Power supply for halogen lamp having an inverter and output circuit
View Patent ↗A circuit ( 10 ) for powering a halogen lamp ( 20 ) comprises an inverter ( 100 ) and an output circuit ( 300 ). During operation, the inverter ( 100 ) and output circuit ( 300 ) provide a magnitude-limited current to the halogen lamp such that the lamp power during an initial period is substantially less than the lamp power during a steady-state operating period. Preferably, output circuit ( 300 ) is a non-isolated circuit that includes a current-limiting inductance, and inverter ( 100 ) includes a frequency control circuit (R 2 ,C 4 ,C 7 ,Q 3 ,R 3 ,R 5 ,C 8 ) for operating the inverter at a higher frequency during the initial period so as to reduce stress upon the lamp filament and preserve the operating life of the lamp. The circuit ( 10 ) is especially suitable for powering low voltage halogen lamps.
1. A circuit for powering at least one halogen lamp, the circuit comprising:
an inverter; and
an output circuit coupled between the inverter and the at least one halogen lamp;
wherein the inverter and output circuit are operable to provide a magnitude-limited current to the at least one halogen lamp, such that the lamp power during an initial period is substantially less than the lamp power during a steady-state operating period;
wherein the inverter comprises:
first and second inverter input terminals for receiving a substantially direct current (DC) voltage source;
an inverter output terminal, at which the inverter provides a substantially alternating current (AC) output voltage;
first and second inverter switches coupled to the inverter input terminals and to the inverter output terminal; and
an inverter control circuit coupled to the first and second inverter switches, and operable:
(i) during a first period, to commutate the inverter switches at a first frequency; and
(ii) during a second period following the first period, to commutate the inverter switches at a second frequency, wherein the second frequency is lower than the first frequency;
and wherein the inverter control circuit comprises:
an inverter driver circuit coupled to the first and second inverter switches, the inverter driver circuit including a DC supply input, a first frequency control input, and a second frequency control input;
a startup circuit coupled between the first and second inverter input terminals and the DC supply input of the inverter driver circuit;
a bootstrapping circuit coupled between the output terminal of the inverter and the DC supply input of the inverter driver circuit; and
a frequency control circuit coupled to the bootstrapping circuit and to the first and second frequency control inputs of the inverter driver circuit, wherein the frequency control circuit is operable;
(i) during the first period, to direct the inverter driver circuit to commutate the inverter switches at the first frequency; and
(ii) during the second period, to direct the inverter driver circuit to commutate the inverter switches at the second frequency.
2. The circuit of claim 1 , wherein the output circuit is non-isolated.
3. The circuit of claim 1 , wherein the output circuit comprises:
first and second output connections adapted for coupling to the at least one halogen lamp;
a current-limiting inductance coupled between the inverter output terminal and the first output connection; and
at least one of:
a first direct current (DC) blocking capacitance coupled between the first inverter input terminal and the second output connection; and
a second direct current (DC) blocking capacitance coupled between the second output connection and circuit ground.
4. The circuit of claim 1 , wherein the bootstrapping circuit is a charge-pump circuit.
5. The circuit of claim 1 , wherein the bootstrapping circuit comprises:
a bootstrapping capacitor coupled between the inverter output terminal and a first node;
a first diode coupled between the first node and circuit ground;
a second diode coupled between the first node and a second node; and
a third diode coupled between the second node and the DC supply input of the inverter driver circuit.
6. The circuit of claim 1 , wherein the frequency control circuit comprises:
a frequency-determining resistance coupled between the first and second frequency control inputs of the inverter driver circuit;
a first frequency-determining capacitance coupled between the second frequency control input of the inverter driver circuit and circuit ground;
a second frequency-determining capacitance coupled to the second frequency control input of the inverter driver circuit; and
a frequency-shifting circuit coupled to the bootstrapping circuit and to the second frequency-determining capacitance, and operable, during the second period, to effectively place the second frequency-determining capacitance in parallel with the first frequency-determining capacitance.
7. The circuit of claim 6 , wherein the frequency-shifting circuit comprises:
an electronic switch having a gate terminal, a drain terminal, and a source terminal, wherein the drain terminal is coupled to the second frequency-determining capacitance, and the source terminal is coupled to circuit ground;
a first biasing resistance operably coupled between the second node and the gate terminal of the electronic switch;
a second biasing resistance coupled between the gate terminal of the electronic switch and circuit ground; and
a timing capacitance coupled between the gate terminal of the electronic switch and circuit ground.
8. The circuit of claim 7 , wherein the electronic switch is a N-channel field-effect transistor.
9. The circuit of claim 7 , wherein the frequency-shifting circuit further comprises a positive-temperature-coefficient (PTC) resistance coupled between the second node and a third node, and coupled in series with the first biasing resistance.
