iW1688
Low-Power Off-line Digital PWM Controller
Allow a maximum forward voltage drop at the Schottky diode Selecting a switching MOSFET:
of 0.4 V. Substitute TON and TRESET into equation (10.8):
For flyback converter the MOSFET will be selected so that
93×11.3
5.4×9.95
VDS is higher than the switching voltages plus reflecting
secondary and spike voltages.
N =
=19.6
N = 20
VDS > VBULK(MAX ) + NVOUT +VSPIKE
(10.13)
Primary and secondary turns:
Allow VSPIKE = 100V. From equation (10.13):
By Faraday law, the number of primary turns will be:
VDS > 372 + 20 + 5.4 +100 = 580V
VIN = NPR × AE ∆B / ∆t ×10−8
(
)
And IDS > IPRPK = 355mA
Where V = volt; N = turns; AE = cm²; ∆B = gauss; t = second
Select a MOSFET = 600 V/0.4 A
1
Secondary current is a triangle of peak amplitude
ISECPK = IPRPK x N at a maximum duration of TRESET. It’s RMS
on a period TP is:
NPR = VIN
∆B
∆t
×10−8
AE
(10.9)
(10.10)
(10.11)
N
TRESET
TP
NPR
(10.14)
ISECRMS = IPRPK ×
×
NSEC =
3
N
Substitute IPRPK = 355mA into equation (10.14)
NPR ×VAUX
NAUX =
VOUT
355× 20
9.95
25
ISECRMS =
×
= 2578mA
3
Select a maximum flux density ∆B = 2500 Gauss and AE =
0.2 cm². From equation (10.9), (10.10) and (10.11):
The working voltage of the rectifier diode is included output
voltage plus reverse voltage at the TON period. A 20%
marginal voltage is suitable for safety:
93
NPR =
2500
11.3
0.2×
×10−6 ×10−8
VBULK(MAX )
NPR = 210Turns
VDIODE =120%
+VOUT
N
Since N = 20 and NSEC = 210/20 = 10.5 Turns
372
20
VDIODE =1.2
+ 5 = 27.3V
Choose NSEC = 10 Turns and NPR = 200T, Set VAUX = 13V
13
NAUX =10×
= 24Turns
Typically we select the rated current of a Schottky diode
rectifier that is twice the output current value. With a known
value for ISECRMS and VDIODE the rectifier diode will be selected
as :
5.4
Primary RMS current: Primary current is a triangle of peak
amplitude IPRPK at maximum duration TON. It’s RMS on a
period TP is:
Diode = 5A/ 40V
IPRPK
TON
TP
This RMS current occured at 130% of the output load.
Therefore the wire size will based on ISECRMS/1.3.
IPRRMS =
(10.12)
3
Substitute IPRPK = 355 mA into equation (10.12)
355
3
11.3
25
IPRRMS =
×
=137mA
MK-4AA003-E
03/16/06
PAGE 11
PRELIMINARY
ETC [ ETC ]
