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

From: ChrisGibboGibson (chrisgibbogibson_at_aol.com)
Date: 12/14/04 

Date: 14 Dec 2004 10:34:37 GMT

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.
>

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

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