Generate -100V DC

D

Denis Gleeson

Guest
Hello All

I placed a similar question some time back but I didn’t give
sufficiently detailed requirements. Im hoping that more details may
solicit some responses.

I need to generate somewhere –55 to –100V DC from a 12V
source. I need to supply 100mA and cant use a transformer. I also cant
waste a lot of power in doing this so I need high efficiency.

I attempted a design with the LM3578 from Nat Semi but found I was
generating a lot of heat in the pass transistor (I was consuming 130mA
with no load) and not getting below about –25V.

Ive looked at some devices from Maxim and will test them soon.


Any suggestions for circuits or devices would be appreciated. And If
anyone can guess why I was consuming 130mA with no load on my test
cct, that would be interesting.

Regards

Denis
 
"Denis Gleeson" <dgleeson-2@utvinternet.com> wrote in message
news:184c35f9.0405310318.76165eed@posting.google.com...
Hello All

I placed a similar question some time back but I didn’t give
sufficiently detailed requirements. Im hoping that more details may
solicit some responses.

I need to generate somewhere –55 to –100V DC from a 12V
source. I need to supply 100mA and cant use a transformer. I also cant
waste a lot of power in doing this so I need high efficiency.

I attempted a design with the LM3578 from Nat Semi but found I was
generating a lot of heat in the pass transistor (I was consuming 130mA
with no load) and not getting below about –25V.

Ive looked at some devices from Maxim and will test them soon.


Any suggestions for circuits or devices would be appreciated. And If
anyone can guess why I was consuming 130mA with no load on my test
cct, that would be interesting.

Regards
The LM3578, should do what you want fine. However there are some major
Click to expand...
'caveats', which apply to _all_ such circuits. Firstly, the very high
instantaneous currents, and gain in some parts of the circuit, really do
make it very unlikely to work, unless built on a PCB, or hand wired, with
good solid tracks. If you use things like plug-in 'prototype' boards, such
circuits will generally not work properly. You may have had a problem of
this sort. Secondly, such circuits generally do require a little load. Your
system may have been driving just one cycle, then overvoltaging the output,
and then sitting for a long time to recover, giving a 'silly' apparent
voltage. Your output transistor, may have been unsuitable for the task.
Again the currents involved can be high, and chosing the right transistor
makes a huge difference to the overall efficiency.
Look instead at the LM2577. This has it's own transistor built-in.
Your high heat with no load, and failure to get above 25v (? you say
below?), would probably suggest the inductor/transistor was not correctly
selected.

Best Wishes
 
Denis Gleeson wrote:
Hello All

I placed a similar question some time back but I didn’t give
sufficiently detailed requirements. Im hoping that more details may
solicit some responses.

I need to generate somewhere –55 to –100V DC from a 12V
source. I need to supply 100mA and cant use a transformer.
(snip)
Click to expand...

I don't understand this requirement, when the correct transformer can
solve so many of your problems. Can you elaborate on this
requirement?

--
John Popelish
 
Denis Gleeson wrote...
I need to generate somewhere -55 to -100V DC from a 12V source.
I need to supply 100mA and cant use a transformer. I also cant
waste a lot of power in doing this so I need high efficiency.

I attempted a design with the LM3578 from Nat Semi but found
I was generating a lot of heat in the pass transistor (I was
consuming 130mA with no load) and not getting below about -25V.

anyone can guess why I was consuming 130mA with no load ...
Click to expand...
You want to generate a negative voltage from a positive one,
without using a transformer, right? The LM3578 can do this in
its "inverting regulator" mode, see datasheet fig 20. But you
won't be able to have more than -1V output (see note 2) unless
you add a HV transistor wired in cascode mode, see fig 21. If
you use a 120V-rated (or higher) transistor you'll be able to
get up to -100V output.

Note, to provide 100mA at 100V, you'll draw 833mA average from
your 12V supply. This translates to an inductor current ramp
with a 1.66A peak. Or actually 1.89A peak, taking into account
the 88% duty cycle with a 12:100V voltage ratio.

There's an efficiency issue with the LM3578 used as an inverting
regulator, due to its high Emitter Saturation Voltage, over 1.2V,
wasting 10% of your power right up front. So you'll need over 2A
peak capability. Oops! Sorry, the LM3578 is limited to 750mA.


Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
Winfield Hill wrote...
Note, to provide 100mA at 100V, you'll draw 833mA average from
your 12V supply. This translates to an inductor current ramp
with a 1.66A peak. Or actually 1.89A peak, taking into account
the 88% duty cycle with a 12:100V voltage ratio.

