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

From: Walter Lolham (wlolham_at_civica.com.au)
Date: 12/14/04 

Date: Tue, 14 Dec 2004 17:53:24 +1100

Mac wrote:
> On Tue, 14 Dec 2004 15:42:40 +1100, Walter Lolham wrote:
>
>
>>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.
>
>
> I think you could replace the switch mode power supply altogether with an
> off-the-shelf DC to DC converter. This would be the easiest thing for you
> to do.
>
> --Mac
>

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

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