ALTAIR04-900 Datasheet, PDF (12/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR04-900
5.1
Power section and gate driver
The power section guarantees safe avalanche operation within the specified energy rating
as well as high dv/dt capability. The Power MOSFET has a V(BR)DSS of 900 V min. and a
typical R_DS(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 power MOSFET cannot be turned on
accidentally.
5.2
High voltage startup 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 V_DC typically.
When the HV current generator is ON, the Icharge current (5.5 mA typical value) is delivered
to the capacitor on the V_CC pin.
With reference to the timing diagram of Figure 10, when power is applied to the circuit and
the voltage on the input bulk capacitor is high enough, the HV generator is sufficiently
biased to start operating, thus it draws about 5.5 mA (typical) from the bulk capacitor. Most
of this current charges the bypass capacitor connected between the Vcc pin and ground and
make its voltage rise linearly.
As the Vcc voltage reaches the start-up threshold (13 V typ.) the chip starts operating, the
internal power 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 is able to power itself directly from the rectified mains: when the voltage on the V_CC
pin falls below Vcc_restart (10.5V typ.), during each MOSFET’s off-time the HV current
generator is turned on and charges the supply capacitor until it reaches the V_CCOn
threshold.
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 CC regulation, when the flyback voltage
generated by the auxiliary winding alone may not be able to keep Vcc above VCCrestart.
At converter power-down the system loses regulation as soon as the input voltage falls
below V_Start. This prevents converter’s restart attempts and ensures monotonic output
voltage decay at system power-down.
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Doc ID 18211 Rev 2