VIPER35 datasheet(19/44 Pages) STMICROELECTRONICS

19 / 44 page

DocID026980 Rev 4

19/44

VIPer35

Operation description

The device is a high performance low voltage PWM controller chip with an 800 V,

avalanche-rugged power section.

The controller includes the PWM logic, ZCD logic for quasi-resonant operation, oscillator,

start-up circuit with soft-start, current limiting circuit with adjustable set-point, burst mode

management, brown-out circuit, UVLO circuit, auto-restart circuit and thermal protection

circuit.

The current limit set-point can be reduced by ZCD pin. Burst mode operation guarantees

high performance in standby mode and meets energy-saving standards.

All fault protections are built-in auto-restart mode with very low repetition rate to prevent the

IC overheating.

8.1

Power section and gate driver

The power section is given by an avalanche-rugged N-channel MOSFET, which guarantees

safe operation within the specified energy rating as well as high dv/dt capability. The power

MOSFET has a BVDSS of 800 V min. and a typical RDS(on) of 4.5 Ω at 25 °C. The integrated

senseFET structure allows a virtual loss-less current sensing.

The gate driver is designed to supply a controlled gate current during both turn-on and turn-

off in order to minimize common-mode EMI. Under UVLO conditions an internal pull-down

circuit holds the gate low in order to ensure that the power section cannot be turned on

accidentally.

8.2

High voltage start-up generator

The HV current generator is supplied through the DRAIN pin and it is enabled only if the

input bulk capacitor voltage is higher than VDRAIN_START threshold, 80 V DC typically.

When HV current generator is on, IDDch1 current (3 mA typical value) is delivered to the

capacitor on VDD pin. During auto-restart mode after a fault event, the current is reduced to

IDDch2 (0.6 mA, typ.) in order to have a slow duty cycle during the restart phase.

8.3

Power-up and soft-start

When the input voltage reaches the device start threshold, VDRAIN_START, the VDD voltage

begins growing due to IDDch1 current (see Table 7) coming from the internal high voltage

start-up circuit. If the VDD voltage reaches VDDon threshold, the power MOSFET starts

switching and the HV current generator turns off.

The IC is powered by the energy stored in the capacitor on VDD pin, CVDD, until the self-

supply circuit (typically an auxiliary winding of the transformer and a steering diode)

develops a voltage so high to sustain the operation.

CVDD capacitor must be correctly sized to avoid fast discharge and keep the required

voltage higher than VDDoff threshold. In fact, an insufficient capacitance value could

terminate the switching operation before the controller receives any energy from the

auxiliary winding.

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