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 was to use a HV buck converter to step the voltage to 16V and then a forward converter (half-bridge) to step down to 5V and provide safety isolation.
Two questions:
Do you know an integrated IC than can buck 25W 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.
What about converting directly from the input to 5V by using the forward transformer. It will be quite low duty-cycle, so does anyone have experience with this? (also the transformer design will be quite different)
I would consider a non isolated Buck Converter, bucking down to 150V. Follow this with a non-isolated Boost Converter, boosting up to 75V. Follow this 75V to 150V input bus with an isolated forward converter that can easily handle the 2X input voltage range. The first two stages would normally pass or convert as required and operate at a max duty of 5:1. The two intermediate bulk caps need to handle only 150V max. Going directly from 16V-600V in a forward converted is very difficult! It would take a 800V MOSFET and duty as low as 1.3%.
Sorry, no idea there since I never use "fat" converter chips. Their production lifetime is often low and they are hardly ever 2nd sourced. Also, IMHO having power components and control circuitry on the same chip isn't so great.
Done that recently but the other way around, 10-15V in and a few hundred volts out. Half-bridges and similar architectures do not like to be regulated but they can be. So, yesterday it was ready and I had to fire it up. Only about 5W but the principle of operation is always similar. For fun and to test stability I turned the HV converter down to close to zero. The output was still nicely regulated but of course the duty cycle looked like s..t. Similar to an idling Harley-Davidson with really fouled-up spark plugs. Even the trusty old Tek 2465 had trouble triggering onto the fuzz. Without the metal enclosure it would probably have been an EMI nightmare.
Regulating a bridge converter can be compared to some really old rotary engine aircraft from almost a century ago. The cylinders spun around and the "crankshaft" was bolted to the fuselage. Since a throttle was not available and the engine would always deliver full power the only way to land them (and survive) was to reduce power by cutting the ignition to several cyclinder. Rat-tat-poof-rat-tat-BANG-poof ...
As Harry mentioned, with such wide input ranges you need FETs that can push a lot of current around and at the same time can stomach a kilovolt or so. Spend some time on the snubber design to make sure the spikes never ever exceed the breakdown limits.
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Regards, Joerg
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I thought they cut the ignition to all cylinders. Not so!
Here's some more info for those aspiring to be fighter pilots: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 8 October 2004, 03:07 PM
By Tripehound
Rotary throttle
Not a throttle like in your car, but a way of increasing or decreasing the amount of fuel mixture fed to the engine, in three separate steps.
There was an engine air intake pipe that used something like a ball valve to regulate the amount of air going to the engine. This was generally operated via a linkage from a handle in a "throttle quadrant" located on the left side of the cockpit. The amount of fuel going to this intake pipe was regulated by a "fine fuel" valve from the pressurized fuel tank, or from a gravity tank, to a jet inside the ball valve, where the incoming fuel was atomized. The air ball valve acted like a Venturi tube, supplying suction to help move the fuel. This "fine fuel" valve was operated by a separate handle located in or near the "throttle quadrant". The Germans seemed to favor locating the "fine fuel" control on the control column and operating the valve by a twist knob via a Bowden(?) cable. Castor oil was carried in a separate tank and pumped from this tank thru a sight flow regulator in the cockpit to the fuel mixture feed line as it entered the crankcase.
Both air and fuel had to be increased or decreased in steps to avoid leaning or flooding, either one resulting in a dead engine. As the aircraft climbed and the air became less dense, the fuel had to be cut back to avoid flooding and a dead engine. As the aircraft descended the fuel had to be increased to avoid leaning out and a dead engine. Oil regulation didn't seem to be quite as demanding of immediate attention as the air/fuel mix, but still necessary.
The coup button, or "blip" switch, allowed quick changes in engine speed by cutting ignition to all cylinders. On landing, for example, the engine was adjusted to take-off power then the coup button used to cut the engine for brief intervals to decrease air speed for landing. If the button was held too long the unburned fuel/oil mix would flood the engine to where it wouldn't restart when the button was let go, and/or worst case the expelled unburned fuel mix would catch fire in the cowling.
Wooden airplanes and iron men!
Gregory -
Have a look at this thread from a few years back.
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(It's three pages - too long to post but quite interesting.)
Tripehound's explanation is very good. I would only add that some makes and models of rotaries had different methods of control in addition to the mixture adjustments and coup button. The 160hp Gnome, for example, could be set to fire all 9, 7, 5, or just 3 cylinders on each revolution. The cylinders that were not firing still cycled, but had no spark. They filled with fuel/oil and then expelled it, unburnt, into the cowl. Messy and primitive, but it worked. Mostly.
There are photos in Mike Vine's Return to Rhinebeck of ORA's repro rotary-powered Caudron G3 being overhauled. The mechanic has the nacelle up on sawhorses with the cowl and engine removed - you would not believe the GUNK on the firewall. It's as thick as cake frosting.
Last edited by EricGoedkoop; 9 October 2004 at 07:36 AM.
It depended on the model and brand of aircraft engine.
When I did parachuting in Belgium they still used the word "coupe" to tell the pilot to cut power. Of course, him being the bush pilot type he often didn't and then you found out that air does have some mass ... jump ... *THWACK*
Yes, and here is a nice link with a picture of the Gnome engine:
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What he probably meant with "Mostly" is that when the blipping wasn't done right you could land with a trail of fire and black smoke. The more "sophisticated" engines would blip alternatively on various cylinders, maybe to reduce the chance of the big bang followed by a fire.
[...]
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Regards, Joerg
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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?
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.
If we assume the input ac is rectified to a DC link voltage and then bucked down by a converter to 24V (primary conv), then normally the duty-cycle would be minimum 8% and that should be no problem.
The half-bridge converter runs off this 24V voltage and bucks down to
5V (secondary conv). In principle I could adjust the 24V voltage up to increase the ratio of the secondary conv and ease the losses of the primary conv.
With respect to the primary buck, I found the Viper100a which seems to be able to be used in a buck configuration. Alternatively it could be done by standard PWM ic, high-side driver and a N-channel MOSFET - which could be all second sourced components.
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.
[...]
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Regards, Joerg
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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 the buck converter. 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 50Hz or
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.
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Other idea: Why not have two switchers? One that runs off the 230VAC rail and another that runs off the battery rail. Assuming the 230VAC is desired to take precedence you could then run the 16V switcher on idle and it only takes over when the 230VAC rail fails to deliver. You could even completely disable the 230VAC switcher in case that dreaded spike comes along and let the 16V take over for those 100msec.
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Regards, Joerg
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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.
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Yep, normally the power factor is only looked upon when the power from the product is above 50W (75W in some special cases).
Moreover, to reduce the peak current, the caps should be the minimum value possible to allow for more "on-time" during the ac cycle. Minimum cap size is the specified hold-time of the SMPS
Yeah, but you know how the guys in Brussels operate. Pretty soon they might institute PFC requirements for bicycle lamps ;-)
That could possibly be done by slowly charging a larger electrolytic via a resistor and providing a diode across that resistor. So it won't mess with the PFC but can still tide you over the specified number of cycles.
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