AC to DC circuit switching using function generator, need help

jegandren · 5th August 2008, 02:54 PM

hi all,

From the circuit attached, lets say if the PFC ic is not connected to the AC to DC power stage. but the signal to the gate of the mosfet is given from a fuction generator of about 100Khz, i noticed the mains trips, suspecting the dV/dt would be high, so a snubber circuit is also attached. but yet the mains trips off. Actually when the swicthing occurs the circuit suppose to boost the voltage. please advise.

Thank You

Fob · 5th August 2008, 04:05 PM

This is PFC, not boost converter. The signal must be modulated with 50Hz sine wave pulses. If you reduse inductance to proper di/dt value then may achieve boost.

jegandren · 5th August 2008, 04:17 PM

yes it is a PFC indeed...but if we look at the topology its actually 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.

but before connecting to the pfc ic (UCC3817) but the signal to the gate of the mosfet is given from a fuction generator just for testing purpose of about 100Khz, i noticed the mains trips... why?
please advise

Fob · 5th August 2008, 04:27 PM

You have too large inductance for this frequency. She/he act like resistance wL

jegandren · 5th August 2008, 04:34 PM

so ur suspecting that the large inductance has caused the inductor to saturate and create a short circuit...so may be i should try with 100uH for a 100KHz switching frequency...is it if a lower inductance is used ..so it is lesser prone to saturation of the inductor. please advise.

Fob · 5th August 2008, 04:37 PM

How much power you need?. Uo, Io, fs

jegandren · 5th August 2008, 04:40 PM

output current = 6A
output voltage = 380V
switching frequency = 100 KHz

Fob · 5th August 2008, 04:53 PM

L = Ul*dt/di = ((380-270)*2.5uS)/(2*6A)=22uH

So you duty is 25% on 100kHz and DO NOT live this WITHOUT load!!!! You need current mode controll circuit.......

jegandren · 5th August 2008, 04:59 PM

hi ,

can i know where did you refer for this formula, yet im not very clear about it. sorry for the trouble mate..

Fob · 5th August 2008, 05:06 PM

The voltage acros inductor is proportional to speed of current change di/dt. This proportion is called inductance L. I suggest you to find some books and please dont play with 380 Volts before read them. This is a frendly advice, with all my respect to you. Please be carefull.

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5th August 2008, 04:05 PM	#2
Fob diyAudio Member Join Date: Jul 2008	This is PFC, not boost converter. The signal must be modulated with 50Hz sine wave pulses. If you reduse inductance to proper di/dt value then may achieve boost.

5th August 2008, 04:17 PM	#3
jegandren diyAudio Member Join Date: Feb 2008	yes it is a PFC indeed...but if we look at the topology its actually 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. but before connecting to the pfc ic (UCC3817) but the signal to the gate of the mosfet is given from a fuction generator just for testing purpose of about 100Khz, i noticed the mains trips... why? please advise

5th August 2008, 04:27 PM	#4
Fob diyAudio Member Join Date: Jul 2008	You have too large inductance for this frequency. She/he act like resistance wL

5th August 2008, 04:34 PM	#5
jegandren diyAudio Member Join Date: Feb 2008	so ur suspecting that the large inductance has caused the inductor to saturate and create a short circuit...so may be i should try with 100uH for a 100KHz switching frequency...is it if a lower inductance is used ..so it is lesser prone to saturation of the inductor. please advise.

5th August 2008, 04:37 PM	#6
Fob diyAudio Member Join Date: Jul 2008	How much power you need?. Uo, Io, fs

5th August 2008, 04:40 PM	#7
jegandren diyAudio Member Join Date: Feb 2008	output current = 6A output voltage = 380V switching frequency = 100 KHz

5th August 2008, 04:53 PM	#8
Fob diyAudio Member Join Date: Jul 2008	L = Uldt/di = ((380-270)2.5uS)/(2*6A)=22uH So you duty is 25% on 100kHz and DO NOT live this WITHOUT load!!!! You need current mode controll circuit.......

5th August 2008, 04:59 PM	#9
jegandren diyAudio Member Join Date: Feb 2008	hi , can i know where did you refer for this formula, yet im not very clear about it. sorry for the trouble mate..

5th August 2008, 05:06 PM	#10
Fob diyAudio Member Join Date: Jul 2008	The voltage acros inductor is proportional to speed of current change di/dt. This proportion is called inductance L. I suggest you to find some books and please dont play with 380 Volts before read them. This is a frendly advice, with all my respect to you. Please be carefull.

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