLC converter (400vin, 2000W, 180Vout), there are 8 x TO220 power diodes. All have the cathode pin connected to their metal tab.
Where the cathode is connected to a “quiet node” (non switching node)…..we will simply directly screw the TO220 tab to the metal heatsink, and connect the heatsink “leg” to the same “quiet node”….and use no insulating spacer.
However, some of the diodes have the cathode connected to a switching node………..so we will have to use an insulating spacer between diode & heatsink, & then connect the heatsink to the nearest “quiet node”, eg the local ground………
However……..the insulating tab is going to mean an increase of the Rth(JA) for the diode….as such, I was wondering, since this version is just a cheap prototype (not going for approvals testing)…..can we get away with just directly screwing the diode to the heatsink, and just leave the heatsink floating? (so the heatsink will be connected directly to a switching node). I appreciate this is bad……but isn’t it only bad for approvals?….and regarding noise which could interfere with circuit operation, well, it wont make any difference?
