AN1889

Application description

9/33

The first choice to be made is the transformer turn ratio NP / NS, where NP and NS are

respectively the number of primary and secondary windings. The calculation of turn ratio is

correlated to the maximum voltage rating of the transistor to be used as primary switch. In

is given by:

Equation 1

obviously

Equation 2

Must be chosen from the designer taking care that the total voltage at point T in Figure 5

does not exceed the maximum breakdown voltage of the device chosen as power switch

according to the Equation 1.

into account the voltage capabilities offered by the standard transistors available. It is worth

noticing that the higher the fly-back voltage the higher the max duty cycle acceptable: higher

duty cycle at fixed output power, leads to lower IRMS with a consequent overall better

efficiency at primary side and an easier design of wide input range converter.

In this case the use of ESBT, having devices with breakdown voltage capability as high as

1700 V can help in that. For the realized demo board with STC03DE170HV we can fix:

●

Margin=250 V

●

Equation 1 we can now easily calculate the transformer turn ratio:

Table 1.

Converter specification data and fixed parameters

Symbol

Description

Values

Rectified minimum Input voltage

250

Rectified maximum Input voltage

750

Output voltage

24

Auxiliary output voltage

15

Maximum output power

48

η

Converter efficiency

80%

F

Switching frequency

50 kHz

Reflected fly back voltage

500 V

Max over voltage limited by clamping circuit

200 V

V

T

V

dcmax

N

P

N

S

------- V

0

V

Fdiode

,

+

() V

Spike

min

arg

++

+

=

N

P

N

S

------- V

0

V

Fdiode

,

+

() V

1

=

= the fly-back voltage