From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 12 Feb 2020 15:05:47 -0800
Subject: Re: [EE] TL431+NPN as rated series regulator? Circuit questions.
Thread-Topic: [EE] TL431+NPN as rated series regulator? Circuit questions.
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To answer the question of why:

I have a device that operates from 15 to 45 volts nominally - safety
and reliability are of concern with this device. A design requirement
is, where possible, that the device only use "jelly-bean" parts for
which there will be pin compatible replacements 30 years from now.

That device has an IC that draws 1 amp at a low duty cycle. That IC is
rated to withstand no more than 55 volts and needs to operate very
near the applied power supply voltage. My device is subject to
+/-800V/800A (1 ohm source impedance) 200us long pulses on its power
input. It is also subject to 75V ("0 ohm" source impedance) 500ms
pulses with a fast rising edge. That 55V rated IC would die if the
pulses were allowed to reach its power pins. I am using the TL431
series regulator to create a voltage rail that follows the power
supply voltage up to the set maximum of 45V. Above 45V the series
regulator will limit the voltage and dissipate the extra potential has
heat.

The TL431 series regulator in combination with other circuit elements
is part of a strategy to avoid damage from the 800V/75V pulses. I
found standard regulators to be unsatisfactory with regards to voltage
rating, power handing, and long term availability.

I would love a surface mount LM317 type device that was rated for 300V
or higher, a running current of 1 amp, and a power dissipation of 2
watts or more. If the said device has at least 5 pin compatible
alternates from different manufacturers I would go for it in a
heartbeat.

Unfortunately, I am basically required to absorb the rising edge of
the 800V pulse with a TVS/MOV type device. The voltage/current curve
of said device allows the supply voltage to rise quite substantially
(to about 250V) before the voltage sensitive circuitry can
"disconnect" itself from the power input.

On Wed, Feb 12, 2020 at 12:44 PM Gordon Williams <gwilliams@ncf.ca> wrote:
>
> For my own education, why would you want to use a circuit like this
> rather than some typical 3 pin voltage regulator?
>
> Seems like TL431 has all the disadvantages, cost, part count, design and
> testing time, etc. in this application.
>
> Gordon Williams
>
>
>
> On 2020-02-11 9:42 p.m., Harold Hallikainen wrote:
> > I think C1 provides a "soft start" by pulling the input to the error
> > amplifier up on power up. I've seen a similar trick used on POLA module=
s.
> > It's interesting that the data sheet shows this capacitor but does not
> > explain it.
> >
> > I think the resistor from the base to emitter of the second transistor
> > ensures that the second transistor does not conduct due to leakage of t=
he
> > first.
> >
> > Harold
> >
> >
> >> Hello everyone,
> >>
> >> The TL431 datasheet shows the attached circuit has an example of how
> >> to create a series regulator.
> >>
> >> I would like some help understanding the purpose of C1, RA, and RB.
> >>
> >> I assume RA is to prevent oscillation?
> >> What is C1's purpose? Why is C1 from cathode to reference? (to prevent
> >> oscillation?)
> >> What is RB's purpose? (to improve turn off time?)
> >>
> >> --
> >> Jason White
> >> --
> >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> >> View/change your membership options at
> >> http://mailman.mit.edu/mailman/listinfo/piclist
> >>
> >
> --
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