AN2835 Datasheet, PDF (7/21 Pages) STMicroelectronics – This application note describes the electronic lamp ballast for 70 W high intensity discharge

AN2835
The selected solution
realized by controlling the peak inductor current of the dual-buck converter. Assume the
input voltage of the buck converter is V_bi, the output voltage is V_bo, the duty cycle is D,
input peak current is I_{in_pk}, as the buck converter is working at a critical discontinuous
mode, and the average input current is:
Equation 1
And the duty cycle is:
I_in
=
1_--
2
I_in-p
k
⋅
D
Equation 2
The input power becomes:
D = ^V-----^b---^o-
V_bi
Equation 3
P_in = V_bi ⋅ I_in
Thus the relationship between the input power (P_in), input peak current (I_{in_pk}) and output
voltage (V_bo) is:
Equation 4
If the lamp is operating with a constant current source, once input peak current (I_{in_pk}) is
selected, we observe the input power (P_in) is proportional to the output voltage (V_bo).
Despite the power losses, the output power is also proportional to the output voltage.
Figure 5. Input power, output voltage and input peak current
!-V
In Figure 5, there are three different I_{in_pk} curves, this helps to choose the proper input peak
current according to the different types of lamp. After warming-up, the lamp voltage
increases slowly to the minimum value of the rated power, the duty cycle increases
accordingly. And then the input peak current decreases. In order to power up the lamp in
steady-state, the circuit changes from constant current control function to constant power
control.
Doc ID 15073 Rev 1
7/21