VIPer22A-E, VIPer22ADIP-E, VIPer22AS-E
Operations
4.4
Startup sequence
Figure 7. Startup sequence
This device includes a high voltage start up current source connected on the drain of the
device. As soon as a voltage is applied on the input of the converter, this start up current
source is activated as long as V is lower than V
. When reaching V
, the start up
DD
DDon
DDon
current source is switched OFF and the device begins to operate by turning on and off its
main power MOSFET. As the FB pin does not receive any current from the optocoupler, the
device operates at full current capacity and the output voltage rises until reaching the
regulation point where the secondary loop begins to send a current in the optocoupler. At
this point, the converter enters a regulated operation where the FB pin receives the amount
of current needed to deliver the right power on secondary side.
This sequence is shown in Figure 7. Note that during the real starting phase t , the device
ss
consumes some energy from the V capacitor, waiting for the auxiliary winding to provide a
DD
continuous supply. If the value of this capacitor is too low, the start up phase is terminated
before receiving any energy from the auxiliary winding and the converter never starts up.
This is illustrated also in the same figure in dashed lines.
Doc ID 12050 Rev 2
11/21
STMICROELECTRONICS [ ST ]
