LM317 in a high-voltage circuit.

[pawihte]

The specs for the LM317 says that, since it uses a floating
circuit, it can be used at high voltages as long as the
input-output differential is within max ratings. But practical
loads will place a capacitance at the output of the LM317, with
many having a physical capacitor. That capacitance will
momentarily short the output to ground at switch-on, causing the
LM317's max voltage rating to be exceeded. An example would be a
fast-rising 100V input and a 75V output with a large cap at the
output. Is this likely to damage the LM317?

 

[John Fields]

On Fri, 22 Oct 2010 12:17:40 +0530, "pawihte" <pawihte@mail.invalid>
wrote:

The specs for the LM317 says that, since it uses a floating
circuit, it can be used at high voltages as long as the
input-output differential is within max ratings. But practical
loads will place a capacitance at the output of the LM317, with
many having a physical capacitor. That capacitance will
momentarily short the output to ground at switch-on, causing the
LM317's max voltage rating to be exceeded. An example would be a
fast-rising 100V input and a 75V output with a large cap at the
output. Is this likely to damage the LM317?

---
From:

http://www.national.com/ds/LM/LM117.pdf

"Besides replacing fixed regulators, the LM117 is useful in a
wide variety of other applications. Since the regulator is “floating”
and sees only the input-to-output differential voltage,
supplies of several hundred volts can be regulated as long as
the maximum input to output differential is not exceeded, i.e.,
avoid short-circuiting the output."

One would think so.
---
JF

 

[pawihte]

John Fields wrote:

On Fri, 22 Oct 2010 12:17:40 +0530, "pawihte"
pawihte@mail.invalid
wrote:

The specs for the LM317 says that, since it uses a floating
circuit, it can be used at high voltages as long as the
input-output differential is within max ratings. But practical
loads will place a capacitance at the output of the LM317,
with
many having a physical capacitor. That capacitance will
momentarily short the output to ground at switch-on, causing
the
LM317's max voltage rating to be exceeded. An example would be
a
fast-rising 100V input and a 75V output with a large cap at
the
output. Is this likely to damage the LM317?

---
From:

http://www.national.com/ds/LM/LM117.pdf

"Besides replacing fixed regulators, the LM117 is useful in a
wide variety of other applications. Since the regulator is
"floating"
and sees only the input-to-output differential voltage,
supplies of several hundred volts can be regulated as long as
the maximum input to output differential is not exceeded, i.e.,
avoid short-circuiting the output."

One would think so.

I have the NatSem linear databook in both CD and printed forms.
But it doesn't say anything about momentarily exceeding the
differential voltage rating which seems to be inevitable in many
practical applications such as the one I described.

 

[Phil Allison]

"pawihte"

I have the NatSem linear databook in both CD and printed forms. But it
doesn't say anything about momentarily exceeding the differential voltage
rating which seems to be inevitable in many practical applications such as
the one I described.

** Not inevitable at all, if you add some simple protection for the IC.

Never heard of Zener diodes ???

One wired across the input and output pins might help.

Wot a fuckwit ......




..... Phil

 

[George Herold]

On Oct 22, 8:51 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"pawihte"



I have the NatSem linear databook in both CD and printed forms. But it
doesn't say anything about momentarily exceeding the differential voltage
rating which seems to be inevitable in many practical applications such as
the one I described.

 ** Not inevitable at all, if you add some simple protection for the IC..

 Never heard of  Zener diodes ???

 One wired across the input and output pins might help.

 Wot a fuckwit ......

....   Phil

I was going to say that he might also put a bit of resistance before
the output capacitance.

George H.

 

[pawihte]

Phil Allison wrote:

"pawihte"

I have the NatSem linear databook in both CD and printed
forms. But
it doesn't say anything about momentarily exceeding the
differential
voltage rating which seems to be inevitable in many practical
applications such as the one I described.

** Not inevitable at all, if you add some simple protection for
the
IC.
Never heard of Zener diodes ???

One wired across the input and output pins might help.

