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In Reply to: Thanks for your concern, but . . . posted by Ray Moth on May 8, 2007 at 00:45:42:
>>I take it that your suggestion to ramp up the voltage slowly would not be satisfied by an RC time constant on the gate, to allow it to reach the reference voltage from the zeners slowly?<<This can be done but it means using a large C in order to achieve a high enough time constant with a low enough value of R. The time constant needs only to be large enough to limit inrush current caused by having to charge the OP Caps.
>>That may explain why MOFET serial regulators usually have a series resistor of ~100 ohms>>
The series (100R) that people hang off the gate is to prevent HF oscillation ... similar to a grid stopper. It's also important to place a zener from S to D to limit potential over-voltage to a safe level.
BTW, there was a comment made by JS about the voltage rating of the Mosfet. If the INPUT voltage is ramped up slower than the regulator's ability to charge the output C then barring a short or sudden instant VERY heavy load on the PS the voltage rating needn't be all that high because the differential across any of its terminals will never be greater than the input V minus the OP V at any given moment. Staging the input voltage is also a reasonable technique here.
Rgds,
Naz
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Follow Ups
- Re: Thanks for your concern, but . . . - Naz 00:30:30 05/09/07 (4)
- Re: Thanks for your concern, but . . . - Ray Moth 00:52:56 05/09/07 (3)
- Re: Thanks for your concern, but . . . - Naz 06:41:47 05/09/07 (2)
- Perhaps I meant the drain! - Ray Moth 14:43:44 05/09/07 (1)
- That's what I thought ... - Naz 15:56:59 05/09/07 (0)
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