Operation VIPER28
14/31 DocID15028 Rev 6
7 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 B
VDSS
of 800 V min. and a maximum R
DS(on)
of 7 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 V
DRAIN_START
threshold, 80 V DC typically.
When the HV current generator is ON, the I
DDch1
current (3 mA typical value) is delivered to
the capacitor on the V
DD
pin. During auto-restart mode after a fault event, the current is
reduced to I
DDch2
(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, V
DRAIN_START
, the VDD voltage
begins to grow due to the I
DDch1
current (see Table 8: Supply section) coming from the
internal high-voltage startup circuit. If the VDD voltage reaches the V
DDon
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, C
VDD
, 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 C
VDD
capacitor must be correctly sized to avoid fast discharge and keep the required
voltage higher than the V
DDoff
threshold. In fact, an insufficient capacitance value could
terminate the switching operation before the controller receives any energy from the
auxiliary winding.