Inrush-current
limitation
UM2076
24/43
DocID029457 Rev 1
not necessary to ensure a soft-start for turn-on delays much lower than a fourth cycle. In
the firmware, a minimum value of 3 ms is defined by the
Phase_Control_ON_Max_µs
,
which sets the maximum ON time of SCRs (7 ms, refer to directive definitions in the
firmware).
Figure 4: "Inrush current at STEVAL-ISF003V1 start-up on 230 V
line (1 mF output DC capacitor)"
) shows an example of such progressive DC capacitor
charging. The test is performed at start-up when the STEVAL-ISF003V1 board is
connected to a 230 V 50 Hz grid, while the output DC capacitor is fully uncharged (i.e., its
initial voltage is null), with a 1mF output DC capacitor connected.
The electrical parameters in the figure below are defined in
(arrow head gives the hot-point of the voltage). As
no PFC is used, VDC is actually the voltage across the 1mF capacitor connected
to the HVDC output.
Figure 17: SCR current zoom for the highest peak current during start-up
With the MCU firmware as the default program:
First SCR turn-on is set to 150 µs before next line zero voltage, as the first gate
current pulse lasts 50 µs. This allows the gate current to be removed 100 µs before
next half cycle. This value is set by
SCRs_OFF_Delay_us
in the firmware. This 100
µs time margin takes into-account the ZVS signal delay (which could equal up to 50
µs, see
Section 9: "AC voltage monitoring and zero-voltage synchronisation"
) and the
delay required to un-saturate transistor Q3 or Q5 which drives the SCR1 and SCR2
(50 µs), respectively. Total delay time equals 150 µs, called
ICL_TRIAC_OFF_Delay_µs
in the firmware.
SCR turn-on then progressively occurs 50 µs sooner when the MAX_INRUSH
CURRENT potentiometer is set to the DEFAULT position. This minimum step value is
defined in the directive section of the firmware (refer to
Step_Phase_Control_Min
).
The DEFAULT position corresponds to the slower output DC capacitor charge, thus
the shortest
Step_Phase_Control_µs
value. When the MAX_INRUSH CURRENT
