buck converter

[Axher]

Hi, I'm in the design process of a non-isolated buck converter to get 70Vdc from the 115vac line and I have some trouble controlling my mosfet. I'm trying to control the mosfet from a microcontroller using a hcpl-3180 from avago technologie. The mosfet is on the high side, so I need a floating supply. Anyone can suggest a schematic how to connect it or suggest me another way of doing it thanks

 

[Skogsgurra]

Look for boot-strap circuits. They are used to pump up the gate driver's voltage. Should be in any basic appnote on switchers. Just google for it.

Gunnar Englund

http://www.gke.org

 

[Axher]

Do I need a boot-strap circuits if I use a isolated 15v source for the mosfet gate driver ?

 

[Skogsgurra]

No. Not if the supply has enough voltage and can be tied to the MOSFET source. But then, I wouldn't call that floating. It is on mains voltage, more or less.


Gunnar Englund

http://www.gke.org

 

[LionelHutz]

You don't need a full floating supply. You likely have or can easily create a non-isolated 15V that's connected to the -ve side of the converter input. Basically, you put a diode from the 15V to the Vcc pin, cathode to Vcc. Then, stick a fairly large capacitor between Vcc and Vee. Any time the MOSFET is off its source is basically at the common potential so the capacitor gets charged. Then, you use the cap as the power source when you turn the MOSFET on.

Look up International Rectifier high voltage gate drivers for a better understanding of what I'm trying to describe.

 

[Skogsgurra]

Yes Lionel. That's a boot-strap circuit.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

The IR21** series of IC's is a nice way to make the problem go away - takes care of the interfacing for you. Worth a look on the International Rectifier website.


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I don't suffer from insanity. I enjoy it...

 

[LionelHutz]

Yes Lionel. That's a boot-strap circuit.

Wow scogsgura, I'm sorry I actually took the time to explain how the circuit works instead of leaving it up to Axher to find the right bootstrap circuit example. There's only dozens of applications for a "bootstrap" circuit.

Axher - to answer your last question. You would not need the bootstrap circuit if your isolated supply is available before the converter begins to operate. Otherwise, you would need it. But, the circuit can operate off of the bootstrap circuit alone so the isolated supply is somewhat overkill.

IR pushes this particular circuit configuration with their driver IC's but it could be done just as easily with that HCPL-3180 part. The IR parts have a number of app notes and voltage limits that you have to pay attention to or you can run into trouble applying their driver IC's.

 

[Axher]

Thanks everyone,

so if I understand correctly if I want to use the HCPL-3180 (

http://www.avagotech.com/products/product-detail.jsp?navId=H0,C1,C5231,C4945,C4988,P93297

) instead of using a IR2117 and adding a extra optocoupler to isolated my microcontroller I would have to had a bootstrap to my circuit to use it on the high side right ?

 

[LionelHutz]

The HCPL-3180 is an isolator and driver IC. If you add a diode and capacitor then that is all you need to have a complete floating driver circuit.

What is the extra optocoupler isolating?

I'm just suggesting you look at the IR parts to see what they're doing if you don't understand how to get your floating power supply.