Application information

STCH02

12/24

DocID028766 Rev 1

3.1

Gate driver

The gate driver of the power MOSFET is designed to supply a controlled gate current during

both turn-on and turn-off in order to minimize the common mode EMI. Under UVLO

conditions an internal pull-down circuit holds the gate low in order to ensure that the power

MOSFET cannot be turned on accidentally.

3.2

Frequency jittering for EMI reduction

Although the STCH02 device works in the QR mode and the switching frequency is already

modulated at twice of the mains frequency, dedicated frequency jittering circuitry is

embedded inside the IC to further reduce the EMI filtering. A proprietary frequency jitter

technique is implemented in the controller, based on the injection of a modulating signal at

9 kHz (above the feedback loop bandwidth) with 50% duty cycle on the current sense

signal: this signal is a square waveform that modulates the amplitude of the peak primary

current. The percentage of this amplitude is set as a default at 5%. As the peak current

reduces with decreasing load levels, the effect of this modulation automatically attenuates at

lower loads, where the energy of EMI noise is highly reduced.

3.3

High voltage start-up generator

Based on a 650 V rated depletion MOSFET embedded into the startup cell, the HV current

generator is supplied through the DRAIN pin and is enabled only if the voltage on the HV pin

is higher than the HVSTART threshold (50 V typical value).

With reference to the timing diagram in Figure 6, when the power is applied to the circuit and

the voltage on the input bulk capacitor is high enough, the HV generator is sufficiently

to ground or during a restart after protection triggering.

While the generator is off, there are virtually no losses across the HV startup cell, except

a few hundreds nA of the leakage current through the depletion MOSFET.

(typically an auxiliary winding of the transformer and a steering diode) develops a voltage

high enough to sustain the operation.

In this way, the self-supply circuit develops a voltage high enough to sustain the operation of

the device. This feature is useful especially during the CC regulation, when the flyback

operative range.