STMicroelectronics Double output SMPS for power line application using ALTAIR04-900 primary controller EVLALTAIR900-M1 EVLALTAIR900-M1 Fiche De Données

12/29

Doc ID 18211 Rev 2

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.

 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 

, 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.