I have used a floating heat sink that is attached to the HV DC common as suggested by Brian. A big disadvantage, I found, to connecting the heat sink electrically to chassis is that enough switching current can be capacitively coupled from the MOSFET drain to chassis to be an EMI problem. If the heat sink is allowed to float you can use a standard TO220 package. On this small part heat sink paste or a thermally conductive pad is not needed. The paste is a mess in production. I also like the ISOPAKs when electrical insulation is needed. There is quite a bit of thermal resistance but if you don't mind the part running a bit hotter they're great. I used to limit my junctions to 100 deg C but later on upped that to 125 deg C. This sure helped make the designs more compact. Another attractive part is the TO218 Isopak. An IGBT's are sure easy to use with the drawbacks being the tail current and the high thermal resistance. Sound like you have a fun project to design. Dave Cuthbert
From: [email protected] [mailto:[email protected]]On Behalf Of Brian O'Connell Sent: Friday, January 09, 2004 10:39 AM To: [email protected] Subject: RE: Bonding Power semiconductors to Chassis Warning: I am assuming that all LVD safety standards reference the same basic standards. There was an 1999 IEC60950 CTL/OSM decision that said the plastic package of a semiconductor can be considered Basic Insulation. Let me know if you need to know the ref no. Two caveats: the package must be rated for the peak and RMS working voltages, and the transistor must pass the respective dielectric withstand test for Basic Insulation. If the transistor has a metal tab, creepage and clearance distances must be observed, and the sheet insulator must be rated for the respective dielectric withstand test voltage. Or, you could float the heatsink, and provide required spacings from the metal and mounted components to secondary and chassis. Or, (conflict of interest alert) you could buy an off-the-shelf component power supply that has already been investigated by an NCB to IEC 60950-1. luck, Brian -----Original Message----- From: McBurney, Ian [Allen & Heath UK] [ mailto:[email protected]] Sent: Friday, January 09, 2004 8:10 AM To: [email protected] Subject: Bonding Power semiconductors to Chassis Dear members; I am in the process of developing an off-line flyback switchmode PSU. The main switching MOSFET requires an aluminium heatsink. The MOSFET I have selected is a TO220 ISOPAK that will be screwed to the heatsink. If I use this device and an insulating pad, is it permitted to bolt the heatsink to the metal chassis of the product? I wish to do this to aid the efficiency of the heatsink. The product is class 1 i.e. must be connected to earth (ground) with the mains cable. The safety standard the product will be tested to is IEC60065. Many thanks; Ian McBurney. Allen & Heath Limited. email: [email protected]
