Switched mode power supply
A switched mode power supply comprising: a rectified voltage terminal and a rectified ground terminal for receiving a phase-cut rectified voltage; a power switch and an inductive element connected in series between the rectified voltage terminal and the rectified ground terminal; a switch controller having a floating ground terminal and a dimming input terminal, wherein the dimming input terminal is for receiving a dimming voltage signal indicative of a required dimming level, wherein the switch controller is configured to control the switch in accordance with the dimming voltage signal and wherein the inductive element is connected between the rectified ground terminal and the floating ground terminal; and a dimming level controller configured to set the dimming voltage signal in accordance with the rectified phase-cut voltage with respect to the floating ground terminal.
1. A switched mode power supply, comprising:
a rectified voltage terminal and a rectified ground terminal for receiving a phase-cut rectified voltage;
a power switch and an inductive element connected in series between the rectified voltage terminal and the rectified ground terminal;
a switch controller having a floating ground terminal and a dimming input terminal, wherein the dimming input terminal is for receiving a dimming voltage signal indicative of a required dimming level, wherein the switch controller is configured to control the switch in accordance with the dimming voltage signal and wherein the inductive element is connected between the rectified ground terminal and the floating ground terminal; and
a dimming level controller configured to set the dimming voltage signal as an average of a portion of the rectified phase-cut voltage with respect to the floating ground terminal over one or more half cycles of the phase-cut rectified voltage.
2. The switched mode power supply of claim 1 , wherein the dimming voltage signal is provided with reference to the floating ground.
3. The switched mode power supply of claim 1 , further comprising a potential divider with a first divider resistor and a second divider resistor, wherein the potential divider is connected in series between the rectified phase-cut voltage terminal and the floating ground terminal, and wherein the dimming voltage signal is available at a node between the first divider resistor and the second divider resistor.
4. The switched mode power supply of claim 3 , further comprising a Zener diode and an averaging capacitor connected in series between the rectified phase-cut voltage terminal and the floating ground terminal, the averaging capacitor having a first plate and a second plate, wherein the second plate is connected to the floating ground terminal, the first plate is connected to the first divider resistor, the first divider resistor is provided in series with, and between, the Zener diode and the averaging capacitor, and the second divider resistor is connected in parallel with the averaging capacitor.
5. The switched mode power supply of claim 3 , further comprising a bias capacitor with a first plate and a second plate, the first plate of the bias capacitor connected to a junction between the Zener diode and the first divider resistor and the second plate of the bias capacitor connected to the rectified ground terminal.
6. The switched mode power supply of claim 1 , wherein the average dimming voltage signal is:
V
dim
(
avg
)
=
2
·
V
mains
·
R
div
1
R
div
1
+
R
div
2
·
(
cos
φ
-
cos
(
π
-
α
)
π
-
(
π
-
α
-
φ
)
·
(
V
Z
+
2
·
V
D
+
V
ESR
2
·
V
mains
)
π
)
where
α
=
sin
-
1
(
V
Z
+
2
·
V
D
+
V
ESR
2
·
V
mains
)
,
α≦φ≦180, φ represents a dimmer phase cut angle, V mains is a root mean squared line voltage at the input terminals, V D is a voltage dropped by a bridge rectifier, V Z is a Zener stand-off voltage and V ESR is the voltage dropped over the inductive element.
7. The switched mode power supply of claim 1 , further comprising an additional resistor connected between the dimming input terminal and a reference voltage in order to provide a fixed offset to the dimming voltage signal.
8. The switched mode power supply of claim 1 , further comprising filtering circuitry connected between the power switch and the rectified voltage terminal or between the rectified voltage terminal and the rectified ground terminal.
9. The switched mode power supply of claim 1 , wherein the inductive element has a resistance between 0.1 and 20 ohms.
10. The switched mode power supply of claim 1 , comprising a buck-boost converter.
11. The switched mode power supply of claim 1 , wherein the dimming level controller and the switching controller are provided in a single package.
12. The switched mode power supply of claim 11 , wherein the power switch is also provided in the single package.
13. A solid state lighting unit comprising the switched mode power supply of claim 1 .