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Trying to improve on the transformer efficiency, I changed the configuration of the rectifier from half wave to full-wave. Below shows the configurations I used enter image description here

Transformer turn ratio is enough so that the input side on the LDO is maintained well above drop-voltage + output voltage. The load on the LDO consumes about 130mA.

I was expecting twice the efficiency improvement, but the result is not what I expected. With secondary side current consumption remaining the same, I only saw 10mA improvements on the primary side.

With half wave rectification, the primary was consuming 110mA, and with full wave rectification, the primary was consuming about 100mA.

The transformer is very hot, and the hotter it gets, the current consumption grows more on the primary side.

Is there anything I am missing?

Addition :

I add the component value as following

The single diode is MRA4007, which has about 0.7V forward voltage The bridge diode is MB6S which has 1V voltage drop per bridge.

Transistor is simple PZT2222 which is simple BJT.

The pulse I am using is 18V when high, and 0 V when low.

The frequency driving is about 31KHz. I changed the frequency to 45KHz, but the current

consumption rather increased. I think there is some kind of loss when switching.

The resistor value, I tried using 3.3K to 10K Ohm, but i saw little improvements.

The capacitors are 10uF plus 0.1uF in parallel before the LDO.

I will try to measure inductance of the transformer and let you know

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    \$\begingroup\$ We need more details to provide a fuller answer. Add component values, load, switching frequency etc. \$\endgroup\$ Jun 30, 2015 at 11:39

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You haven't mentioned the load current, in addition to Kevin's list of missing information.

The problem could be :

  • primary inductance too low (not enough turns on the primary),
  • core saturation (too many turns on the primary) or
  • both at the same time.

Since you can't simultaneously increase and decrease the primary turns, the last of these requires either a higher switching frequency, a bigger core, a different core material, or a combination of all these.

I'm suspecting core saturation because of your comment that heating worsens the problem (you'll probably find core permeability reduces with increasing temperature, reducing the primary inductance thus increasing the current, causing thermal runaway). Does freezer spray reduce the current?

Once the basics are right you can further improve efficiency by (for example) increasing wire diameter if I^2*R losses are important, or interleaving primary and secondary windings if leakage inductance is too high, but you're not at that stage yet.

Better information as Kevin asked will help improve the answers. At least, tell us

  • the switching frequency,
  • the primary inductance (measure across both legs of primary with secondary OPEN circuit)
  • the leakage inductance (measure across both legs of primary with secondary SHORT circuit)

and anything you can about the core.

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    \$\begingroup\$ What frequency are you driving the circuit at? If the frequency or the inductance of the transformer is not appropriate there will be a large magnetizing current and high hysteresis losses in the transformer. Did you wind it yourself? How many turns, what core material, what are the dimensions of the core? What diodes are being used? Diodes such as 1N4001-1N4007 are very slow and will reduce efficiency. The half wave output rectifier will result in a DC bias in the transformer which can saturate it. The bridge rectifier on the output will drop a higher voltage and reduce the efficiency. \$\endgroup\$ Jul 1, 2015 at 4:21

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