Gilly wrote:Hi Jack
Something must be wrong to blow FETs at the 200Watt level......all the time....
Is it only just one side that blows....?
How many go bang ! ..... 1 or 2......or the whole lot...!
K9ACT wrote:My Class D is running pretty well now but I still blow FETs when I push it to 200W and much over 100% modulation so I have been just running it at 100W and not sure where to go from here.
I have the Overload Board and the bag of parts and I guess I can figure out how to hook it up but the instructions on the schematic assume a 45V carrier and I need the numbers for 30V. Also what is meant by 12V at input port in this setup?
One of the notes says that if it trips on normal voice peaks, nudge up the voltage cal a bit but this seems like inviting the problem I am now having.
I am thinking that I will just begin by shutting down the HV on overload but I need some advice on where best to do this.
As there is less current in the primary, this seems like the easiest place but it may not be the best for some reason I am not aware of. On the other hand, maybe it needs to be in the DC side.
Jack
steve_qix wrote:The overload board will protect everything from overcurrent. If you have a parasitic and if that is causing devices to fail in the RF amplifier, the overload board will protect the modulator and other components from damage, but it can't do anything about the parasitics (IF this is what is causing the problem).
Gilly wrote:Is it always the same FET in the same Module or is it random amongst them...
K9ACT wrote:steve_qix wrote:The overload board will protect everything from overcurrent. If you have a parasitic and if that is causing devices to fail in the RF amplifier, the overload board will protect the modulator and other components from damage, but it can't do anything about the parasitics (IF this is what is causing the problem).
If I understand, it will prevent the drudgery of replacing FETs including the ones in the RF deck but I have to get rid of the parasitics to run it. (How do we teach this spell checker how to spell?)
If the problem is parasitics, it only happens at high levels of modulation and displays no other symptoms than blowing fets.
I am still waiting to hear from you on those high voltage Transient suppressors. I would at least like that base covered.
What do I need to do to get 10 of them?
I have everything but the semiconductors on the Overload Board but there is a heat sink in the package with no clue where it goes. It would fit the regulators but there is no hole in the board for the mounting tab. Was this an accidental inclusion or am I supposed to use it.
One more question, why is it necessary to shut down the PWM generator if the HV is shut down?
js
steve_qix wrote:
The heat sink goes on the 5 volt regulator. This is documented somewhere, but I forget where right off..
kf1z wrote:If you use "reset low", then you should have to put +5v on it to start pulse-train etc...
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