Datasheet

SLUU087B

10-Watt Flyback Converter Using the UCC3809

12 Gate Drive

At 15 volts gate to source voltage the IRFR220 has a total gate charge of approximately 14 nC. The UCC3809

is capable of sourcing 400-mA of peak-drive current which would result in a turn-on time of 35 ns. To limit the

peak current through the IC, an external resistor is placed between the totem-pole output of the IC and the gate

of the MOSFET. The minimum value of this resistor is determined by:

GATE

VDD(min) * V

SAT

GATE(peak)

This small-series resistor also damps any oscillations caused by the resonant tank of the parasitic inductances

in the traces of the board and the FET’s input capacitance. A pulldown resistor is added to the gate drive to insure

the MOSFET gate does not get charged to its turnon threshold during device start up. Adding a fast-switching

diode and smaller value resistor in parallel with the gate resistor helps to control the current the IC needs to sink

during turnoff and protects the output stage of the device. These components also help to reduce turnoff losses

which tend to dominate the switching losses in discontinuous current-mode (DCM) converters.

13 Output Diode

The output diode in a flyback converter is subject to large peak and rms current stresses. The 10-W flyback

converter described here has measured peak-secondary currents as high as 18 A with an rms value of

approximately 7 A. Schottky diodes are recommended because of their low-forward voltage drop and the virtual

absence of minority carrier reverse recovery. The secondary-side Schottky rectifier was selected to meet the

working peak-reverse voltage, the peak repetitive-forward current, and the average forward current of the

application. The working peak-reverse voltage, V

REV

, or blocking voltage, is calculated according to the

following equation:

REV

IN(max)

) V

RDS(on)

)

OUT

The reflected peak-primary current constitutes the peak repetitive-forward current through the diode. Because

all current to the output capacitor and load must flow through the diode, the average-forward diode current is

equal to the steady-state load current. Power loss in the Schottky is the summation of the conduction losses

and reverse leakage losses. Conduction losses are calculated using the forward voltage drop across the diode

and the average-forward current. Reverse leakage losses are dependent upon the reverse-leakage current, the

blocking voltage, and the on time of the FET.