App. note SLUP206 - Efficiency calculation of a synchronous buck

Mohammed Rahim

Hello team,

Customer is referring to slup206 to calculate the switching power loss of a buck converter, but can't duplicate the results given in the appnote.

In Table 2 on page 16, the switching loss of a switch MOSFET is given (e.g. 0.364W for the Si4836DY MOSFET). But using the equation for PSWswitching on page 9 gives a result of 18W. He thinks the error may be in calculating Ig, which he calculated as Qg*Fs.

PSWswitching = Vin*Fs*(ISWpk*(QgdSW + QgsSW)/Ig + (QossSW + QossSR)/2)

PSWswitching = 3.3*600k*(10.96*(5.8n + 4.44n)/(20n*600k) + (9.24n + 9.24n)/2) = 18W

Could it be something else? I appreciate any help on this.

Thanks in advance,

Mohammed

over 13 years ago

0 Na Kong over 13 years ago

TI__Genius 17240 points

Hi Mohammed,

I think the caluclation of Ig is not right. Qg*Fs is the average gate charging current. However, in the aforementioned equation, (QgdSW+QgsSW)/Ig is used to calculate the turn-on and turn-off time (switching time) of the high-side MOSFET.

Therefore, Ig = (Vgtdrv - Vplateau)/(Rghi + Rg + Rgi), where Vplateau is the miller plateau voltage of Vgs.

Regards,

0 Mohammed Rahim over 13 years ago in reply to Na Kong

TI__Prodigy 575 points

Hello Na,

Thanks for the quick response. Just a follow-up question: how do we find Rghi and Rgi? I can't seem to find these values in the gate controller/MOSFET datasheet. Are these just estimated values? Sorry if I'm asking an obvious question.

Thanks,

Mohammed

0 Na Kong over 13 years ago in reply to Mohammed Rahim

TI__Genius 17240 points

Hi Mohammed,

Rghi generally is given in the datasheet of the driver or controller IC which has driver integrated.

Rgi generally is given in the datasheet of the MOSFETs and the symbol probably is RG for series gate resistance.

Regards,

0 Mohammed Rahim over 13 years ago in reply to Na Kong

TI__Prodigy 575 points

Thanks, Na. I assumed the Rgi was separate from the Rg (referring to Fig. 11 in the app note).

Regards,

Mohammed

0 Na Kong over 13 years ago in reply to Mohammed Rahim

TI__Genius 17240 points

HI Mohammed,

My understanding, Rg is the external gate resistance. Sometimes, Rg is added intentionally to slow down the turn-on and turn-off speed of the FET.

Regards,

0 Mohammed Rahim over 13 years ago in reply to Na Kong

TI__Prodigy 575 points

Ok, I see what you mean. Thanks a lot for your help, Na.

Regards,

Mohammed

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App. note SLUP206 - Efficiency calculation of a synchronous buck