LED designs that use a critical conduction mode (CRM) power factor corrected (PFC) controller may benefit from an upgrade. The control method used by most of today’s CRM PFC controllers is current mode (CM), but new controllers that use voltage mode (VM) are worth considering. They reduce power losses and noise, offer special features just for LED lighting, and are available as drop-in replacements for CM controllers.
Figure 1 gives a sample circuit design for each method.
Power losses associated with MOSFET switching
The main difference between current mode and voltage mode is how the controller decides when to turn off the power MOSFET.
In current mode, the MOSFET is turned off when the current sense resistor detects that the inductor current meets the desired reference current. The inductor current is the MOSFET drain current. The reference current is a percentage of the AC line voltage, which is sensed through a resistor divider network and fed to the control IC via the multiplier pin. Sensing the inductor current and the reference current, and then comparing the two, creates a fairly significant power loss. The signal value has to be high enough to perform compensation and control, and the over-current protection voltage needs to be above the signal, at somewhere between 1.2 and 1.4V.
In voltage mode, the MOSFET drain current is only used for over-current protection and inductor-saturation protection. This means the over-current protection voltage is much lower, at about 0.8V. This reduces the power losses in the current sense resistor by 25 to 50 percent. Also, there are modest power savings from the input and output sense resistors. The overall result is higher efficiency, especially in low-power applications. In addition, proper optimization of the resistance can result in a level of quasi-resident switching for the MOSFET.
Issues associated with the compensation network
Another significant difference between current mode and voltage mode is the placement of the compensation network. In Figure 1, the compensation network is represented by the capacitor Ccomp.
In current mode, Ccomp is placed between the output of the error amplifier and the resistor divider used to sense the output voltage. When the power supply is turned on, it takes a certain amount of time and current before the high-resistance output sense resistors have an effect on the charge of Ccomp. This creates an over-voltage condition. Turning on the power can also create an initial peak voltage that is quite high, and that can cause the output voltage to ring. The over-voltage condition and the output ringing can result in audible noise when the lamp is first turned on.
In voltage mode, Ccomp is between the output of the error amplifier and ground. Current doesn’t need to flow to the output voltage at the over-voltage protection limit and because one node is connected to GND and the other to the error amp output, the circuit is less susceptible to layout-induced noise.
Special VM features for LED lighting
One of Fairchild’s new VM PFC controllers, the FL7930C, includes special features just for LED lighting. An integrated optimizer improves total harmonic distortion (THD) while eliminating external components. This frees up an external pin for use as a PFC-ready pin. The pin simplifies power sequencing by telling the downstream DC/DC converter that the output voltage is at 89 percent of the set bus voltage.
A soft-start feature, with zero overshoot, ensures safe operation and restart by protecting the circuit from high current surges or high-voltage spikes if the AC line voltage is momentarily interrupted. Soft-start also improves reliability by lowering the voltage stress on the bulk capacitors as well.
The FL7930C pinout makes it a drop-in replacement for CM controllers like the FAN7527B. The FL7930C can be used on the existing board, without layout changes, but this takes away the ability to use the PFC-ready function in pin 2.
Figure 2 shows what needs to change to make the board work with the PFC-ready function.
In LED lighting, where the key factors are high efficiency, high power factor (PF), and low THD, CM CRM PF controllers are a common choice for the driver. Upgrading to a VM CRM PFC controller can provide an improved design with fewer power losses, less noise, better THD ratings, simpler power sequencing, and safer operation. Fairchild’s drop-in replacements provide an easy way to make the change.
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