Dimming control method
A dimming control method and a dimming circuit are used to control the brightness of a LED. The dimming circuit includes a power conversion unit having an input end, an output end, an inductor, and a switch. When the dimming current signal is higher than or equal to a first current threshold, the switch is controlled such that the power conversion unit operates in a continuous Conduction mode or a boundary Conduction mode; when the dimming current signal is lower than the first current threshold, the switch is controlled such that the power conversion unit operates in a discontinuous Conduction mode; and when the dimming current signal is lower than a second current threshold, the switch is controlled such that the power conversion unit operates in a chopping control mode.
1. A dimming control method applied to a dimming circuit for controlling the brightness of a light emitting diode, the dimming circuit comprising a power conversion unit having an input end, an output end, an inductor, and a switch, the power conversion unit receiving an input voltage from the input end, and converting the input voltage into an output voltage output from the output end by operation of the switch, wherein the dimming control method comprises:
generating an input voltage sampling signal by detecting the input voltage of the power conversion unit;
generating an output voltage sampling signal by detecting the output voltage of the power conversion unit;
generating a current sampling signal by detecting a current flowing through the switch in the power conversion unit;
receiving a dimming signal to calculate and generate a dimming current signal; and
outputting a control signal to a driving circuit to drive and control the switch to turn on or turn off according to the input voltage sampling signal, the output voltage sampling signal, the current sampling signal, and the dimming current signal; wherein,
the dimming current signal is compared with a first current threshold and a second current threshold,
when the dimming current signal is higher than or equal to the first current threshold, the switch is controlled such that the power conversion unit operates in a continuous conduction mode or a boundary conduction mode;
when the dimming current signal is lower than the first current threshold, the switch is controlled such that the power conversion unit operates in a discontinuous conduction mode, wherein when the dimming current signal is lower than the second current threshold, the switch is controlled such that the power conversion unit operates in a chopping control mode.
2. The dimming control method according to claim 1 , comprising:
setting a peak current reference according to the dimming current signal;
comparing the current sampling signal with the peak current reference, and controlling the switch to be switched from an on state to an off state when the current sampling signal is equal to the peak current reference.
3. The dimming control method according to claim 2 , comprising:
setting a ripple current reference, and calculating an off time of the switch according to the output voltage sampling signal, the ripple current reference, and an inductance of the inductor;
controlling the switch to be switched from the off state to the on state after the off time, such that the power conversion unit operates in the continuous conduction mode.
4. The dimming control method according to claim 3 , wherein the off time of the switch is calculated by the following formula:
T
off
=
I
pp
·
L
V
out
;
wherein T off is an off time of the switch, V out is the output voltage, I pp is the ripple current reference, and L is the inductance of the inductor.
5. The dimming control method according to claim 4 , wherein the ripple current reference is equal to a corresponding peak current reference when the dimming current signal is equal to the first current threshold.
6. The dimming control method according to claim 3 , further comprising:
setting a fixed switching period when the dimming current signal is higher than or equal to the second current threshold and lower than the first current threshold; and
calculating an on time of the switch according to the input voltage sampling signal, the output voltage sampling signal, the dimming current signal, and the inductance of the inductor, and adjusting an output current by adjusting a duty cycle of the control signal, thereby controlling the switch to make the power conversion unit to operate in a constant frequency discontinuous conduction mode.
7. The dimming control method according to claim 6 , wherein,
the on time of the switch is calculated by the following formula:
T
on
=
2
·
L
·
V
out
·
I
dim
f
s
·
V
in
·
(
V
in
-
V
out
)
;
wherein T on is the on time of the switch, L is the inductance of the inductor, V in is the input voltage, V out is the output voltage, I dim is the dimming current signal, and f g is a switching frequency of the switch.
8. The dimming control method according to claim 6 , further comprising:
when the dimming current signal is lower than the second current threshold, setting the duty cycle of the control signal to be fixed, and adjusting the number of pulses of the control signal by chopping to adjust the output current, thereby controlling the switch to make the power conversion unit to operate in the chopping control mode; and
in the chopping control mode, the duty cycle of the control signal is fixed to a corresponding duty cycle when the dimming current signal is equal to the second current threshold.
