What’s All This Common-Base Stuff, Anyway? (.PDF Download)

May 16, 2017
What’s All This Common-Base Stuff, Anyway? (.PDF Download)

In 2007, a power-conversion startup company was promoting a wall-wart schematic that had an external FET driving an external high-voltage transistor (Fig. 1). I note the startup company has been absorbed by the great folks at Power Integrations, and I see no mention of this part or schematic on its website. Indeed, Power Integrations confirmed it has obsoleted the part, and would recommend its LinkSwitch-4 device, which drives a HV transistor directly via the base.

When I first saw this external FET circuit in 2007, I did not understand the reason, thinking that it would be easier to put the FET inside the package or have the internal FET drive a bigger external FET. I was also confused as to why the driver FET was connected to the HV transistor in a common-base configuration. I tried asking a PR person from the startup company, but he really did not have any definitive answer. I then wrote a bunch of pals asking what was going on.

To my question of “why don't they just drive the base directly with the chip?” Bob Pease wrote, “I think I can answer that. The base of the low-beta NPN would be harder to drive than driving the FET through the emitter. With the base driven, you would need to drive more base current and more Miller current. The FET helps avoid that.”


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