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AN4105

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

Fairchild Semiconductor 

AN4105

APPLICATION NOTE

3.3 Dealing With Leading Edge Noise

Among the measures taken to reduce leading edge noise, the

most commonly used technique is the RC filter. As shown in

Figure 14a the RC filter is effective against the noise, but it

has the disadvantage that it distorts the current sensing signal

so that accurate current sensing becomes difficult.

Furthermore, a large RC value may be difficult to implement

on an IC and may even require a bigger chip. The technique

of leading edge blanking, as presented in Figure 14b and

14c, overcomes the distortion disadvantage of the RC

technique and works as follows. Since the problem noise

arises just after turn on, if a circuit is inserted that ignores the

current sensing line for a fixed time just after turn on,

operation can continue normally regardless of the noise.

Whatever the details of the location and type of circuit used,

the basic idea is to maintain a minimum turn on time i.e.,

touse the shortest turn on time that cannot be terminated

once turn on starts. Duty ratio control with a minimum turn

on time is implemented through a non linear control method

having a very wide control range relative to a linear control.

The non linear control operates such that if load conditions

require a turn on time of 400ns when the minimum turn on

time is set at 500ns, then one switching cycle will turn on at

800ns. The next cycle will be missed, ensuring that the

average turn on time is 400ns. In this case every other cycle

is missed. This is pulse skipping. In this case the switching

frequency will be half that of a linearly controlled system,

thereby improving SMPS efficiency at light loads. The input

power is therefore minimised. The Fairchild KA34063 dc/dc

converter is an example of a non linear control IC.

3.3.1 Burst Mode Operation

The aforementioned method can be viewed as an example of

burst mode operation. Burst mode operation, by reducing the

switching frequency, is one of the most useful ways to

improve SMPS efficiency at light loads and to reduce the

standby input power of household appliances, etc. Note that

burst mode operation is not a burst oscillation (as in ringing

choke conversion circuits), which can bring about reliability

problems. There are mainly two types of true burst mode

operation: one type lowers the switching frequency equally.

The other switches at normal frequency for a fixed time and

stops the control IC operation for a large number of cycles.

Even though in the first method the control IC continues to

consume power, the output voltage ripple is minimized.

The second method can be a useful way to reduce the

minimum input power at standby (since obviously the

standby power is greatly reduced when the IC is stopped).

Indeed, it is often used in cell phones to reduce the dc/dc

converter’s power consumption in standby mode. However,

it has the disadvantage of a larger output voltage ripple.

Currently, Europe restricts a household appliance’s standby

input power to less than 5W, and in time it will be required

to be less than 3W. For such needs, burst mode operation

will be a powerful method to satisfy the requirement for

reduced standby input power.

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©2002 Fairchild Semiconductor Corporation

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