**Part Number:** UCC38C43

Hi there,

I consider to use UCC38C43 in simple flyback design to drive Vishay mosfet SIR872A, and have some questions about how can we calculate the peak and average gate current of the mosfet.

Design Spec: Vdrv(3843) =12V, Fs=80KHZ, Rgate=10ohm, Vin=20V, Vout=60V, turns-ration=1:1. DCM. Duty cyle = 10%~40%.

The breif sch of drive part and mosfet part is as below :

I learned that driving mosfet is mainly about quickly provide Qg(tot) and peak current capability is important for driver IC selection. Here are some of my thoughts and calculation on peak and gate current.

1. The max. current should be limited by R(Hi_3843) + Rgate, approx. 12V/(10ohm+10ohm) = 0.6A.

2. Igate(average) ≈ Q(tot) * Fs = 47nC*80Khz = 3.76mA, which means 3843 itself actually provides quite small power, so most of the transit large gate drive current (charges) should be provided by C_bypass of 3843 VDD pin. Based on this conclusion, if 3843 VDD pin is not properly decoupled, 3843 may not capable of outputing large peak current as high as 1A.

3. by TI slua618 equation(18), we can calculate C_bypass value without knowing the gate peak current. C_bypass = [(0.1mA*40%/80KHZ) + 47nC] / (5%*12V) = 80nF, so 1uF ceramic should be far enough.--------------------Note from Colin:Vrop=100mV ; and Make sure that you use ceramic caps with a low votlage co-efficient of capacitance - parts with V > 25 rating should be ok.

4. Finally, one of my confusion is, **how shold we calculate (or estimate as precise as possible) the Igate(peak) value? Or is it necessary to do this ?**

Someone on internet recommends taking 1/100*Ts as the turn-on **transition** process time t-on, then derive Igate(peak) = Q(tot) / ton_transistion, and I think we should also minus the mosfet turn-on delay time (or rising time??) based on this.

In my design this would be Igate(peak) ≈ 47nF/ [0.01*12.5us - 30ns(max)] = 0.497A < 0.6A < 1A(3843 capability). **But I am not sure whether this " 1/100*Ts ≈ turon on transition time" is reasonable or not, in other word, how could we determine the transition time requirement? **

5. Ps: One Ti note slua882 does mentioned a simplified way to get Igate(peak), but I think the assumption and simplifications may be kind of rough....

I hope TI experts could help review my thinking process of peak and gate gate current, any corrections and advice are appreciated. Thanks!