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

[Walter Lolham]

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.

 

[Mac]

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

 

[Walter Lolham]

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

 

[John Woodgate]

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

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?

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.

 

[Frank Bemelman]

I read in sci.electronics.design that Walter Lolham
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.
--

You could squeeze out a bit more using schottkey diodes

 

[ChrisGibboGibson]

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

 

[Allan Herriman]

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.

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:
http://www.opussolutions.com/modules.php?name=News&file=article&sid=23

Regards,
Allan

 

[Frithiof Andreas Jensen]

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).

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

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.

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

see f.ex. http://www.morex.com.tw/60dcpower.htm - and note that the input to
the DC/DC board is indeed 12 V DC

That's why I want to just modify the existing one and adapt
it to work with a 12 volt power source.

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

 

[Walter Lolham]

Frithiof Andreas Jensen wrote:
....

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.

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.