ALTAIR04-900TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ALTAIR04-900TR' PDF : 29 Pages View PDF
Application information
ALTAIR04-900
5.1
Power section and gate driver
The power section guarantees the safe avalanche operation within the specified energy
rating as well as high dv/dt capability. The MOSFET has a V(BR)DSS of 900 V min. and a
typical RDS(on) of 16 Ω.
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 MOSFET cannot be turned on
accidentally.
5.2
High voltage start-up generator
Figure 10 shows the internal schematic of the high voltage start-up generator (HV
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 VStart threshold, 50 VDC typically. When
the HV current generator is on, the Icharge current (5.5 mA typical value) is delivered to the
capacitor on the Vcc pin.
With reference to the timing diagram in Figure 10, when power is applied to the circuit and
the voltage on the input bulk capacitor is high, the HV generator is sufficiently biased to start
operating, thus it draws about 5.5 mA (typical) from the bulk capacitor. This current charges
the bypass capacitor connected between the Vcc pin and ground and rises its voltage
linearly.
As the Vcc voltage reaches the start-up threshold (13 V typ.) the chip starts operating, the
internal MOSFET is enabled to switch and the HV generator is cut off by the Vcc_OK signal
asserted high. The IC is powered by the energy stored in the Vcc capacitor.
The chip powers itself directly from the rectified mains: when the voltage on the Vcc pin falls
below Vccrestart (10.5V typ.), during each MOSFET off-time, the HV current generator turns
on and charges the supply capacitor until it reaches the VccOn threshold.
In this manner, the self-supply circuit develops a high voltage to sustain the operation of the
device. This feature is useful during CC regulation, when the flyback voltage generated by
the auxiliary winding alone, may not be able to keep Vcc above Vcc restart.
At converter power-down, the system loses regulation as soon as the input voltage falls
below VStart. This avoids converter restart attempts and assures monotonic output voltage
decay at system power-down.
12/29
DocID18211 Rev 3
Share Link: 
