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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. Noise Considerations at switch Turn On

3.1 SMPS Current Sensing

Whether an SMPS is current mode or voltage mode

controlled, or uses some form of non linear control, it

requires the protection afforded by current sensing

capabilities. Even though most current sensing is done using

a sensing resistor or a current transformer, there are

instances where a MOSFET is used by the SMPS to further

reduce current sensing losses. The Fairchild Power Switch

(FPS) switching element is a SenseFET. This minimises any

power losses in the sense resistor, which is integrated onto

the controller chip.

3.2 Current Sensing Waveform Noise After

Turn On

A leading edge spike is present on the current sense line

when the SMPS switching device turns on. It arises from

three causes, as shown in Figure 13: (1) reverse recovery

current; (2) charge/discharge current of the MOSFET shunt

capacitance; and (3) MOSFET gate operating current.

(1) Reverse recovery current is generated when the SMPS

operates in continuous conduction mode (CCM; see Section

4.1). If the MOSFET is turned on while the rectifier diode is

conducting, the diode, during its reverse recovery time will

act like a short circuit. Therefore a large current spike will

flow in the MOSFET. There are three ways to reduce the

magnitude of this (reverse recovery) current: use a fast

recovery diode; reduce the MOSFET’s gate operating

current at turn on; or, increase the transformer leakage

inductance.

(2) The total capacitance on the MOSFET’s drain side

includes the parasitic capacitor Cds between the drain and the

source terminals, the junction capacitance of the snubber

diode, and the capacitance of the transformer windings. At

turn on, the total equivalent capacitor discharges through the

MOSFET.

(3) As shown in the diagram, the MOSFET gate operating

current also flows through the current sensing resistor.

Vin

Vo

Vin

Vo

Vin

Vo

Rg

Control

IC

Rf

Rs

Cf

Rg

Control

IC

Rf

R

Cf

s

Rg

Control

IC

Rf

Rs

Cf

Reverse Recovery Current

Charge/Discharge Current of

the equvalent capacitor

MOSFET Gate Operating

Current

Figure 13. Current sensing waveform noise after turn on.

10

©2002 Fairchild Semiconductor Corporation

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