help needed on attached circuit

jegandren · 17th July 2008, 04:47 PM

Hi all,

Attached is a AC - DC circuit with a power factor corrector IC. From the attachment when a pulse is given to the gate of the MOSFET, so current would flow from drain to source. My question is wouldn't it short the circuit and lead to fatal circuit blow.

TechGuy · 17th July 2008, 10:07 PM

Not unless the controller causes the inductor to saturate. When the Mosfet is switched on, the inductor resists the flow of current, limiting that amount of power drawn. The inductor will restrict current flow until it reaches saturation. So as long as the mosfet is switched off before the inductor saturates, it will operate without a problem.

jtkraj · 21st July 2008, 11:11 AM

If you go through the Buck-Boost topology. you will understand and have basic idea about how these kind of circuit operates.

you will find a hell lot of articles in Web to learn about the PFC operation.

golam · 21st July 2008, 02:57 PM

Hi Jegandren,

No, the circuit will not be shorted. The schematic you have given is the schematic of a Non-isolated flyback circuit. The non-isolated flyback turns on the MOSFET for a fraction D of the switching period, which, since it produces a voltage across the inductor, causes current to ramp up, storing energy in the inductance. When the MOSFET turns off, the inductor current goes through the diode and into the output capacitor and load. The non-isolated flyback should have a single output (there is no way to make more than one). The output is not isolated from the input and the output cannot be made less than the input voltage - even if you turn the MOSFET completely off, the output will equal the input (minus a diode drop).

Frequently people call the non-isolated flyback a 'boost' converter, which is not appropriate.

Thanks.

mflorin · 22nd July 2008, 05:59 PM

If you're going to use L4981A, then check controller's datasheet and all available application notes (www.st.com). You'll find all the information you need (including basic stuff).
See also the PFC free software that will help you design a PFC stage based on L4981A.

21st July 2008, 02:57 PM	#4
golam diyAudio Member Join Date: Jun 2008	Hi Jegandren, No, the circuit will not be shorted. The schematic you have given is the schematic of a Non-isolated flyback circuit. The non-isolated flyback turns on the MOSFET for a fraction D of the switching period, which, since it produces a voltage across the inductor, causes current to ramp up, storing energy in the inductance. When the MOSFET turns off, the inductor current goes through the diode and into the output capacitor and load. The non-isolated flyback should have a single output (there is no way to make more than one). The output is not isolated from the input and the output cannot be made less than the input voltage - even if you turn the MOSFET completely off, the output will equal the input (minus a diode drop). Frequently people call the non-isolated flyback a 'boost' converter, which is not appropriate. Thanks. __________________ Microcontroller is the future in Electronics. মাইক্রোকন্ট্রলারই ইলেক্ট্রনিক্সের ভবিষ্যৎ।

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17th July 2008, 10:07 PM	#2
TechGuy diyAudio Member Join Date: Apr 2006 Location: Connecticut	Not unless the controller causes the inductor to saturate. When the Mosfet is switched on, the inductor resists the flow of current, limiting that amount of power drawn. The inductor will restrict current flow until it reaches saturation. So as long as the mosfet is switched off before the inductor saturates, it will operate without a problem.

21st July 2008, 11:11 AM	#3
jtkraj diyAudio Member Join Date: Jun 2008	If you go through the Buck-Boost topology. you will understand and have basic idea about how these kind of circuit operates. you will find a hell lot of articles in Web to learn about the PFC operation.

22nd July 2008, 05:59 PM	#5
mflorin diyAudio Member Join Date: Jun 2005 Location: IASI	If you're going to use L4981A, then check controller's datasheet and all available application notes (www.st.com). You'll find all the information you need (including basic stuff). See also the PFC free software that will help you design a PFC stage based on L4981A.

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