Short circuit ballast protection

• US 6,667,584 B2
• Filed: 10/15/2002
• Issued: 12/23/2003
• Est. Priority Date: 10/18/2001
• Status: Expired due to Fees

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Hooijer, Christofher Daniel Charles, Langeslag, Wilhelmus Hinderikus Maria
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): A, Minh D
• Application Number:
  US10/270,964
• Publication Number:
  US 20030076055A1
• Time in Patent Office: 434 Days
• Field of Search: 315/58, 315/224, 315/291, 315/209.R, 315/307, 313/493, 313/113.A
• US Class Current: 315/224
• CPC Class Codes: H05B 41/2851 for protecting the circuit against abnormal operating conditions
• CPC Class Codes: H05B 41/2855 against abnormal lamp operating conditions
• CPC Class Codes: H05B 41/2985 against abnormal lamp operating conditions

Abstract

A fluorescent lamp preheating circuit heats electrodes prior to lamp ignition while utilizing a sense resistor to detect and correct short-circuit conditions caused by improper wiring. The circuit uses few components, each of which is independent of parameters such as switching frequency, duty-cycle, and filament resistance, and is therefore compact, cost-effective, and efficient. The preheating circuit can be switched off when the electrodes are warmed sufficiently in order to conserve energy and prevent thermal overload, or when short-circuit conditions are encountered.

Claims

1. A ballast circuit adapted to be connected to at least one fluorescent lamp, each lamp having two ends with a filament at each end, each filament connected to a lamp electrode, further comprising:

a circuit part for generating a supply voltage;

a preheating circuit powered by the supply voltage, configured to preheat the lamp electrodes;

a protection circuit, configured to detect short-circuit conditions in the preheating circuit, and comprising;

a sensing resistor coupled to the preheating circuit, configured to sense a current flowing through the preheating circuit;

a control signal circuit connected across the sensing resistor, configured to convert the sensed current into a control signal, comprising

a low-pass filter, configured to prevent transitory currents from triggering a control means; and

a rectifying diode;

the control means connected between said preheating circuit and the sensing resistor, configured to disable the supply voltage when the control signal exceeds the value of a preset trigger level.

2. The circuit of claim 1, wherein said control means is a transistor.

3. The circuit of claim 2, wherein the transistor is a MOSFET.

4. The circuit of claim 3, further comprising a clamping diode configured to maintain a voltage across the MOSFET substantially at a bus voltage.

5. The circuit of claim 1, wherein the control signal is a rectified average DC-voltage.

6. The circuit of claim 1, wherein the preheating circuit comprises:

a filament heating transformer including a primary winding and at least two secondary windings; and

a DC blocking capacitor connected in series with the filament heating transformer, configured to remove DC voltage bias from the supply voltage.

7. The circuit of claim 6, further comprising:

a filament sensing circuit, configured to detect the absence of a lamp filament;

a capacitor positioned in series with one of the secondary windings of the filament heating transformer to prevent current flow through the secondary winding when the absence of a lamp filament is detected.