Transformer primary side lamp current sense circuit

A circuit for sensing a current in a secondary winding of a transformer comprising:

• a. a resistor coupled in series with a primary winding of the transformer;
• b. a transconductance amplifier coupled to the resistor for forming a first current signal wherein the first current signal is representative of a current in the primary winding;
• c. a capacitor coupled to receive the first current signal;
• and
• d. means for preventing the capacitor from receiving the first current signal during periods when a voltage signal applied to the primary winding is below zero.

A method of sensing a current in a secondary winding of a transformer comprising the steps of:

• a. monitoring a current signal representative of a current in the primary winding of the transformer;
• b. monitoring a voltage signal representative of a voltage across the primary winding wherein the voltage across the primary winding is substantially a sinusoid centered substantially at ground level;
• c. integrating the current signal over 180 degrees of the voltage signal;
• d. determining a desired power level to be delivered to a load coupled to the secondary winding;
• e. adding a quantity to a result of the step of integrating wherein the quantity is representative of the desired power level;
• and
• f. delivering power to a circuit coupled to the primary winding based upon a result of the step of adding.

A circuit for delivering power to a fluorescent lamp comprising:

• a. a transformer having a primary winding and a secondary winding wherein the fluorescent lamp is coupled to the secondary winding;
• b. a resonant circuit coupled to the primary winding;
• c. a power source coupled to the resonant circuit;
• and
• d. a controller circuit for controlling a level of power delivered to the resonant circuit based upon a desired level of brightness of the fluorescent lamp and based upon a level of current in the lamp wherein the controller comprises:
• (1) means for monitoring a voltage across a resistor coupled in series with the lamp;
• (2) means for monitoring a voltage across the primary winding;
• (3) means for charging a capacitor coupled to the controller with a current representative of the voltage across the resistor during periods when the voltage across the primary winding is positive;
• (4) means for adjusting the voltage across the capacitor based upon the desired level of brightness;
• and
• (5) means for generating a series of pulses delivered to the resonant circuit wherein a width of the pulses is representative of the voltage across the capacitor.

A circuit for sensing a current in a secondary winding of a transformer comprising:

• a. a resistor for monitoring a current in a primary winding of the transformer wherein at least a portion of the current flows through the resistor;
• b. means for integrating coupled to the resistor for integrating a signal formed across the resistor;
• and
• c. means for selectively enabling the means for integrating wherein the means for integrating is enabled during periods wherein a voltage applied to the primary winding is positive and wherein the means for integrating is disabled during periods when the voltage applied to the primary winding is negative.

A circuit for sensing a current in a secondary winding of a transformer comprising:

• a. means for monitoring a current in a primary winding of the transformer;
• b. means for integrating a signal formed by the means for monitoring, the means for integrating coupled to the means for monitoring;
• and
• c. a comparator for enabling the means for integrating during selected half-cycles of the voltage applied to the primary winding and for disabling the means for integrating during alternate half-cycles of the voltage applied to the primary winding wherein the comparator is coupled to the means for integrating.

A circuit for sensing a current in a secondary winding of a transformer comprising:

• a. a resistor for monitoring a first current in a primary winding of the transformer wherein at least a portion of the first current flows through the resistor;
• b. means for forming a second current coupled to the means for monitoring wherein the second current is representative of the first current during periods wherein a voltage applied to the primary winding is positive and wherein the second current is zero during periods when the voltage applied to the primary winding is negative and wherein the means for forming the second current comprises:
• (1) a transconductance amplifier having an inverting input, a non-inverting input and a transconductance output wherein the inverting input is coupled to a reference voltage level, the non-inverting input is coupled to the resistor and the transconductance output is coupled to the capacitor;
• (2) means for selectively preventing the transconductance output from charging the capacitor;
• and
• (3) means for sensing a voltage representative of a voltage across the primary winding coupled to control the means for selectively preventing;
• and
• c. a charge storage element coupled to receive the second current.