9. The dimming control method according to claim 3 , further comprising:
when the dimming current signal is higher than or equal to a third current threshold and lower than the first current threshold,
calculating a switching period of the switch according to the input voltage sampling signal, the output voltage sampling signal, the dimming current signal, and the ripple current reference;
calculating an on time of the switch according to the input voltage sampling signal, the output voltage sampling signal and the ripple current reference;
adjusting an output current according to the switching period and the on time, thereby controlling the switch to make the power conversion unit to operate in a variable frequency discontinuous conduction mode.
10. The dimming control method according to claim 9 , wherein,
the switching period of the switch is calculated by the following formula:
T
s
=
I
pp
2
·
L
·
V
in
2
·
I
dim
·
V
out
·
(
V
in
-
V
out
)
;
wherein T s is the switching period of the switch, I pp is the ripple current reference, L is the inductance of the inductor, V in is the input voltage, I dim is the dimming current signal, and V out is the output voltage; and
the on time of the switch is calculated by the following formula:
T
on
=
I
pp
·
L
V
in
-
V
out
;
wherein T on is the on time of the switch, I pp is the ripple current reference, and L is the inductance of the inductor.
11. The dimming control method according to claim 9 , further comprising:
setting the switching period to be fixed when the dimming current signal is higher than or equal to the second current threshold and lower than the third current threshold; and
calculating the on time of the switch according to the input voltage sampling signal, the output voltage sampling signal, and the dimming current signal, and adjusting the output current by adjusting a duty cycle of the control signal, thereby controlling the switch to make the power conversion unit to operate in a constant frequency discontinuous conduction mode.
12. The dimming control method according to claim 11 , wherein,
the on time of the switch is calculated by the following formula:
T
on
=
2
·
L
·
V
out
·
I
dim
f
s
·
V
in
·
(
V
in
-
V
out
)
;
wherein T on is the on time of the switch, L is the inductance of the inductor, V in is the input voltage, V out is the output voltage, I dim is the dimming current signal, and f g is a switching frequency of the switch.
13. The dimming control method according to claim 11 , further comprising:
when the dimming current signal is lower than the second current threshold, setting the duty cycle of the control signal to be fixed, and adjusting the number of pulses of the control signal by chopping to adjust the output current, thereby controlling the switch to make the power conversion unit to operate in the chopping control mode; and
in the chopping control mode, the duty cycle of the control signal is fixed to a corresponding duty cycle when the dimming current signal is equal to the second current threshold.
14. The dimming control method according to claim 9 , wherein in the discontinuous conduction mode, the peak current reference is fixed to a corresponding peak current reference when the dimming current signal is equal to the first current threshold.
15. The dimming control method according to claim 2 , further comprising:
generating a zero-crossing detecting signal by detecting a current on the inductor of the power conversion unit, and when a zero-crossing point of the current is detected, controlling the switch to be switched from the off state to the on state, such that the power conversion unit operates in the boundary conduction mode.
16. The dimming control method according to claim 15 , further comprising:
setting a switching period of the switch to be fixed when the dimming current signal is higher than or equal to the second current threshold and lower than the first current threshold; and
calculating an on time of the switch according to the input voltage sampling signal, the output voltage sampling signal, the dimming current signal, and an inductance of the inductor, and adjusting an output current by adjusting a duty cycle of the control signal, thereby controlling the switch to make the power conversion unit to operate in the discontinuous conduction mode.
17. The dimming control method according to claim 16 , wherein,
the on time of the switch is calculated by the following formula:
T
on
=
2
·
L
·
V
out
·
I
dim
f
s
·
V
in
·
(
V
in
-
V
out
)
;
wherein T on is the on time of the switch, L is the inductance of the inductor, V in is the input voltage, V out is the output voltage, I dim is the dimming current signal, and f g is a switching frequency of the switch.
18. The dimming control method according to claim 16 , further comprising:
when the dimming current signal is lower than the second current threshold, setting the duty cycle of the control signal to be fixed, and adjusting the number of pulses of the control signal by chopping to adjust the output current, thereby controlling the switch to make the power conversion unit to operate in the chopping control mode.
19. The dimming control method according to claim 18 , wherein in the chopping control mode, the duty cycle of the control signal is fixed to a corresponding duty cycle when the dimming current signal is equal to the second current threshold.