FA5604 Datasheet, PDF (30/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(11)Loss calculation
IC loss must be confirmed to use the IC within the ratings.
Since it is hard to directly measure IC loss, some examples of calculating approximate IC loss are given below.
(11-1) Calculation example 1
Suppose the supply voltage is VCC, IC current consumption is ICC, the total gate charge of the power MOSFET
is Qg, and the switching frequency is fosc. Total IC loss Pd can be calculated by:
Pd ≈ VCC×(ICC+Qg×fosc) ·············· (14)
This expression calculates an approximate value of Pd, which is normally a little larger than the actual loss.
Since various conditions such as temperature characteristics apply, thoroughly verify the appropriateness of the
calculation under all applicable conditions.
Example:
When VCC=18V, ICC=3mA (max.) is obtained from the specifications. Suppose Qg=80nC and fosc=190kHz.
Pd ≈ 18V× (3mA+80nC×190kHz) ≈ 328mW
(11-2) Calculation example 2
The IC loss consists of the loss caused by operation of the control circuit and the loss caused at the output circuit
to drive the power MOSFET.
○1 Loss at the control circuit
The loss caused by operation of the IC control circuit is calculated by the supply voltage and IC current
consumption. When the supply voltage is VCC and IC current consumption is ICC, loss Pcon at the control circuit
is:
Pcon=VCC×ICC ················· (15)
Example:
When VCC=18V, ICC=1.6mA (max.) is obtained from the specifications. The typical IC loss is given by:
Pcon=18V × 1.6mA ≈ 29mW
○2 Loss at the output circuit
The output circuit of the IC is a MOSFET push-pull circuit. When the ON resistances of MOSFETs making up the
output circuit are Ron and Roff, the resistances can be determined as shown below based on VCC=18V obtained
from the output characteristics shown in the specifications:
Ron=15Ω (typ), Roff=7Ω (typ)
When the total gate charge of the power MOSFET is Qg, the switching frequency is fosc, the supply voltage is
VCC and gate resistance is Rg, the loss (Pdr) caused at the IC output circuit is given by:
Pdr

1
2
VCC  Qg

fosc

 _

Ron
Rg  Ron

Roff
Rg  Roff



·······················(16)
When gate resistance differs between ON and OFF as shown in Fig.31, the loss is given by:
Pdr

1
2
VCCQg 
fosc



Rg1
Ron
Rg2 
Ron

Roff
Rg1 Roff



···············(17)
Example:
When VCC=18V, Qg=80nC, fosc=190 kHz and Rg=10, the typical IC loss is given by:
Pdr

1
2
18V
 80nC
190kHz

 15


10

15

7
10  7



=138mW
Total loss
The total loss (Pd) of the IC is the sum of the control circuit loss (Pcon) and the output circuit loss (Pdr) calculated
previously:
Pd=Pop+Pdr ················ (18)
Example:
The standard IC loss under the conditions used in ○1 and ○2 above are:
Pd=Pcon+Pdr=29mW+138mW=167mW
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
30
http://www.fujielectric.co.jp/products/semiconductor/