User guide

AN375
AN375REV4 19
Reflected voltage V
Reflected
was initially selected to be 170V. To avoid incomplete layers in the transformer
winding voltage V
Reflected
was reduced to 165.6V. Use Equation 8 on page 10 to verify the FET breakdown
voltage V
Breakdown
. See Equation 29:
The low power and cost constraints attached to the design example make the RC snubber circuit a desirable
choice. The initial snubber resistance R
Snubber
is calculated by equating the energy E
leakage
stored in the
leakage inductance to the power P
leakage
dissipated by the snubber resistor during the entire switching
period TT at the peak of the line voltage. Set the leakage energy E
leakage
derived in Equation 5 on page 9
equal to the leakage power P
leakage
dissipated in Equation 9 on page 11. See Equation 30:
Knowing or by measuring that the leakage inductance is 1% of the transformer primary side inductance the
leakage inductance L
leakage
is calculated using Equation 31:
Use Equation 29 on page 19 to set the clamp voltage V
RC
equal to the zener clamp voltage V
Zener
.
See Equation 32:
To calculate the snubber resistance R
Snubber
, first establish values for peak current I
PK
, switching period TT,
and zener clamp voltage V
Zener
. Peak current I
PK
is derived using Equation 41 on page 20 to equal 513.6mA
and a maximum switching frequency F
SW(max)
of 56.7kHz implies a switching period TT equals 17.6µs. Use
Equation 30 to solve for snubber resistor R
Snubber
. See Equation 33:
Determine the RC time constant using Equation 34:
Calculate the snubber capacitance C
Snubber
using Equation 35:
Since the snubber capacitor C
Snubber
is not a critical component of the RC snubber circuit, the value of the
snubber capacitor C
Snubber
is selected as a standard 1nF.
Step 3) Determine the Flyback Transformer Turns Ratio
Using Equation 10 on page 11, calculate a turns ratio N based on the output voltage V
OUT
. See Equation 36:
V
Breakdown
V
INPK max
V
Zener
V
Minarg
++ 187V 275V 138V++ 600V===
[Eq. 29]
I
PK
2
L
leakage

2
------------------------------------
V
RC
2
TT
R
Snubber
-------------------------
=
[Eq. 30]
L
leakage
0.01 2600H 26H==
[Eq. 31]
V
RC
V
Zener
V
Breakdown
V
INPK max
– V
Minarg
+ 600V 187V 138V+– 275V== = =
[Eq. 32]
R
Snubber
2V
RC
2
 TT
I
PK
2
L
leakage

------------------------------------
2 275V
2
 17.6s
513.6mA
2
26H
--------------------------------------------------------
388k== =
[Eq. 33]
RC
T
line
TT
2
--------------------------
16.667ms 17.6s
2
---------------------------------------------------- 383s== =
[Eq. 34]
C
Snubber
RC
R
Snubber
------------------------
383s
388k
------------------
0.987nF===
[Eq. 35]
N
N
P
N
S
-------
V
Reflected
V
OUT
V
F
+
----------------------------
165.6V
27.9V 0.9V+
----------------------------------------5.75== = =
[Eq. 36]