LTC3108 - Need help understanding the schematic

[shawnmk123]

Hello,

I have some difficulty understanding how the LTC3108 works.

Referring to the circuit in the first page of the datasheet, a TEG supplies power to the LTC3108 IC through a transformer. But I don't understand how a DC voltage (The TEG gives DC voltage as its output) can be transformed using a transformer, since transformers need AC input?

Please help.

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I understand it as the MOSFET is controlled from the secondary side through the capacitors and inductance of the Transformer, both of which decide the timing (or frequency of switching).

But that itself poses a difficult question: How did they achieve this in such a small space? I mean the FET is in the primary side but the Gate drive is on the secondary side. Does that not mean they need something to drive the MOSFET which is connected to the primary winding, something like an opto-coupler?

 

[mtwieg]

Hello,

I have some difficulty understanding how the LTC3108 works.

Referring to the circuit in the first page of the datasheet, a TEG supplies power to the LTC3108 IC through a transformer. But I don't understand how a DC voltage (The TEG gives DC voltage as its output) can be transformed using a transformer, since transformers need AC input?

The switching of the internal FET makes it so that their is an AC voltage across the transformer (there is also a DC bias current which is not transferred to the secondary).

I understand it as the MOSFET is controlled from the secondary side through the capacitors and inductance of the Transformer, both of which decide the timing (or frequency of switching).

But that itself poses a difficult question: How did they achieve this in such a small space? I mean the FET is in the primary side but the Gate drive is on the secondary side. Does that not mean they need something to drive the MOSFET which is connected to the primary winding, something like an opto-coupler?

The primary and secondary of the transformer are not isolated (the purpose of the transformer is to step up the TEG voltage, not isolate it), so no optocoupler is necessary.