[SOLVED] High power, small transformer?

Hi
Why is/are the transformer(s) in a 300w pc atx psu way smaller (and probably less expensive) than a 30v 1a (30w) transformer from my electronics shop?

Cecemel

Because it's a 300W power supply, not a 300W transformer.

Because the frequency used in the switching power supply is not 50 Hz, but much, much higher allowing a much smaller mass transformer core.

None of these PC SMPS's can take 300W power continuously it can hardly take 80-100W for a longer duration. Normal CPU's will not consume more than 100W continuously. And since it is high frequency small size transformer is sufficient.

None of these PC SMPS's can take 300W power continuously it can hardly take 80-100W for a longer duration.

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Just ignorant. ATX specification requires full load operation in a 10 to 50 °C ambient temperature range. If you have a no-name power supply not meeting the specifications, dump it in the electronic waste container.

Then how do they do it? Just feed rectified mains into the smps? If they aren't 300w transformers, what are they used for?

Hi,

Read the posts before.
The SMPS uses high frequency.
The higher the frequency, the smaller the transformer.

SMPS additionally uses other method(s) than feeding sine wave input.
Read about SMPS basics.

Klaus

KlausST said:

Hi,

Read the posts before.
The SMPS uses high frequency.
The higher the frequency, the smaller the transformer.

SMPS additionally uses other method(s) than feeding sine wave input.
Read about SMPS basics.

Klaus

Click to expand...

I know, but smps's are dc-dc right? So where does it get that 300w dc from?

Cecemel said:

I know, but smps's are dc-dc right? So where does it get that 300w dc from?

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The 300 Watts comes from multiplying the output voltage with the maximum output current. If multiple outputs, each output wattage is calculated then summed up together.

RobertFalbo said:

The 300 Watts comes from multiplying the output voltage with the maximum output current. If multiple outputs, each output wattage is calculated then summed up together.

Click to expand...

So the combined power of the transformers on the board is 300w, al their primaries are in parallel, connected to mains. And their secondaries go to separate smps's?

Cecemel said:

So the combined power of the transformers on the board is 300w, al their primaries are in parallel, connected to mains. And their secondaries go to separate smps's?

Click to expand...

No. In an SMPS, the mains voltage - 110 or 230V AC - is first converted to DC. The DC voltage is then converted to AC again, but at a much higher frequency. The high frequency AC is fed to a high-frequency transformer which has separate secondary windings for the various output voltages required. The outputs from the secondaries go to their own high-frequency rectifiers and filter circuits.

All of this involves complex circuitry and may seem pointless to the uninitiated. The main reason is that, at high frequencies, the transformer and filter capacitors can be much smaller, lighter and less expensive. By comparison, a 50/60Hz 500-watt transformer would be huge and weigh several kilos.

Another point worth noting is that regulation of the output voltages can be integrated into the AC-DC-AC conversion process.