There's an efficiency issue with the LM3578 used as an inverting
regulator, due to its high Emitter Saturation Voltage, over 1.2V,
wasting 10% of your power right up front. So you'll need over 2A
peak capability. Oops! Sorry, the LM3578 is limited to 750mA.
Click to expand...
This assumes discontinuous inductor-current operation, but if
you work at a high frequency, so the inductor only partially
discharges each cycle, you can get more power from an LM3578.
For example, if the current increases from 650 to 750mA during
the 88% on time, or 700mA average, you may be able to deliver
about 75mA to your -100V load.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
John Popelish <jpopelish@rica.net> wrote:
I don't understand this requirement, when the correct transformer can
solve so many of your problems. Can you elaborate on this
requirement?
Click to expand...
I'm also curious about his 'no transformer' requirement in that one can
argue that with a flyback supply what you really have is a 'tapped inductor'
rather than a transformer. I.e., if for some odd reason he has a design
document stating transformers aren't allowed, he can just call it what it
really is and be OK.

I suspect his real issue is that he wants to be able to purchase his parts
off-the-shelf, and of course that's much more doable with just inductors
than transformers.
 
I suspect his real issue is that he wants to be able to purchase his parts
off-the-shelf, and of course that's much more doable with just inductors
than transformers.
Click to expand...
A royer converter into a 6V or 12V center tapped transfirmer, should get him
where he wants to go.
 
In article <184c35f9.0405310318.76165eed@posting.google.com>,
Denis Gleeson <dgleeson-2@utvinternet.com> wrote:
[...]
I need to generate somewhere –55 to –100V DC from a 12V
source. I need to supply 100mA and cant use a transformer. I also cant
waste a lot of power in doing this so I need high efficiency.
Click to expand...

I'd be tempted to try something like the LT1070 from linear in a circuit
like this:
--------------------------!!------- etc
!
!-----------------!!---------
! !
-----))))))------------!!-----))))))-----!<--------))))))-----!<-- etc
! ! !
LT1070's / V ---
Switch ! --- ---
GND ! !
GND GND

By making the output a tripler, you make the cicuit place less demands on
the diodes and inductors but require 3 and 4 of each. This may be a good
trade off. At 33V a Schottky diode can be used. At 100V use have to use
a SiC Schottky or a fast silicon diode.
--
--
kensmith@rahul.net forging knowledge
 
"Winfield Hill" <Winfield_member@newsguy.com> wrote in message
news:c9ffmj02e9n@drn.newsguy.com...
Denis Gleeson wrote...

I need to generate somewhere -55 to -100V DC from a 12V source.
I need to supply 100mA and cant use a transformer. I also cant
waste a lot of power in doing this so I need high efficiency.

I attempted a design with the LM3578 from Nat Semi but found
I was generating a lot of heat in the pass transistor (I was
consuming 130mA with no load) and not getting below about -25V.

anyone can guess why I was consuming 130mA with no load ...

You want to generate a negative voltage from a positive one,
without using a transformer, right? The LM3578 can do this in
its "inverting regulator" mode, see datasheet fig 20. But you
won't be able to have more than -1V output (see note 2) unless
you add a HV transistor wired in cascode mode, see fig 21. If
you use a 120V-rated (or higher) transistor you'll be able to
get up to -100V output.
Click to expand...


Greetings Win.

Are we looking at the same figure 21 of the same datasheet?

When I look at this one:

http://cache.national.com/ds/LM/LM1578A.pdf

I notice the external transistor Q1 of figure 21 is a PNP device with it's
base being driven by the internal NPN device in the LM3578. It seems to me
in this configuration the saturation voltage and maximum current capability
of the NPN device aren't very important.

Of course this circuit arrangement isn't especially great at efficiency
since both the PNP saturation voltage and base drive current is all waste
which is unfortunate.

But it does seem like it should work provided the right parts selection and
barring any other unforeseen problems.
 
Fritz Schlunder wrote...
Are we looking at the same figure 21 of the same datasheet?
http://cache.national.com/ds/LM/LM1578A.pdf

I notice the external transistor Q1 of figure 21 is a PNP device with it's
base being driven by the internal NPN device in the LM3578. It seems to me
in this configuration the saturation voltage and maximum current capability
of the NPN device aren't very important.

Of course this circuit arrangement isn't especially great at efficiency
since both the PNP saturation voltage and base drive current is all waste
which is unfortunate.
Click to expand...
You are right, I glanced too quickly and assumed it was a cascode.
So my whole analysis goes out the window. Sheesh!

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
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