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AN4105

Design Considerations for Switched Mode Power Supplies Using A Fairchild Power Switch (FPS) in a Flyback Converter

Fairchild Semiconductor 

APPLICATION NOTE

AN4105

5. The Transformer

5.1 Why a Transformer Is Needed

There are three reasons for needing a transformer in a power

conversion circuit. The first reason is safety. A transformer

affords electrical isolation between the primary and

secondary sides, as shown in Figure 19a. In addition, a true

ground on the output side helps prevent electric shock. The

second reason is for voltage conversion. For example, if a

dc/dc converter (such as the buck converter shown in Figure

19b) switching at 50kHz is used to obtain 5V from 100V.

The duty cycle would only be 5%. Using a 50kHz switching

frequency, the control circuit may have only about 1µs to

act, which is not an easy task. Even if this were possible, the

internal voltage and current for each element would be very

large reducing efficiency. The problem is aggravated at high

output current. In the above example, using a transformer to

lower the voltage to 10V would then allowing an on time of

about 10µs. This is an advantageous strategy for lowering

cost and raising efficiency. The third reason to use a

transformer has to do with high voltages and voltage fluctu-

ations. For example, even though all control for a 1000V

supply is done at the 5V power source on the GND side, a

transformer is necessary if power is needed for current

sensing at the 1000V output terminal or for other control. If

the isolation voltage between the transformer windings is

sufficient, a 1000V potential difference can be safely

maintained between the primary and secondary windings

and power can be delivered. Further, a transformer is also

required when the power GND has a sudden potential

fluctuation as in a half bridge converter, gate drive power

source.

AC220V

& Rectifier

DC

Output

GND

Vin=100V

Vo=5V

Electric Sshocrtk when touched Safe to touch

(a) For sSaaffteyty

100V ----> Duty 5% ----> 5V

50kHz --> Ton=1uS

5V

1000 V

Vin=100V

10:1

Vo=5V

5V

100V --> 10V --> Duty 50% --> 5V

50kHz -> Ton=10uS

(b) For voltage change

(c) For potential fluctuation

Figure 19. Why a transformer is needed.

5.2 The Ideal Transformer

The transformer is a device that uses inductive coupling

between its windings to deliver power or signals from one

winding to the another. This is usually from the primary

winding to the secondary winding. The voltages across the

windings can be raised or lowered with respect to each other,

and, if necessary, the primary and secondary sides can be

isolated from each other. Figure 20 shows an ideal

transformer, a simplistic model useful in describing the

transformer concept. An ideal transformer which is actually

a fictional concept must satisfy the following three

conditions:

1.The coupling coefficient between the windings

is unity (i.e., the leakage flux is zero);

2.The coil loss is zero (the device has no losses).

3.The inductance of each coil is infinite.

©2002 Fairchild Semiconductor Corporation

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