From: Winfield Hill
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
Date: 18 Jan 2003 09:12:57 -0800
Organization: Rowland Institute
References: <email@example.com> <firstname.lastname@example.org>
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> Winfield Hill wrote:
>> The Intersil HIP5600 is an adjustable three-terminal regulator
>> that can be thought of as a 500V version of the LM317. I've
>> used it in many of my lab instrument designs and found it to
>> be reliable and, like the LM317, an elegant design simplifier.
>> For example, here's a simple adjustable 0 to 400V regulator,
>> with a fixed-voltage feature.
>>. +480V ___ HIP5600
>>. ---| |---+---+-----------(O) HV out
>>. |___| | |
>>. | | 1.5k
>>. | | | adjust, 0 - 400V
>>. '---- | --+----o->o--------,
>>. | o--, |
>>. 10uF === 300V | 500k
>>. 450V | fixed | pot
>>. | 380k |
>> The switch allows one to select a fixed 300V output, or to
>> adjust the voltage to any value between 1.2 and 400 volts.
>> But this circuit has one potential problem that can cause
>> a component failure. As a design quiz, tell us what the
>> problem is, and find a solution that doesn't significantly
>> affect any of the circuit's functionality. Hint, the '5600
>> isn't the vulnerable part.
> If the unit is switched from the fixed 300V position over to the
> pot and the pot is turned right down (ie to give 1.2 volts) then
> the reg might not like having (albeit very briefly) 300 volts on
> it's output (from the cap) with (presumable) only a cuppla volts
> on the pass transistor base (assuming an NPN pass tranny). But
> you say the vulnerable part isn't the '5600.
> So that leaves the 480 volt spike across the cap at the time of
> switch over (cure a cap from switch common to ground?), the large
> current spike in the cap when switching from either extreme to
> the other, or perhaps the voltage rating of the Rs, wattage-wise
> 0.25 watt is sufficient but these are only rated for 250 volts
> and this will be exceeded. I can't think of anything else.
Gibbo, you're very close to identifying the flaw, closer than
any of the other respondents so far. Take one more look.