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Current drawn by FET gate drive in buckboost LED driver?

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grizedale

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Hello,

Please help me find Current required to drive FET in buckboost converter

I am using SiS892DN mosfet in 1MHz buckboost converter LED driver.

Vin = 27V
Vout = 58V with respect to ground (though leds are returned to vin)
Fsw = 1MHz
Controller = LT3755
FET gate drive voltage = 7.15V
Iled = 0.45A (string of eight leds)

FET DATASHEET:
https://www.vishay.com/docs/66590/sis892dn.pdf

..The datasheet does not specify Qg for my gate drive voltage of 7.15V. Also, the datasheet gives Qg for 50V = Vds, but mine is 58V.

So do you know what the current suppyling the gate drive will be...or rather , do you know what will be the "particular" Qg for my particular application?

I need to find out so i can check the controller (LT3755) does not overheat.
 
Last edited:

I'm gonna take a wild shot at this.

The datasheet appears to give Qg for Vds = 50V, Vgs = 10V and Vds = 50V, Vgs = 4.5V where the gate charge is typically 14.2nC and 6.7nC respectively. It can probably be assumed that the gate charge in your case (7.15V) will result in a Qg somewhere there between.

Since coulombs are C = IxT. And let's say we need to discharge a worst case ~14.2nC in a time period of 1% of the switching period (1 Mhz) which is 10e-8 then we'd need 1.46A during that period to discharge 14.2nC.

I'm not familar with exactly how fast you need to switch the mosfet in order to minimize switching losses but maybe this will help.
 

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