Forward converter having a primary-side current sense circuit

Abstract

A load control device for controlling the amount of power delivered to an electrical load (e.g., an led light source) comprises an isolated forward converter comprising a transformer, a controller, and a current sense circuit operable to receive a sense voltage representative of a primary current conducting through to a primary winding of the transformer. The primary winding is coupled in series with a semiconductor switch, while a secondary winding is adapted to be operatively coupled to the load. The forward converter comprises a sense resistor coupled in series with the primary winding for producing the sense voltage that is representative of the primary current. The current sense circuit receives the sense voltage and averages the sense voltage when the semiconductor switch is conductive, so as to generate a load current control signal that is representative of a real component of a load current conducted through the load.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is
and C_PARASITIC is the total parasitic capacitance between the junction of the FETs Q210, Q212 and circuit common.

As previously mentioned, the controller increases and decreases the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 for controlling the FETs Q210, Q212 of the forward converter 140 to respectively increase and decrease the intensity of the LED light source. Due to hardware limitations, the controller may be operable to adjust the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 by a minimum time step T_STEP, which results in a corresponding step I_STEP in the load current I_LOAD. Near the high-end intensity L_HE, this step I_STEP in the load current I_LOAD may be rather large (e.g., approximately 70 mA). Since it is desirable to adjust the load current I_LOAD by smaller amounts, the controller is operable to “dither” the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2, e.g., change the on times between two values that result in the magnitude of the load current being controlled to DC currents on either side of the target current I_TRGT.

FIG. 9 shows an example waveform of a load current conducted through an LED light source (e.g., the load current I_LOAD). For example, the load current I_LOAD shown in FIG. 9 may be conducted through the LED light source when the target current I_TRGT is at a steady-state value of approximately 390 mA. A controller (e.g., the controller 150 of the LED driver 100 shown in FIG. 1 and/or the controller controlling the forward converter 240 and the current sense circuit 260 shown in FIG. 2) may control a forward converter (e.g., the forward converter 140, 240) to conduct the load current I_LOAD shown in FIG. 9 through the LED light source. The controller adjusts the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 to control the magnitude of the load current I_LOAD to between two DC currents I_L-1, I_L-2 that are separated by the step I_STEP (e.g., approximately 350 mA and 420 mA, respectively). The load current I_LOAD is characterized by a dithering frequency f_DITHER (e.g., approximately 2 kHz) and a dithering period T_DITHER as shown in FIG. 9. For example, a duty cycle DC_DITHER of the load current I_LOAD may be approximately 57%, such that the average magnitude of the load current I_LOAD is approximately equal to the target current I_TRGT (e.g., 390 mA for the example of FIG. 9).

FIG. 10 shows an example waveform of the load current I_LOAD when the target current I_TRGT is being increased with respect to time. As shown in FIG. 10, the controller 150 is able to adjust the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 to control the magnitude of the load current I_LOAD to between two DC currents I_L-1, I_L-2 that are separated by the step I_STEP. The duty cycle DC_DITHER of the load current I_LOAD increases as the target current I_TRGT increases. At some point, the controller is able to control the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 to achieve the desired target current I_TRGT without dithering the on times, thus resulting in a constant section 400 of the load current I_LOAD. As the target current I_TRGT continues to increase after the constant section 400, the controller is able to control the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2 to dither the magnitude of the load current I_LOAD between the DC current I_L-2 and a larger DC current I_L-3.

However, the constant section 400 of the load current I_LOAD as shown in FIG. 10 may cause the human eye to detect a visible step in the adjustment of the intensity of the LED light source. Therefore, when the controller is actively adjusting the intensity of the LED light source, the controller is operable to add a periodic supplemental signal (e.g., a ramp signal I_RAMP or sawtooth waveform) to the target current I_TRGT. FIG. 11 shows example waveforms of the ramp signal I_RAMP and the resulting load current I_LOAD when the ramp signal is added to the target current I_TRGT. Note that these waveforms are not to scale and the ramp signal I_RAMP is a digital waveform. The ramp signal I_RAMP is characterized by a ramp frequency f_RAMP (e.g., approximately 238 Hz) and a ramp period T_RAMP. The ramp signal I_RAMP may have, for example, a maximum ramp signal magnitude I_RAMP-MAX of approximately 150 mA. The ramp signal I_RAMP and may increase with respect to time in, for example, approximately 35 steps across the length of the ramp period T_RAMP. When the controller adds the ramp signal I_RAMP to the target current I_TRGT to control the on times T_ON of the drive control signals V_DRIVE1, V_DRIVE2, the resulting load current I_LOAD has a varying magnitude as shown in FIG. 11. As a result, the perception to the human eye of the visible steps in the intensity of the LED light source as the controller is actively adjusting the intensity of the LED light source is reduced

When the target current I_TRGT returns to a steady-state value, the controller may stop adding the ramp signal I_RAMP to the target current I_TRGT. For example, the controller may decrease the magnitude of the ramp signal I_RAMP from the maximum ramp signal magnitude I_RAMP-MAX to zero across a period of time after the target current I_TRGT has reached a steady-state value.

