The current to drive light emitting diodes (LED) for lighting and other applications is commonly provided by a switched mode power supply or other switched mode power converter. Moreover, a single switched mode power converter may be able to provide the current required for multiple LEDs or LED strings. In some applications it is desirable to be able to separately control or dim such individual LEDs or LED strings. It is well known to provide bypass switches in order to provide this control function. In circumstances when all the bypass switches connected to a switched mode power converter operating as an LED current generator are conducting, such that all the LEDs are off, it is feasible to also turn off the current generator in order to save power.
The power savings that can be obtained by switching off a current generator for multiple of strings of LEDs can be substantial. This is illustrated in FIG. 1. FIG. 1 shows the variation of the driver efficiency with varying light level. Dashed line 1 indicates the driver efficiency where the current generator is always on. Contrastingly, dot-dashed line 2 indicates the efficiency obtainable by selectively switching off the current source, where solid line 3 indicates the on-off duty cycle of the current source so selectively switched. The system modelled in this figure has two bypass switches connected to a single current source. The LEDs connected in parallel with the two bypass switches, that is to say, the two LED channels, are both switched using pulse width modulated (PWM) signals. Both of the LED channels are 100% ON at the 100% light level, but are 50% out of phase. This represents a worst case situation. Thus, the solid curve 3 indicates the percentage of the PWM duty cycle for which the current generator is required to be on. The efficiency for a system which does not turn off the current source is shown in dashed line 1, whereas dot-dashed line 2 shows the system efficiency when the current generator is turned off when not required.
As shown in FIG. 1 a system efficiency improvement from approximately 12% to above 80% may be obtained for a 1% light level. For smaller or zero phase shift between the two bypass switches, and, or alternatively, for PWM duty cycle(s) smaller than 100%, even larger efficiency improvements are possible.
The efficiency at partial load, that is to say less than 100%, of solid state LED lighting systems is becoming increasingly important from an integral energy efficiency point of view, or total cost of ownership. With the increasing cost of power, this trend is becoming visible in other areas such as mains-connected consumer systems like personal computers and televisions, professional infrastructure systems such as router stations and server banks, as well as automotive applications. Methods and systems which contribute to power saving for a current generator combined with LED bypassing is thus of significant commercial interest.
Three basic methods of operating a switched mode power supply are illustrated in the current vs. time graphs of FIG. 2. FIG. 2(a) illustrates continuous conduction mode (CCM) operation. In this mode the current through the power supply inductor is always larger than zero. A second mode of operation is illustrated in FIG. 2(b). This is the so called boundary conduction mode (BCM), which is also sometimes referred to as critical conduction mode. In this mode of operation the current through the inductor is allowed to fall to zero; however it immediately starts rising again, although in practice it is typically controlled such that it goes a bit negative to allow for power-efficient zero-voltage (or zero-current) turn-on of the control switch. A third mode of operation is illustrated in FIG. 2(c). In this mode, termed discontinuous conduction mode (DCM), the current is pulsed; that is to say, the current rises to a maximum and then falls to zero, and there is a delay before the start of the next current pulse when the current starts to rise again. From the figure the origin for the term “boundary” conduction mode is apparent: this mode represents the boundary between continuous conduction mode and discontinuous conduction mode.
Most current generators operate in continuous conduction mode. If they have been turned off in order to save power and one of the bypass switches stops conducting, the LED current generator needs to be turned on again. Unfortunately, a current converter operated in CCM requires some time for the current to ramp up again; thus the current generator needs to turn on prior to the time when the bypass switch stops conducting. Although it is possible to implement this, additional circuitry is required, which adds to the complexity and cost of the generator.
This situation is illustrated in FIG. 3. This figure shows the pulse width modulation (PWM) signals 301 and 302 for two LED strings. The logical signal NOT OR, 303, corresponds to the time when the converter may be switched off since both LED strings are turned off and consequently no current is required. Thus current is not required when neither bypass switch 301 is conducting (during period 311), nor bypass switch 302 is conducting (during period 312). As shown, during part of the PWM cycle 305, the converter-off signal 303 is high and the converter current 304 is allowed to fall to zero. However, as shown in trace 304, there is a delay between the falling edge of the converter-off trace 303, that is moment t0, and the availability of the full converter current 304, that is moment t1. This delay, being the ramp up time 306, depends heavily on the specific implementation and the application, and may be dependent upon such factors as the inductor, the switching frequency, the input or output voltages, and so forth. No fixed value can thus be determined a priori. The problem may be passed on from the current generator designer to an application engineer through offering a user-adjustable ramp-up lead time. Alternatively, the required lead time may be automatically detected as proposed in applicant's co-pending European patent application no. EPO 8102752.6. This, however, requires a relatively complex circuit, especially when the PWM inputs are not generated on chip.
There thus remains an ongoing need to provide a switched mode power converter for LED applications which provides for high efficiency partial load operation.