Use of AC power transformer at the input os SMPS

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ysba

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Hi,

What do you guys think of using an AC power transformer at the input of a SMPS?

I know this idea may sound weird... but it came from the following:

We are working in a solution to replace a SMPS that fails very often. Since cost and size are not very important for this design, we thought in put an AC power transformer at SMPS's input. The transformer will feed the SMPS with half AC line voltage. Thus, if any voltage spike reaches the unit, it will be reduced by half at the SMPS. The idea is to protect the SMPS.

What do you think? Is this effective or unnecesary?

Thanks.
 

If you're sure that the failures are caused by voltage stress on the input, then it may work. Keep in mind that the SMPS will have to draw about twice as much current, operate at twice the duty cycle, etc. That may cause more problems.

If it's failing due to brief spikes on the AC line then a surge arrestor and filter is probably a far better solution.
 
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    ysba

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Hi,

What do you guys think of using an AC power transformer at the input of a SMPS?

I know this idea may sound weird
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Hi ysba

Who told that it may sound weird ?!



Sorry but you're going through the worst way ! The spike could be easily damped by using a lot of things e.g varistors , filters , surge current and instantaneous voltage protections using triac or ......



Best Luck
Goldsmith
 

I think it is weird because one of the main advantages of SMPS against AC transformer supplies is to reduce its size and weight. So using an AC transformer plus a SMPS seems a little pointless for me.
 

If you aren't doing a serious design, anything goes, but these days avoiding primary transformers is pretty standard to reduce cost, weight and size then there is a PFC encouraged in many countries.

Traditional 2-stage converters have independent control loops for the PFC stage and the step-down stage, allowing each converter output to be regulated on its own.
Look for a half-bridge resonant (HBR) converter topology with significant more efficiency and EMI advantages for the second stage conversion over a more conventional flyback topology.
 

We are using a flyback converter with PFC in single stage. It is a 12V 60W output SMPS.

It is a pretty serious design, but cost, size and weight are not so important.

The main reason for this thread is that my boss came with the transformer idea, and I don't agree with him. So I'm trying to find arguments to convince him to not spend extra work and time to build the transformers. He thinks that the transformer at the input will protect the SMPS stage from voltage transients. It may be true, but I think that varistors and line filters will do the job efficiently.

All your posts were good. I'll speak with him as soon as possible.

If any of you have another consideration, please let me know

Thanks a lot!
 

A good isolation transformer *sometimes* helps if your AC line has excessive common-mode noise, and a poor or non-existent earth connection.

This is most often caused by poor electrical installations in areas without proper electrical codes.

We once sold some equipment to semi-rural areas in a Central America country, and had to use an isolation transformer.
 

Using a line transformer is certainly not a solution I would consider first for an actual product.

What specific evidence do you have about the failure mode? How much headroom is there between the peak AC input voltage and the max tolerable input voltage of the supply?
 

Well, I used the info I got from this thread to argue with my boss, trying to convince him to not use a line transformer in order to avoid extra work and decrease product's size and weigth.

Our task is to remove the old SMPSs, and replace with a new one designed by us. The idea of the line transformer came from my boss, and he is not changing his mind. We actually don't know why exactly the old suplies failed, but he thinks this will make our design be more robust.

I guess I don't have much more to do. Anyway, thanks for all the answers!
 

Wait, so you're redesigning the SMPS, but he still wants to use a line transformer in the new design? That's awful.
 

Rather wouldn't it be still more easy t find why the SMPS fails frequently? while fed directly? More over SMPS supplies are widely used even in life saving medical equipment which are expected to work for a life time tackling all the AC line situations in various conditions.
 

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