10. A circuit for powering a halogen lamp having a rated operating filament voltage that is substantially less than a voltage provided by a conventional alternating current (AC) power line voltage source, the circuit comprising:
an inverter, comprising:
first and second inverter input terminals for receiving the substantially direct current (DC) voltage source;
an inverter output terminal;
first and second inverter switches coupled to the inverter input terminals and to the inverter output terminal; and
an inverter control circuit coupled to the first and second inverter switches, the inverter control circuit comprising:
an inverter driver circuit coupled to the first and second inverter switches, the inverter driver circuit including a DC supply input, a first frequency control input, and a second frequency control input;
a startup circuit coupled between the first and second inverter input terminals and the DC supply input of the inverter driver circuit;
a bootstrapping circuit coupled between the output terminal of the inverter and the DC supply input of the inverter driver circuit; and
a frequency control circuit coupled to the bootstrapping circuit and to the first and second frequency control inputs of the inverter driver circuit, wherein the frequency control circuit is operable:
(i) during a first period, to direct the inverter driver circuit to commutate the inverter switches at the first frequency; and
(ii) during a second period following the first period, to direct the inverter driver circuit to commutate the inverter switches at the second frequency that is substantially less than the first frequency; and
a non-isolated output circuit coupled between the inverter and the halogen lamp.
11. The circuit of claim 10 , wherein the rated operating filament voltage of the halogen is on the order of about 40 volts.
12. The circuit of claim 10 , wherein the circuit occupies a physical volume that is small enough to allow the circuit to be situated within a lamp base portion of the halogen lamp.
13. The circuit of claim 10 , wherein the non-isolated output circuit comprises:
first and second output connections adapted for coupling to the halogen lamp;
a current-limiting inductance coupled between the inverter output terminal and the first output connection; and
at least one of:
a first direct current (DC) blocking capacitance coupled between the first inverter input terminal and the second output connection; and
a second direct current (DC) blocking capacitance coupled between the second output connection and circuit ground.
14. The circuit of claim 10 , wherein the frequency control circuit comprises:
a frequency-determining resistance coupled between the first and second frequency control inputs of the inverter driver circuit;
a first frequency-determining capacitance coupled between the second frequency control input of the inverter driver circuit and circuit ground;
a second frequency-determining capacitance coupled to the second frequency control input of the inverter driver circuit; and
a frequency-shifting circuit coupled to the bootstrapping circuit and to the second frequency-determining capacitance, and operable, during the second period, to effectively place the second frequency-determining capacitance in parallel with the first frequency-determining capacitance.
15. The circuit of claim 14 , wherein the frequency-shifting circuit comprises:
an electronic switch having a gate terminal, a drain terminal, and a source terminal, wherein the drain terminal is coupled to the second frequency-determining capacitance, and the source terminal is coupled to circuit ground;
a first biasing resistance operably coupled between the bootstrapping circuit and the gate terminal of the electronic switch; and
a second biasing resistance coupled between the gate terminal of the electronic switch and circuit ground.
16. A circuit for powering at least one halogen lamp, the circuit comprising:
an inverter, comprising:
first and second inverter input terminals adapted to receive a substantially direct current (DC) voltage source;
an inverter output terminal;
first and second inverter switches coupled to the inverter input terminals and to the inverter output terminal;
an inverter control circuit coupled to the first and second inverter switches, the inverter control circuit comprising:
an inverter driver circuit coupled to the first and second inverter switches, the inverter driver circuit including a DC supply input, a first frequency control input, and a second frequency control input;
a startup circuit coupled between the first and second inverter input terminals and the DC supply input of the inverter driver circuit;
a bootstrapping circuit coupled between the output terminal of the inverter and the DC supply input of the inverter driver circuit; and
a frequency control circuit coupled to the bootstrapping circuit and to the first and second frequency control inputs of the inverter driver circuit, wherein the frequency control circuit comprises:
a frequency-determining resistance coupled between the first and second frequency control inputs of the inverter driver circuit;
a first frequency-determining capacitance coupled between the second frequency control input of the inverter driver circuit and circuit ground;
a second frequency-determining capacitance coupled to the second frequency control input of the inverter driver circuit; and
a frequency-shifting circuit, comprising:
an electronic switch having a gate terminal, a drain terminal, and a source terminal, wherein the drain terminal is coupled to the second frequency-determining capacitance, and the source terminal is coupled to circuit ground;
a first biasing resistance operably coupled between the bootstrapping circuit and the gate terminal of the electronic switch;
a second biasing resistance coupled between the gate terminal of the electronic switch and circuit ground; and
a timing capacitance coupled between the gate terminal of the electronic switch and circuit ground; and
an output circuit, comprising:
first and second output connections adapted for coupling to the at least one halogen lamp;
a current-limiting inductance coupled between the inverter output terminal and the first output connection; and
at least one of:
a first direct current (DC) blocking capacitance coupled between the first inverter input terminal and the second output connection; and
a second direct current (DC) blocking capacitance coupled between the second output connection and circuit ground.
17. The circuit of claim 16 , wherein the bootstrapping circuit comprises:
a bootstrapping capacitor coupled between the inverter output terminal and a first node;
a first diode coupled between the first node and circuit ground;
a second diode coupled between the first node and a second node; and
a third diode coupled between the second node and the DC supply input of the inverter driver circuit.
18. The circuit of claim 16 , wherein the frequency-shifting circuit further comprises a positive-temperature-coefficient (PTC) resistance coupled between the second node and a third node, and coupled in series with the first biasing resistance.