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 15:42:40 +1100

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.

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