On the 2230 which uses the same basic design, the preregulator output
is fixed. On the 2246, it looks like Q2208 adjusts the preregulator
output to keep the same voltage across the switching transistors and
power pass element Q2214.
On Thu, 03 Jan 2013 22:04:38 -0000, "Albert" <aodiversen@...>
wrote:
You are right. At first sight I thought that the 44 V would be regulated independently always to that value, but it's not. This also destroys my argument that the sum of saturation voltage and FET voltage must be more or less constant.
Albert
--- In TekScopes@..., "Mover" wrote:
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.
--- 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
David Hess