DC to DC converter question

[sal_park]

Hi,

I've got a DC-DC converter setup (LM2576HV) to supply 5v from a 42v battery. I was wondering how I would go about getting a 5v supply from say a 100v battery ? I can't see any DC to DC converters rated that high (perhaps my google skills are failing) or is there some other way of doing this easily ? The only way I can think of is to have 2 LM2576HV in series across the 100v supply (hoping they will see ~50v input each) and then taking their combined output as the input to a 3rd LM2576 to get the 5v.

I'm quite concerned about heat generation so don't want to use a dropper resistor so am hoping there is another way.
(I need about 400mA supply current if that has any bearing on the answer)

Any suggestions greatly received


sal_park

[c4757p]

1) Why on earth do you need to run something off a 100V battery? That's a lot of volts! Kind of dangerous... 

2) One easy thing you could to is to try running a cheap cell phone charger off it. It's a bit of a low voltage, so it might not like it, but it's worth a try. They can give 400mA, too.

[sal_park]

1) Why on earth do you need to run something off a 100V battery? That's a lot of volts! Kind of dangerous... 

It's for an electric bike

[Psi]

You can use a flyback type arrangement and run the switchmode controller from the powersupply output.

This creates a problem, since the thing cannot start until there's volts on the output but there wont be volts until it starts.
To get round this issue you add a high value resistor like 200K between the highvoltage and mcu vcc to 'bootstrap' the system and start the mcu,

[w2aew]

I would assume that the 100V battery is some series combination of lower voltage batteries.  If that assumption is correct, then you could simply run your DC-DC converter off of a subset of the batteries.  Should be perfectly fine, especially considering that your load is quite low compared to the electric bike load.

[ejeffrey]

You could have a small 6 or 12 V battery that starts up the electrical system, including powering a DC-DC converter. The low-voltage battery would then be recharged by the HV battery.  This is the approach taken by the Toyota Prius, and I assume other hybrid/electric cars.  The main battery is disconnected when the car is off for safety.  There is a small auxiliary 12 volt lead-acid battery that is responsible for powering the low-voltage electronics and most importantly closing the relay contacts on the HV battery so that it can start the car (the HV battery is required to start the gasoline engine).  Once the DC-DC converter is started, it powers the 12 volt electronics and recharges the battery.

[sal_park]

The low-voltage battery would then be recharged by the HV battery.

Great idea, but how do I do this ? I can't (for example) just stick 100V across a 12v battery. Well I could, but I suspect a fire or some other kind of high excitement event would ensue
Good suggestion to take a smaller voltage from some of the cells, but I'd rather not do this as it will make the install more complex for some people.

I'm building a bike computer (OS H/W of course) and my bike battery is 42V, but it seems there are plenty of people with higher voltage batteries (e.g. 72V) so I'm figuring if I can design in from the start dealing with powering it from a higher voltage source then it would be a better way to do it.

thanks for the help so far guys, every days a school day   

[ejeffrey]

The problem with high voltage DC-DC converters is that the controller chips need a 'logic' supply to run their internal oscillators, comparators, and fet drivers.  In low voltage applications, the controller chip is just powered from the input voltage.  However, if you have an auxiliary low-voltage supply, the converter chip never sees the high voltage.  Only the MOSFETs and catch diodes have to be rated for the high voltage.  So if you have a way to provide some low voltage, pretty much any SMPS circuit with external FETs can be used.

The two classic solutions to this problem are an independent auxiliary supply or bootstrapping.  Bootstrapping is to have the circuit powered from a low voltage winding on its main transformer.  That works once the circuit is started, but it cant actually turn on.  To solve this problem, you use a resistive dropper from the main high voltage source that charges a capacitor with enough stored charge to start up the circuit.  Once running, the SMPS controller is powered by its own output.

[bilko]

take a look at www.supertex.com some of their buck boost chips can take up to 450VDC input to power the logic

[amyk]

A universal input ATX PSU will probably run with a 100VDC input.

If you're fine with having some isolation too, look at the some of these.

[lewis]

And there's the TOPswitch range of off-line SMPS controllers, farnell sell some transformers designed for these. Have a look at the datasheets and they tell you which controller IC to use. Here's a good start: http://uk.farnell.com/jsp/search/browse.jsp?N=2031+203947&Ntk=gensearch&Ntt=topswitch&Ntx=mode+matchallpartial

[IanB]

As others have mentioned, you could take a look at a standard cell phone charger that outputs 5 V DC at up to 500 mA (or even 1000 mA in some cases). Typical devices will operate down to 90 V AC, but since the first thing they do is rectify the input they often work just as well from DC as from AC.