BRIDGELESS AC TO DC FLYBACK CONVERTER SIMPLIFIED

Sashanksasi B

hi,

we are working on something which should convert (300-450) AC voltage to dc voltage of 12V without using a rectifier circuit (no diodes connected to primary) (flyback with ac input with no rectifier circuit and no auxiliary coil only operated in buck mode with primary and secondary coil and the primary circuit must function only in the positive half cycle)

given components are a capacitor, a switch(n channel MOSFET or transistor), a flyback transformer, load ,

can you get me the design equations corresponding to this configuration of flyback transformer and capacitor and the input and the output equations

over 5 years ago

0 Benjamin Lough over 5 years ago

TI__Mastermind 27145 points

Hi Sashanksasi,

Please attach a schematic for the topology you are proposing.

Best Regards,

Ben Lough

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

Design equations of L1, L2, Co.Let the output power be Po, switching frequency be Fsw, primary and secondary currents be Ip, Is. input voltage is Vac, the output voltage is Vo. ripple voltages and currents are (delta-V and delta-I). duty cycle is D and the maximum duty cycle be Dmax. number of turns in primary and secondary is (Np, Ns)

0 Benjamin Lough over 5 years ago in reply to Sashanksasi B

TI__Mastermind 27145 points

Hi Sashanksasi,

This circuit will not work with an AC input unfortunately. The body diode of the MOSFET will conduct when the AC input is negative and it will place the AC voltage amplitude across the MOSFET body diode + the primary winding in parallel with the RCD clamp. At 300Vac to 450Vac, this current will be quite large and likely destroy the power stage. Any particular reason for not using a diode bridge?

Best Regards,

Ben Lough

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

sir if I use a switch with PWM pulse and duty ratio always less than 50%(only buck mode) will it works?

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

basically we are trying to reduce the components as low as possible and is the design equations for components are same as with dc?

0 Benjamin Lough over 5 years ago in reply to Sashanksasi B

TI__Mastermind 27145 points

Hi Sashanksasi,

Unfortunately it will not work even if the duty cycle is <50%. You will still have body diode conduction during the negative portion of the AC line cycle. The equations for change slightly when going from DC input to AC input but they are largely the same. It depends on why type of flyback you use (QR, DCM, CCM, AM-FM modulation, etc).

Best Regards,

Ben Lough

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

I wish to operate it in CCM, and how to mitigate the effects of body diode conduction, is there any other basic devices with which I can operate this circuit, and I would be thankful to you if you can tell me changes to be made to make this circuit work. Thank you sir

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

can you just post me the design equations of the components(L1,L2,Co) for my circuit

0 Benjamin Lough over 5 years ago in reply to Sashanksasi B

TI__Mastermind 27145 points

Hi Sashanksasi,

The most direct method to make the circuit work is to include the diode bridge. For the power stage calculation I suggest using the power stage designer tool. You can calculate your power stage for CCM flyback using the tool:http://www.ti.com/tool/POWERSTAGE-DESIGNER

The calculations used by the tool can be found here:https://www.ti.com/seclit/ug/slyu036/slyu036.pdf

Best Regards,

Ben Lough

0 Sashanksasi B over 5 years ago in reply to Benjamin Lough

Prodigy 80 points

sorry sir, but the main constraint to our project is to remove the diode bridge and form the circuit and operate it in only buck mode. so if you could help me solve this I would be happier. thank you, sir. my main problem is to find the governing design equations for the components in my topology

0 Benjamin Lough over 5 years ago in reply to Sashanksasi B

TI__Mastermind 27145 points

Hi Sashanksasi,

I'm sorry about this circuit will not work without some form of rectification on the primary.

Best Regards,

Ben Lough

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BRIDGELESS AC TO DC FLYBACK CONVERTER SIMPLIFIED