While FIG. 11 shows the ramp signal I_RAMP (i.e., a sawtooth waveform) that is added to the target current I_TRGT, other periodic waveforms could be used.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims

1. A load control device for controlling the amount of power delivered to an electrical load, the load control device comprising:

an isolated forward converter operable to receive a bus voltage and to conduct a load current through the electrical load, the forward converter comprising a transformer that comprises a primary winding coupled in series with a first semiconductor switch and a secondary winding adapted to be operatively coupled to the electrical load, the forward converter further comprising a half-bridge inverter comprising the first semiconductor switch and a second semiconductor switch coupled in series across the bus voltage for generating an inverter voltage at a junction of the two semiconductor switches, the junction of the two semiconductor switches coupled to the primary winding of the transformer through a capacitor, such that a primary voltage across the primary winding has a positive polarity when the first semiconductor switch is conductive and has a negative polarity when the second semiconductor switch is conductive, the transformer operable to transfer power to the secondary winding when either of the semiconductor switches is conductive, the forward converter further comprising a sense resistor coupled in series with the primary winding for producing a sense voltage representative of a primary current conducted through the primary winding;

a controller coupled to the forward converter for controlling the first semiconductor switch to control the load current conducted through the electrical load; and

a current sense circuit operable to receive the sense voltage, the current sense circuit comprising an averaging circuit for averaging the sense voltage when the first semiconductor switch of the forward converter is conductive, so as to generate a load current control signal that is representative of a real component of the primary current.

2. The load control device of claim 1, wherein the current sense circuit comprises a third semiconductor switch for disconnecting the sense voltage from the averaging circuit, the controller operable to render the third semiconductor switch conductive and non-conductive, such that the sense voltage is coupled to the averaging circuit when the first semiconductor switch of the forward converter is conductive.

3. The load control device of claim 2, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch, such that the sense voltage is coupled to the averaging circuit for the on time.

4. The load control device of claim 2, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch such that the sense voltage is coupled to the averaging circuit for the on time plus an additional amount of time when a target amount of power to be delivered to the electrical load is greater less than a threshold amount.

5. The load control device of claim 2, wherein the sense voltage is coupled to the averaging circuit through two series-connected resistors, the third semiconductor switch coupled between the junction of the two resistors and circuit common, such that the sense voltage is coupled to the averaging circuit when the third semiconductor switch is non-conductive.

6. The load control device of claim 1, wherein the controller is operable to control the magnitude of the load current to control the amount of power delivered to the electrical load to a target amount of power.

7. A load control device for controlling the amount of power delivered to an electrical load, the load control device comprising:

an isolated forward converter operable to receive a bus voltage and to conduct a load current through the electrical load, the forward converter comprising a transformer that comprises a primary winding coupled in series with a first semiconductor switch and a secondary winding adapted to be operatively coupled to the electrical load, the forward converter further comprising a sense resistor coupled in series with the primary winding for producing a sense voltage representative of a primary current conducted through the primary winding;

a controller coupled to the forward converter for controlling the first semiconductor switch to control the magnitude of the load current conducted through the electrical load to control the amount of power delivered to the electrical load to a target amount of power; and

a current sense circuit operable to receive the sense voltage and to average the sense voltage when the first semiconductor switch of the forward converter is conductive, so as to generate a load current control signal that is representative of a real component of the primary current;

wherein the current sense circuit is operable to average the sense voltage for an on time when the first semiconductor switch of the forward converter is conductive plus an addition additional amount of time, so as to generate the load current control signal that is representative of the real component of the primary current.

8. The load control device of claim 7, wherein the current sense circuit comprises an averaging circuit for averaging the sense voltage when the first semiconductor switch of the forward converter is conductive, such that the load current control signal is representative of the real component of the primary current, and wherein the averaging circuit comprises a capacitor across which the load current control signal is generated.

9. The load control device of claim 8, wherein the current sense circuit comprises an averaging circuit for averaging the sense voltage when the first semiconductor switch of the forward converter is conductive, such that the load current control signal is representative of the real component of the primary current.

10. The load control device of claim 9, wherein the forward converter comprises a half-bridge inverter comprising the first semiconductor switch and a second semiconductor switch coupled in series across the bus voltage for generating an inverter voltage at a junction of the two semiconductor switches, the junction of the two semiconductor switches coupled to the primary winding of the transformer through a capacitor, such that a primary voltage across the primary winding has a positive polarity when the first semiconductor switch is conductive and has a negative polarity when the second semiconductor switch is conductive, the transformer operable to transfer power to a secondary winding when either of the semiconductor switches is conductive.

11. The load control device of claim 8, wherein the current sense circuit is operable to average the sense voltage for the on time plus the addition additional amount of time when the target amount of power is greater less than a threshold amount.

12. The load control device of claim 11, wherein the addition additional amount of time is a function of the target amount of power.

