VIPER38HD datasheet(14/31 Pages) STMICROELECTRONICS

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Operation

VIPER38

14/31

DocID025703 Rev 2

Operation

The device

is a high-performance low-voltage PWM controller chip with an 800 V, avalanche

rugged power section.

The controller includes the oscillator with jitter, startup circuit with soft-start, PWM logic,

current limiting circuit with adjustable setpoint, second overcurrent circuit, burst mode

management, extra power timer circuit, UVLO circuit, auto-restart circuit and thermal

protection circuit.

The current limit setpoint is set by the CONT pin. Burst mode operation guarantees high

performance in standby mode and contributes to meeting energy-saving standards.

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

the IC from overheating.

7.1

Power section and gate driver

The power section is implemented with an avalanche-rugged N-channel MOSFET, which

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

The power section 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 virtually 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.

7.2

High-voltage startup 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 the VDRAIN_START threshold, 80 V DC typically.

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

the capacitor on the 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.

7.3

Power-up and soft startup

When the input voltage rises to the device start threshold, VDRAIN_START, the VDD voltage

begins to grow due to the IDDch1 current (see Table 7: Supply section) coming from the

internal high-voltage startup circuit. If the VDD voltage reaches the VDDon threshold, the

power MOSFET starts switching and the HV current generator is turned OFF.

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

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

develops a voltage high enough to sustain the operation.

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

voltage higher than the 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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