Question: simple inverter to fire up a switch mode power supply?

I read in sci.electronics.design that Walter Lolham wrote (in ) about 'Question: simple inverter to fire up a switch mode power supply?', on Tue, 14 Dec 2004:

Diode/cap voltage multipliers are horribly inefficient at the multiplication factors you would need. In cat, a Cockroft-Walton multiplier will not multiply at all more than 14 times. The accumulated voltage drops across the diode strings reduce the multiplication factors of additional stages to 1.

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Multipliers are very inefficient.

Transformers can be one of the most efficient devices around. 95% is very common. 98% is quite easily achievable.

You're barking up the wrong tree. A simple high frequency inverter giving the required DC output voltage is your best bet.

Gibbo

"John Woodgate" schreef in bericht news: snipped-for-privacy@jmwa.demon.co.uk...

You could squeeze out a bit more using schottkey diodes ;)

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Thanks, Frank.
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Hi there,

I am not an electronics guru, just know enough to be dangerous and need some help on the following.

In a switch mode power supply, in the first stage the 110/240 volts mains power goes into a bridge rectifier and becomes DC and gets filtered by a large capacitor giving around 150 -> 350 volts DC.

Now to use such a power supply say with a 12 volt battery, an inverter would be the easiest solution. But since inverters are not very efficient (heat, loss in the transformer etc), do you think if the following solution could work?

If I oscillate the 12 volt (similar to an inverter), but rather than put it across a step up transformer, instead I feed it into a diode/cap bridge to multiply the voltage N times to give me the required voltage (say around 180 volts) to charge the large cap in the primary stage of my switchmode power supply. Given the current is sufficient, do you think it could work?

If it does, advantage is that it will be much simpler, cheaper and smaller to build inside the box, on top of being more efficient. Secondly when there is no load or if the load is minimal, battery consumption will be next to nothing as opposed to an inverter which still has to chop the primary of a relatively large transformer, which in theory is a load in itself.

Any thoughts/suggestions would be appreciated. If you know of a suitable circuit that you could point me to, I'll appreciate it even more.

Regards, Walter.

Problem is that the only DC to DC converters that I've seen in the shops converts between 12volt and 24volts (suitable for truck equipment). It's even bigger than an inverter and just as expensive if not more.

The switchmode power supply that I have in mind is like a PC power supply and has many voltages, so it's not easy to build another one altogether. That's why I want to just modify the existing one and adapt it to work with a 12 volt power source.

Thanks anyway. Walter

Wrong Shops - Remember, If *you* have a problem there is a large probability that others suffer the same, therefore often a solution exists.

*IS* is a PC supply? One can buy those for running off the lump-in-the cord power supplies.

see f.ex.

- and note that the input to the DC/DC board is indeed 12 V DC

That's much harder than building a new one!!

Since the OP is clearly not a power supply designer, simply buying an automotive PC supply is probably the best bet. IMO.

Here's an example:

Regards, Allan

- easiest is to get an evaluation board and hack that. that's what they are for.

The simplest approach is to use a Boost or Flyback topology designed for the voltage where the main power is taken and then supply the additional voltages from Auxilliary windings on the core. They will track within a few %.

You need to specify the currents AND the voltages - it's very important, the

33 V f.ex. could be for varactor tuning, pulling at most 10 mA, in which case a capacitive step-up ic may be all one needs and 13.5 - 18 smells of Internal Regulator - in which case you dont really have to do it.

Good way of getting killed and/or blowing stuff up - Use a Lump-in-the-cord or Wall wart for the 240; that way someone will not plug this thing in with the supply set to the wrong voltage and *you* don't have to worry about safety isolation and the insurance not paying up.

Frithiof Andreas Jensen wrote: ....

Thanks for all the suggestions, they were all definitely very helpful. The power supplies suggested seem to be for PC applications. My appologies for not clarifying. The equipment I need power for is a satellite receiver which has to supply power to the LNB between 13.5 and

18 volts. If I remember correctly, there is a ~ +33 volt comming out of the power supply, which a normal PC power supply doesn't have.

If I don't find a suitable power supply, I may have to do some modifications, or use 2 power supplies.

The other reason I wanted to utilize the existing power supply was so that I could easily toggle between a 12VDC or 240VAC power source by simply flicking a 2 pole switch. By using another power supply, this will become a bit more difficult to do.

Thanks again for all replies.

Cheers,

-- Walter

Frithiof Andreas Jensen wrote: ....

What I meant is that if I was going to do it that way, there'd be 2 plugs. The existing one which is 240VAC and an additional plug for the

12VDC. The switch would only allow one of the power sources, so even if both power sources are connected, they'd isolated at the switch and only one of them would be able to provide power at any time.

Thanks heaps for your suggestions.

Cheers, Walter.