The 2246 has an active protection circuit with a feedback loop that monitors the Drain voltage of the FET and keeps the heat dissipation of the FET under check. The FET also has a huge heat sink. There's no such active protection for the switching transistors, so it is probably safer to allow the FET to take more heat than the transistors. Since driving the transistors harder with a smaller resistor will reduce transistor heat dissipation, your logic of retaining smaller resistors fits right in other than of course if it blows the resistors!
Have others seen overheated and blown resistors?
Priya.
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--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
--- In TekScopes@..., "Francis" wrote:
Hi,
don't forget that the transistors are operating in switching mode,
and therefore should have a Ic/Ib ratio not greater than 10,
which gives 200mA of base current for a 2A collector current.
This can be easily verified by a simple voltage probing on the
leads of the resistors. The base current is even over the above
value. (be careful, this part of the SMPS is connected to the
mains)
I have found 0.3A and over. A simple calculation gives 5.1x0.3x0.3=459mW, which is way above the possible dissipation
of these components. BTW, I not sure that their size can allow
1/4W dissipation. 1/8 looks more realistic....
I must say that the power is not maintained all the time, only
when the transistor is ON, roughly 1/3 of the time. Still too
much in my opinion. I decided to increase the value as follow:
2.7 -> 5.1 and 5.1 -> 10. I don't use TIP41 either, only TIP31
The FET is much cooler, so are the resistors. The PCB says "thanks",
and will be less cooked than with the original values. The
waveforms are perfect.
