Electronics > Repair

Need help with a Metcal MX500

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kff:
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

wraper:

--- Quote from: kff on February 01, 2015, 06:49:44 pm ---Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

--- End quote ---
Mounting tab is not gate but drain. Considering working frequency likely because of the capacitance.

jbryan2:

--- Quote from: kff on February 01, 2015, 06:49:44 pm ---Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

--- End quote ---

Source is tied directly to ground. If the heatsink is tied to ground you had better isolate the tab/drain.

jbryan2:

--- Quote from: kff on February 01, 2015, 06:49:44 pm ---Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

--- End quote ---

Oh, and the gate is tied to ground through L10.  If you don't isolate the tab/drain then all three pins are - glory be! - tied to ground.  It's not going to do much that way.

wraper:

--- Quote from: jbryan2 on February 02, 2015, 02:44:10 am ---
--- Quote from: kff on February 01, 2015, 06:49:44 pm ---Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

--- End quote ---

Oh, and the gate is tied to ground through L10.  If you don't isolate the tab/drain then all three pins are - glory be! - tied to ground.  It's not going to do much that way.

--- End quote ---
As I understand he asked about why 2 layers of insulation, not just one.

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