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sg3524 flyback conveter using external mosfet

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s.manikandan

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hi,

i have to drive the mosfet through sg3524 emitter1 & emitter2 to get 90% duty cycle by shorting two collector terminal and two emitter terminal together. collector powered by input power 12v and emitter is connected to gate terminal of the mosfet through gate resistor directly.

is that right by connecting sg3524 emitter terminal to gate of the mosfet.....?
 

FvM

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is that right by connecting sg3524 emitter terminal to gate of the mosfet.....?
Off-switching speed will be limited by the emitter pull-down resistor. For fast switching, a gate driver stage may be reasonable.
 

Tahmid

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For high frequencies, connect a driver stage instead of driving the MOSFET directly, as FvM has said above.

What frequency are you using?

You may use a simple totem pole driver, a diode-PNP combination, or a dedicated gate driver, such as TC427.
 

s.manikandan

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For high frequencies, connect a driver stage instead of driving the MOSFET directly, as FvM has said above.

What frequency are you using?

You may use a simple totem pole driver, a diode-PNP combination, or a dedicated gate driver, such as TC427.

Sg3524 controller gives 12v output and 100 mA drive current . This is much enough for our mosfet switch, because the charge is 6nC. so the controller can able to give its average current and as per datasheet if we put 10ohms gate resistor ,we can get 1.2A peak current .

we are thinking to reduce the cost as much and size of the power supply SINCE ITS A OPEN CIRCUIT ,FIXED DUTY CYCLE FLYBACK CONTROLLER .....

NEED YOUR SUGGESTIONS
 

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Sg3524 controller gives 12v output and 100 mA drive current . This is much enough for our mosfet switch, because the charge is 6nC. so the controller can able to give its average current and as per datasheet if we put 10ohms gate resistor ,we can get 1.2A peak current.
I wonder which circuit you imagine, partcularly where you get -100 mA (or even -1.2A) during switch-off. Tahmid suggested two possible solutions, but you're apparently thinking of passive components only. What's the value of your pull-down resistor? Or are you omitting it at all?

P.S.: I also wonder which SG3524 specifies 1.2A peak current? Mine doesn't.
 
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Tahmid

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Sg3524 controller gives 12v output and 100 mA drive current . This is much enough for our mosfet switch, because the charge is 6nC. so the controller can able to give its average current and as per datasheet if we put 10ohms gate resistor ,we can get 1.2A peak current .
How?

we are thinking to reduce the cost as much and size of the power supply SINCE ITS A OPEN CIRCUIT ,FIXED DUTY CYCLE FLYBACK CONTROLLER .....

NEED YOUR SUGGESTIONS
Like already mentioned above, you'll need to use a driver. Maybe you can get the MOSFET to switch on quickly enough. But, what about switching off? Without a driver, switching off will be dependent on the pull-down resistor. Since cost is such a large factor, use a discrete solution: totem-pole or diode-transistor combination. It wouldn't add to the cost much. And it's essential.

If it's possible, you might look into switching to SG3525 and you may be able to omit the output driver since the SG3525 has internal totem-pole drivers.

By fixed duty cycle, do you mean open-loop (with no feedback)?

Since you're using flyback, why not use a controller such as UC3845? As far as I know, it's cheaper than SG3524 and you can easily use it for flyback converter.

Hope this helps.
Tahmid.
 

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