Hi All-
I'm creating a circuit to handle an 8A charge current with a BQ24650 chip, and as part of design verification, I want to be sure I'm calculating the switching losses of the FET correctly. To be sure I have it correctly, I've calculated the switching losses here for the 'typical' switching circuit used in TI's EVAL board.
Are these calculations correct?
Thanks!
Parameters from the design, MOSFET, and BQ24650:
| Design Parameters | Value | MOSFET Parameters | Value | BQ24650 Parameters | Value |
| Output Inductance | 10uH | Rdson | 27 mOhms | Vregn | 6V |
|
Output Capacitance |
15uF | Qg | 4.9nC | Ron | 6 Ohms |
| Switching Freq, Fs | 600kHz | Qgd | 1.5nC | Roff | 1.4 Ohms |
| Vin | 21V | Qgs | 1.4nC | ||
| Vout | 12.6V | Vplt | 2.5V | ||
| Duty (D) | 0.6 | ||||
From equation 17 on page 22 of the BQ24650 Datasheet, I can find that the conduction losses of the top MOSFET are:
D * Ichrg * Ichrg * Rdson
And the switching losses of the top MOSFET are:
0.5 * Vin * Ichrg * (Ton - Toff)*Fs
To calculate Ton and Toff, I need to calculate Qsw (equation 19), Ion (equation 20), and Ioff (equation 20):
Qsw = Qgd + 0.5 * Qgs Qsw = 2.2nC
Ion = (Vregn - Vplt) / Ron and Ioff = Vplt / Roff Ion = 0.58A, Ioff = 1.78A
Ton = Qsw / Ion and Toff = Qsw / Ioff. Ton = 3.7nSec and Toff = 1.2nSec
Using the equation for the power loss from consuction and switching, I find conduction losses of the top FET at 0.064W and switching losses of the top FET at 0.063W.
Equation 21 on page 22 shows that the bottom FET only has conduction losses:
Pbottom = (1-D) * Ichg * Ichg * Rdson. Pbottom = 0.04W.
Are these calculations correct? Have I missed anything? Also, why are there no switching losses for the bottom FET?
Thanks a ton!