High performance LED driver power supply design

Design of drive circuit based on AX6066+A433LED

The AX6066 is a converter with output power between 12 watts and 65 watts with primary feedback. The AX6066 is suitable for AC/DC power supply applications and can meet the application requirements of AC lines with low power consumption and high average operating efficiency under no load conditions. The chip can control the converter to operate in a discontinuous state mode. The discontinuous mode of operation provides a unique safety current limit that is also insensitive to signal jitter on the AC line. Peak current modulation mode does not require slope compensation.

The AX6066 works by driving the source of an external high voltage power tube. This structure is called a common-base, common-emitter (source-level) drive. It highlights the two advantages of fast start-up and the ability to ensure that the control chip is not connected to high voltage without load. There is no effect on the normal operation of a converter with a feedback flyback structure.

The feedback pin accepts current instead of voltage. In an unloaded operating state, this design minimizes the power dissipation of the primary side by avoiding the external resistor's energy consumption from current to voltage.

With constant peak current and varying off-time modulation, the AX6066 has the highest average efficiency between peak power and 22% peak power. The AX6066's internal modulation tends to keep its power between 22% and 100% peak load, eliminating design challenges and making the converter's average efficiency reach the energy star rating.

Common base, common emitter bias and start

The AX6066 uses a common-base, common-emitter drive and bias to control the high-voltage power tube and provides an initial bias at startup. This common-base, common-emissive structure implements a common gate control by low-voltage control of the switches connected between the ground and the source of the high-voltage power tube. There are the following key points to note:

1. The DC voltage of the external high-voltage power tube should be applied.

2. The external high voltage power transistor is driven by the source, not the gate.

3. All current in the initial coil must pass through the internal low voltage drive tube.

The AX6066 integrates a 90mΩ low voltage switch FET with all associated current sensing and drive. The external high voltage power tube is forced to follow the internal fast low pressure drive tube. The drain and gate of the external high voltage drive tube does not affect the speed of the shutdown because the gate is connected to a separate DC supply. This common-base, cascode structure allows the external high-voltage power tube to be turned off quickly, and the switching losses of the FET switch are also reduced.

Feedback function

The modulation and operation of the AX6066 is controlled by the current on the FB pin accepted by the chip. The FB pin is typically used to feed back the output error signal to the modulator inside the chip. The AX6066 accepts the current fed back from the FB pin through the internal current mirror to the internal feedback processing block and then to the frequency modulation and current modulation module. The voltage on the FB pin is constant at 0.7V. The emitter of the transistor must have an output coupling capacitor to filter out the noise on the ac line. The cutoff angle of this filter is at least 10 times the maximum switching frequency of the converter. A 100KΩ resistor is required between FB and GND to eliminate the effects of reverse current flow on the FB pin during converter reset. The optocoupler feedback structure with a smaller current transfer ratio is better than the no-load operation of the feedback structure of the secondary coil.

Modulation mode

In the normal operating mode, the feedback current of the FB pin controls the operating mode of the AX6066. The FB feedback current controls the converter to have three modes of operation: frequency modulation mode, amplitude modulation, and green modulation.