13. The load control device of claim 12, wherein the addition additional amount of time increases as the target amount of power decreases.

14. The load control device of claim 13, wherein the addition additional amount of time increases linearly as the target amount of power decreases.

15. An led driver for controlling the intensity of an led light source, the led driver comprising:

an isolated forward converter operable to receive a bus voltage and to conduct a load current through the led light source, the forward converter comprising a transformer that comprises a primary winding and a secondary winding adapted to be operatively coupled to the led light source, the forward converter further comprising a half-bridge inverter circuit for generating an inverter voltage that is coupled to the primary winding of the transformer through a capacitor for producing a primary voltage across the primary winding, the forward converter further comprising a sense resistor coupled in series with the primary winding for producing a sense voltage representative of a primary current conducted through the primary winding;

a controller coupled to the forward converter for controlling the half-bridge inverter circuit to control the load current conducted through the led light source to control the intensity of the led light source to a target intensity; and

a current sense circuit operable to receive the sense voltage and to average the sense voltage when the magnitude of the primary voltage across the primary winding is positive and greater than approximately zero volts, so as to generate a load current control signal that is representative of a real component of the primary current.

16. The led driver of claim 15, wherein the half-bridge inverter circuit comprises first and second semiconductor switches coupled in series across the bus voltage for generating the inverter voltage at a junction of the two semiconductor switches, the junction of the two semiconductor switches coupled to the primary winding of the transformer through the capacitor, the controller coupled to the forward converter for controlling the first and second semiconductor switches, such that the primary voltage across the primary winding has a positive polarity when the first semiconductor switch is conductive and has a negative polarity when the second semiconductor switch is conductive, the transformer operable to transfer power to the secondary winding when either of the semiconductor switches is conductive.

17. The led driver of claim 16, wherein the current sense circuit comprises an averaging circuit for averaging the sense voltage when the first semiconductor switch of the half-bridge inverter circuit is conductive, such that the load current control signal is representative of the real component of the primary current.

18. The led driver of claim 17, wherein the current sense circuit comprises a third semiconductor switch for disconnecting the sense voltage from the averaging circuit, the controller operable to render the third semiconductor switch conductive and non-conductive, such that the sense voltage is coupled to the averaging circuit when the first semiconductor switch of the half-bridge inverter circuit is conductive.

19. The led driver of claim 18, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch such that the sense voltage is coupled to the averaging circuit for the on time plus an offset time when the target intensity is greater less than a threshold intensity.

20. The led driver of claim 19, wherein the offset time is a function of the target intensity.

21. The led driver of claim 20, wherein the offset time increases as the target intensity decreases.

22. The led driver of claim 21, wherein the offset time increases linearly as the target intensity decreases.

23. The led driver of claim 18, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch, such that the sense voltage is coupled to the averaging circuit for the on time.

24. The led driver of claim 16, wherein the forward converter comprises an energy-storage inductor operatively coupled in series with the secondary winding of the transformer, the energy-storage inductor comprising a partially-gapped magnetic core set.

25. A forward converter for controlling the amount of power delivered to an electrical load from an input voltage, the forward converter comprising:

a transformer comprising a primary winding and a secondary winding adapted to be operatively coupled to the electrical load;

a half-bridge inverter circuit comprising first and second semiconductor switches coupled in series across the input voltage for generating an inverter voltage at a junction of the two semiconductor switches;

a capacitor coupled between the junction of the two semiconductor switches of the half-bridge inverter circuit and the primary winding of the transformer, such that a primary voltage is produced across the primary winding, the transformer operable to transfer power to the secondary winding when either of the semiconductor switches is conductive;

a controller coupled to the half-bridge inverter circuit for controlling the first and second semiconductor switches to control a load current conducted through the electrical load;

a sense resistor coupled in series with the primary winding for producing a sense voltage representative of a primary current conducted through the primary winding; and

a current sense circuit operable to receive the sense voltage and to average the sense voltage when the first semiconductor switch of the half-bridge inverter circuit is conductive, so as to generate a load current control signal that is representative of a real component of the primary current.

26. The forward converter of claim 25, wherein the current sense circuit comprises an averaging circuit for averaging the sense voltage when the first semiconductor switch of the half-bridge inverter circuit is conductive, such that the load current control signal is representative of the real component of the primary current.

27. The forward converter of claim 26, wherein the current sense circuit comprises a third semiconductor switch for disconnecting the sense voltage from the averaging circuit, the controller operable to render the third semiconductor switch conductive and non-conductive, such that the sense voltage is coupled to the averaging circuit when the first semiconductor switch of the half-bridge inverter circuit is conductive.

28. The forward converter of claim 27, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch, such that the sense voltage is coupled to the averaging circuit for the on time.

29. The forward converter of claim 27, wherein the controller renders the first semiconductor switch conductive for an on time, and controls the third semiconductor switch such that the sense voltage is coupled to the averaging circuit for the on time plus an offset time when a target amount of power to be delivered to the electrical load is greater less than a threshold amount.