AX6066+A433 realizes LED drive power supply

LED driver circuit design based on AX6066+A433

AX6066 is a converter with primary side feedback and output power between 12W and 65W. AX6066 is suitable for AC/DC power supply applications, which can meet the application requirements of low power consumption and high average working efficiency of AC line under no-load conditions. The chip can control the converter to work in discontinuous state mode. Discontinuous working mode provides a unique safety current limiting function, which is also insensitive to signal jitter on the AC line. Peak current modulation mode does not require slope compensation.

AX6066 works by driving the source of an external high-voltage power tube. This structure is called common-base, common-emitter (source-level) drive. It highlights the two advantages of fast startup and ensuring that the control chip is not connected to high voltage when there is no load. It has no effect on the normal operation of the converter with feedback flyback structure.

The feedback pin accepts current instead of voltage. Under no-load conditions, this design minimizes power dissipation on the primary side by avoiding external resistors that dissipate energy from current to voltage.

Under constant peak current and variable off-time modulation, the average efficiency of AX6066 is the highest between peak power and 22% of peak power. The internal modulation of AX6066 tends to keep its power constant between 22% and 100% of peak load, eliminating design difficulties and making the average efficiency of the converter reach energy star indicators.

Common-base, common-emitter biasing and startup

AX6066 uses a common base, common emitter drive and bias to control the high voltage power tube and provide initial bias at startup. This common base, common emitter structure realizes a universal gate control by controlling the switch connected between the ground and the source of the high voltage power tube at low voltage. There are the following key points to note:

1. A DC voltage should be applied to the gate of the external high-voltage power tube.

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

3. All the current of the initial coil must pass through the internal low-voltage driver tube.

AX6066 integrates a 90mΩ low voltage switching field effect transistor and all associated current sensing and driving. The external high voltage power tube is forced to follow the internal fast low voltage driver tube. The drain and gate of the external high voltage driver tube will not affect the shutdown speed because the gate terminal is connected to an independent DC power supply. This common base and common emitter structure allows the external high voltage power tube to be turned off quickly, and the switching loss of the field effect transistor switch will also be reduced.

Feedback function

The modulation and operation mode of the AX6066 are controlled by the current received on the FB pin of the chip. The FB pin is usually used to feed back the output error signal to the modulator inside the chip. The AX6066 receives the current fed back from the FB pin through the internal current mirror and feeds it to the internal feedback processing module, and then to the frequency modulation and current modulation module. The voltage on the FB pin is constant at 0.7V. There must be an output coupling capacitor at the emitter of the transistor to filter out the noise on the AC line. The cutoff frequency of this filter should be at least 10 times the maximum switching frequency of the converter. A 100KΩ resistor is required between FB and GND to eliminate the noise effect of the reverse current flowing to the FB pin when the converter is overloaded and reset. The optocoupler feedback structure with a smaller current transfer ratio has better no-load working state than the feedback structure with a larger current transfer ratio relying on the secondary coil.

Modulation Mode

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

The converter operates in frequency modulation with a larger power load (22% to 100% of the peak rated power). The peak current of the high-voltage power tube will reach its maximum programmable value, and the FB feedback current adjusts the output voltage by changing the operating frequency of the switch. The switching frequency is inversely proportional to the on-time of the switch. The switching frequency is modulated in the range from 30kHz (22% of the peak rated power) to 133kHz (100% of the peak rated power), the timing time is constant, and the rated current IDRV is also constant. The maximum programmable high-voltage power tube current IDRV, PK(max) is determined by the resistor connected to the CL pin.

The converter operates in AM mode at medium power levels (2.5% to 22% of peak rated power). The feedback current at the FB pin regulates the output voltage by modulating the rated current of the high-voltage power tube from 33% to 100% of the maximum programmable current value, and the operating frequency of the switch is fixed at around 30KHz. The AX6066 modulates the voltage on the CL pin from 3V to 1V to change the peak current.

The converter operates in green mode at a low load state (0% to 2.5% of peak rated power). The FB feedback current modulates the output voltage by using the FB current to generate a special trigger pulse. At this time, the peak current of the high-voltage power tube is 33% of the maximum programmable current value. The switching frequency including a trigger pulse is about 30kHz. The duration of the trigger process is modulated by the suppression force of the power supply and the feedback of the FB port. The AX6066 maintains energy conservation under low load and no load conditions by reducing the internal bias power consumption during the trigger process.

Primary Current Sensing

AX6066 uses current mirror technology to detect the initial current on the current modulator. All primary coil currents are connected to the external ground through the DRV pin and the driver power tube. The current on the driver power tube is proportional to the mirror image and controlled by the output of a PWM comparator compared with the CL current. At the beginning of each switching cycle, a shutdown pulse T of about 220ns is generated to the internal current limiter module, allowing the driver tube to open without false triggering limit on the main edge. The conventional capacitor discharge current will also appear in this circuit structure.

Zero Crossing Detection

In order to start the next switching cycle, the modulation needs to meet the following three conditions:

1. The previous switch on edge time must be equal to or greater than the processing time of the internal feedback block determined by the feedback current.

2. The previous switch on edge time must be longer than the minimum internal switching period of AX6066 (usually this period is 7.5us, corresponding to a frequency of 133kHz).

3. Quickly follow the ZCD zero-crossing voltage from high to low. The modulation time must be greater than the waiting time from the last zero-crossing detection to the next zero-crossing detection.

Each switching cycle is preceded by at least one zero detection on the ZCD pin. The modulation allows for damped oscillations between modulation pulses if the switching cycle needs to exceed the limit for damped oscillations and allows for longer pauses between modulation pulses under no-load operating conditions.

The switching frequency cannot exceed 133KHz in general. In order to keep the bulk voltage constant when it exceeds the minimum line voltage, the AX6066 will limit the maximum power by controlling the switching frequency.

The current in the AX6066 control inductor is always kept discontinuous. This design prevents current tailing during startup or short circuit conditions and facilitates maximum power control.

The voltage for zero current detection comes from the voltage on the auxiliary coil divided by a resistor, as shown in Figure 1 below. The phase of the voltage on the auxiliary coil is consistent with the phase of the voltage on the secondary output coil. The function of ZCD detection is to detect the demagnetization of the transformer. When the voltage of ZCD changes from high to low, there will be a 20mV ZCD threshold voltage. The voltage of the ZCD pin will be clamped at a drift voltage of -160mV inside the chip. A delay of 50ns to 200ns can be obtained by adding a capacitor CZCD connected to the ZCD port. This delay can keep the switch of the primary coil consistent with the trough of the voltage waveform of the initial coil.

Figure 1: Zero current detection function block diagram