AN829 APPLICATION NOTE

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SEMI-CONDUCTOR KIT

The semiconductor kit consists of an L4981 controller, an STE36N50-DK power module and an STH80N05

power sense (see figure 1).

L4981A/B controller:

The L4981 operates with an input voltage range of 85V to 270V and uses average current mode PWM control,

providing feed forward line and load compensation. Two versions are available: version (A) provides synchro-

nization with the down stream converter, whereas version (B) provides linear frequency modulation, spreading

the RFI noise spectrum.

Both versions incorporate overvoltage and overcurrent protection, soft start and under voltage lockout with pro-

grammable threshold.

Other features include an on chip voltage reference (2%) which is externally available, a typical starting current

of only 0.5mA and separate grounds for the power and signal stages.

L4981 use an optimum current control method. It is an average current control using feed forward line regulation

and variable or fixed switching frequency.

The oscillator simultaneously turns on the power switch and starts the ramp of the PWM current control. The

average inductor current is compared with the current reference by means of the current error amplifier. It op-

erates as an integrator, allowing the circuit to accurately follow the current reference generated by the multiplier.

This current reference is obtained by sinewave modulating the error voltage of the voltage control loop.

A feed forward compensation of the mains voltage has been added to the multiplier in order to keep constant

the voltage control loop bandwidth whatever the mains fluctuation. A fourth multiplier input allows external com-

pensation to be applied to the current modulation.

The oscillator can operate at constant or modulated switching frequency. In applications where modulated fre-

quency is used, the RFI noise spectrum can be spread adjusting the depth of modulation by means of an exter-

nal resistor. Then the maximum inductor current occurs at the minimum operating frequency.

STE30NA50-DK Power Module:

package with very low parasitic inductance and capacitance reduces the component count, and significantly re-

duces transient overvoltages, and EMI and RFI.

As a result, the design safety margin can be relaxed and the voltage rating of the power MOSFET can be just

current and avalanche handling capabilities of the power MOSFET section are specified at 100°C junction tem-

perature, allowing for maximum utilization of the device. The MOSFET is a low gate-charge type and so its drive

requirements are compatible with the 2A peak current capability of the L4981 controller.

ial technology, and is a part of the STTA series. Its low trr (30ns) keeps the MOSFET switching losses to a min-

STH80N05 Power sense:

Using a high density low voltage Power MOSFET for current sensing has many advantages:

– low resistance, typically 10m2