Offline full-bridge SMPS....need help - Page 2

Tekko · 7th December 2005, 12:22 PM

You need to make like in the ESP car smps schem, use a npn and pnp bipolar transistor, one to turn the fets on and one to discharge the gate capacitance to allow the fets to switch as they should, otherwise it will not work. The SG3525 cannot drive the fets directly as its open collector and too low current handling.

Also you need to adjust the voltage feedback for the desired output, this is easiest done with a pot.

sagarverma · 7th December 2005, 12:25 PM

Quote:

Originally posted by Tekko
You need to make like in the ESP car smps schem, use a npn and pnp bipolar transistor, one to turn the fets on and one to discharge the gate capacitance to allow the fets to switch as they should, otherwise it will not work. The SG3525 cannot drive the fets directly as its open collector and too low current handling.

Also you need to adjust the voltage feedback for the desired output, this is easiest done with a pot.

his design omits bjt's.he says sg is capable to directly drive gates(100 ohm res req.).i used that design and smps is happily working in car.

Nejc_car · 7th December 2005, 01:05 PM

Do I have any other choise, but to use trafo for driving gates

sagarverma · 7th December 2005, 01:09 PM

use ESP design,good and works.

Nejc_car · 7th December 2005, 01:13 PM

If you mean for car I allready have it.

sagarverma · 7th December 2005, 01:45 PM

yes.

Eva · 7th December 2005, 01:49 PM

You all have misunderstood the gate drive issue and the point of the whole thread. We were talking about high side drive in a full bridge off-line application, not about gate buffering nor 12V SMPS.

Obviously, such high side MOSFETs require a floating drive scheme, they can't be directly connected to a SG3525.

sagarverma · 7th December 2005, 02:08 PM

Quote:

Originally posted by Eva
You all have misunderstood the gate drive issue and the point of the whole thread. We were talking about high side drive in a full bridge off-line application, not about gate buffering nor 12V SMPS.

Obviously, such high side MOSFETs require a floating drive scheme, they can't be directly connected to a SG3525.

thanks for making this clear.

Nejc_car · 7th December 2005, 02:19 PM

Then my only option is to use trafo for driving gate's of the Mos-Fet's. Is it ok to use turn of 20:20+20 for gate trafo.

Eva · 7th December 2005, 04:12 PM

When designing such a transformer, leakage inductance should be minimised while maximising coupling between windings.

In practice, this is achieved by using the minimum number of turns that guarantees absence of core saturation for the operating frequency, voltage and temperature range, while filling the available width of the coil former (not including creepages). Sometimes, it's useful to wind bifilar or to split the primary in two (series or paralell) halves with the secondaries placed inbetween, etc...

The previous recommendations also apply to power transformers. You should figure out the optimum core size and magnet wire diameter.

The reason for a low leakage inductance is required in gate-drive transformers is that MOSFET turn-off times depend directly on it. When gate buffering is employed leakage inductance is no longer critical, though, but at the expense of an increased component count.

7th December 2005, 01:09 PM	#14
sagarverma diyAudio Member Join Date: Feb 2005 Location: NEW DELHI	use ESP design,good and works. __________________ Never Underestimate The Bandwidth of A Station Wagon Full Of Magnetic Tapes Hurtling Down The Highway...................Andrew S Tanenbaum

7th December 2005, 01:45 PM	#16
sagarverma diyAudio Member Join Date: Feb 2005 Location: NEW DELHI	yes. __________________ Never Underestimate The Bandwidth of A Station Wagon Full Of Magnetic Tapes Hurtling Down The Highway...................Andrew S Tanenbaum

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7th December 2005, 12:22 PM	#11
Tekko diyAudio Member Join Date: Jan 2005	You need to make like in the ESP car smps schem, use a npn and pnp bipolar transistor, one to turn the fets on and one to discharge the gate capacitance to allow the fets to switch as they should, otherwise it will not work. The SG3525 cannot drive the fets directly as its open collector and too low current handling. Also you need to adjust the voltage feedback for the desired output, this is easiest done with a pot.

7th December 2005, 01:05 PM	#13
Nejc_car diyAudio Member Join Date: Jan 2005 Location: ljubljana	Do I have any other choise, but to use trafo for driving gates

7th December 2005, 01:13 PM	#15
Nejc_car diyAudio Member Join Date: Jan 2005 Location: ljubljana	If you mean for car I allready have it.

7th December 2005, 01:49 PM	#17
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	You all have misunderstood the gate drive issue and the point of the whole thread. We were talking about high side drive in a full bridge off-line application, not about gate buffering nor 12V SMPS. Obviously, such high side MOSFETs require a floating drive scheme, they can't be directly connected to a SG3525.

7th December 2005, 02:19 PM	#19
Nejc_car diyAudio Member Join Date: Jan 2005 Location: ljubljana	Then my only option is to use trafo for driving gate's of the Mos-Fet's. Is it ok to use turn of 20:20+20 for gate trafo.

7th December 2005, 04:12 PM	#20
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	When designing such a transformer, leakage inductance should be minimised while maximising coupling between windings. In practice, this is achieved by using the minimum number of turns that guarantees absence of core saturation for the operating frequency, voltage and temperature range, while filling the available width of the coil former (not including creepages). Sometimes, it's useful to wind bifilar or to split the primary in two (series or paralell) halves with the secondaries placed inbetween, etc... The previous recommendations also apply to power transformers. You should figure out the optimum core size and magnet wire diameter. The reason for a low leakage inductance is required in gate-drive transformers is that MOSFET turn-off times depend directly on it. When gate buffering is employed leakage inductance is no longer critical, though, but at the expense of an increased component count.