Author Topic: voltage doubler for AC adapter  (Read 459 times)

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Offline rocmelTopic starter

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voltage doubler for AC adapter
« on: May 04, 2020, 06:56:01 am »
I need a 30v adapter but my junk box only has up to 24v
So I hacked a HP 18V adapter made by Astec (it's actually 17v) in order to get the 30V out

It will be used to charge 25.2V lithium battery pack.
I don't know why they used parts in parallel.
I added the circuit shown shaded. I traced the circuit and & left out some ceramic caps.
The diode is from EHT supply, high speed from an old TV.
Can someone tell me why I'm getting 32v at the added circuit.

 

Offline capt bullshot

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Re: voltage doubler for AC adapter
« Reply #1 on: May 04, 2020, 09:50:00 am »
Most probably it's because your SMPS uses the flyback topology.
With this topology, energy is charged into the magnetic core from the primary and then discharged into the secondary. Voltage regulation is done by adjusting the amount of transferred energy. While the charging phase, the full rectified input voltage is applied to the primary, and the secondary rectifier blocks that voltage. Your additional rectifier works the other way round, so your circuit receives the primary voltage multiplied by the transformer ratio. In a flyback converter, this voltage is not regulated nor has anything in common with the regulated output, it just depends on the input voltage and the transformer ratio (plus ringing).
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