Wot a fuckwit ......

Ah, good ol' Phil. What happens to the zener diode, say a 33V or

39V piece, when you apply +100V (which may have a fairly low
series resistance) at one end and the other end is momentarily
grounded? Zzzzt!?

 

[pawihte]

George Herold wrote:

On Oct 22, 8:51 am, "Phil Allison" <phi...@tpg.com.au> wrote:
"pawihte"



I have the NatSem linear databook in both CD and printed
forms. But
it doesn't say anything about momentarily exceeding the
differential voltage rating which seems to be inevitable in
many
practical applications such as the one I described.

** Not inevitable at all, if you add some simple protection
for the
IC.

Never heard of Zener diodes ???

One wired across the input and output pins might help.

Wot a fuckwit ......

.... Phil

I was going to say that he might also put a bit of resistance
before
the output capacitance.

George H.

But that will defeat the purpose of having a regulated supply -
at least partly.

 

[Michael Black]

On Fri, 22 Oct 2010, pawihte wrote:

The specs for the LM317 says that, since it uses a floating
circuit, it can be used at high voltages as long as the
input-output differential is within max ratings. But practical
loads will place a capacitance at the output of the LM317, with
many having a physical capacitor. That capacitance will
momentarily short the output to ground at switch-on, causing the
LM317's max voltage rating to be exceeded. An example would be a
fast-rising 100V input and a 75V output with a large cap at the
output. Is this likely to damage the LM317?

And how much capacitance? If you're using high voltage, surely these

are no longer low impedance devices, hence you won't see all the high
value capacitors seen on low impedance circuits. Yes, the circuitry
itself will present some capacitance to the regulator, but it being
incidental, how serious can it be?


Michael

 

[Joerg]

pawihte wrote:

The specs for the LM317 says that, since it uses a floating
circuit, it can be used at high voltages as long as the
input-output differential is within max ratings. But practical
loads will place a capacitance at the output of the LM317, with
many having a physical capacitor. That capacitance will
momentarily short the output to ground at switch-on, causing the
LM317's max voltage rating to be exceeded. An example would be a
fast-rising 100V input and a 75V output with a large cap at the
output. Is this likely to damage the LM317?

The rule is that you can't exceed abs max values, not even for a very
short time. Chances are, something avalanches in there and ... tsk ... *BAM*

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

 

[whit3rd]

On Oct 22, 5:51 am, "Phil Allison" <phi...@tpg.com.au> wrote:

[on the concern of capacitive load and HV transient on an LM317]

 ** Not inevitable at all, if you add some simple protection for the IC..
....[Zener diode]
 One wired across the input and output pins might help.

If the output has a capacitor, the Zener protects against turnon
transient by breakdown, and by turnoff transient by forward
conduction.
The app notes DO note the need for a diode there, as I recall.

A millisecond or so of forward overvoltage won't harm the IC
in most cases (it exceeds the protective range of the
shutdown circuitry, but it doesn't overheat in that short a time).

 

[Phil Allison]

"pawihte"

Phil Allison wrote:
"pawihte"

I have the NatSem linear databook in both CD and printed forms. But
it doesn't say anything about momentarily exceeding the differential
voltage rating which seems to be inevitable in many practical
applications such as the one I described.

** Not inevitable at all, if you add some simple protection for the IC.
Never heard of Zener diodes ???

One wired across the input and output pins might help.

Wot a fuckwit ......

Ah, good ol' Phil. What happens to the zener diode, say a 33V or 39V
piece, when you apply +100V (which may have a fairly low series
resistance) at one end and the other end is momentarily grounded? Zzzzt!?

** Applying the +100V DC input should NOT be an instantaneous event and
grounding the output is simply not allowed when using IC regs with excess
input voltage.

Long as the 100V supply takes a few milliseconds to rise, a hefty ( say 5
watt) zener will be fine passing enough current to help charge a 10 uF
electro on the load side.

BTW zeners fail by going short - so the reg IC will survive.



...... Phil