KlausKragelundwrote:
On 27 Nov., 21:32, Joerg <
[email protected]>
wrote:
KlausKragelund wrote:
On 26 Nov., 18:06, Joerg <
[email protected]>
wrote:
KlausKragelund wrote:
On Thu, 20 Nov 2008 06:58:51 -0800 (PST),KlausKragelund
Hi
I need to design a wide input range converter (16VDC to 600VDC)with
two 5V outputs with a total power of 25W.
The supply needs to be double isolated and my first thought wasto use
a HVbuckconverter to step the voltage to 16V and then a forward
converter (half-bridge) to step down to 5V and provide safety
isolation.
Two questions:
1. Do you know an integrated IC than canbuck25W without being forced
to resort to a UC28xx and level shifting/current sensing (should
preferable be current mode controller). Something like the Onsemi
NCP1010, but just higher currents.
2. What about converting directly from the input to 5V by usingthe
forward transformer. It will be quite low duty-cycle, so does anyone
have experience with this? (also the transformer design will bequite
different)
Suggest you reconsider your source's input configuration, even if it
means using two sets of fixed alternative input hardware, or
switched/folded conversion topologies.
No single source would seem to have the arbitrary 40:1 voltage
compliance you've specified, without exhibiting loading
characteristics that would normally be sensibly exploited to reduce
circuit complexity, even if it meant burning a continuous minimum
load.
The high end you've specified implies multipliers of 1.6 for creepage,
clearance and insulation distances and thicknesses, when compared to
240VAC circuitry. Safe limiting and fusing over the range also
approaches the impractical.
'Low duty cycle' could imply high pulsed power, for a 25W continuous
rating. Intermittent operational requirements, on the other hand,
could either nix simpler circuitry or allow liberties to be taken with
thermal ratings. What do you actually mean?
RL- Skjul tekst i anførselstegn -
- Vis tekst i anførselstegn -
Well, the 600VDC comes from an ac overvoltage event on the input
(normally this voltage is 230Vac/310VDC). This overvoltage event is
only present for 100ms maximum, so this will not stress the components
over the lifetime, but anyway, 100ms is an eternity in transient
thermal response time terms.
100msec? Interesting, that sounds just like an aircraft spec. Anyhow, if
100msec is abs max and it is guaranteed that those surges won't appear
machine-gun style you could snip those off with a simple pass transistor
that has enough thermal mass for it. One that is always closed except
when the voltage exceeds 350V or so, whatever a safe spec might be..
[...]
Actually the 100ms is because the control guys cannot shut down their
stuff fast enough, so I am force to live with it.
I did actually think about the snip-off FET, but then that would be
have to be rated for the current at 16V and voltage rated for 700V,so
I could equally well just use a big FET in thebuckconverter. Another
idea was to use the fact that the input is ac, so could just turn on
the big FET for each half-cycle when the voltage passes 24V - to
charge the DC link. But I think the losses will be very high.
The losses don't have to be high especially since it's probably 50Hzor
60Hz AC. The main issue in the AC case are the capacitors. Having to
have all those at the highest expected voltage rating is going put a
real dent into the budget because anything past 350V or maybe 400V will
be large, expensive and not liked by the purchasing folks. I guess in
the end you'll just have to price all this out.
--
I will try to do a simulation of that. Regarding the voltage rating of
the capacitors, I think I can use ones rated for the 230Vac and simply
regard the overvoltage situation as a transient (as it indeed is).
Even better simply only turn the FET on if the voltage on the ac cycle
is say 24V, so this way the caps can be rated only for something like
35VDC and then much less volume.
When the input is DC, then the FET should be on always, so the FET
would just turn on at any voltage below 35VDC to protect the caps and
to function in either ac and DC case (the ac and DC are never present
at the same time and use the same terminals)
Yes, that could work. Especially since you mentioned that both voltage
options come in via the same rail. Of course, in the AC case that will
really mess up the power factor but it might not be of much concern for
a 25W supply.