L6561 Flyback LED Driver

[TRUNG_EMC]

I'm using an L6561 to build a flyback circuit to control an LED with constant current. The input voltage is 220VAC 50Hz. The output controlling the LED is 12V 0.15A. I tested it with 12V 0.7A and the circuit worked very well with a power factor (PF) of 0.96 and THDI <10%. However, when testing with 12V 0.15A, the PF was 0.68 and THDI was around 40%. Could it be that the L6562 doesn't have a good power factor under light load?

[TRUNG_EMC]

Can anyone help me?

[Richard Li]

In AN966-I6561 page 8, noticed Cin related PF and THDI, you may try reduce C1.

[TRUNG_EMC]

Okay, I'll give it a try.

[TRUNG_EMC]

I tried it and it didn't change anything. I measured at the gate and found it was outputting pulses in bursts, not continuously.

[unsigned_char_array]

Sounds like it’s operating in discontinuous mode. Higher inductance or higher switching frequency can prevent that.

[TRUNG_EMC]

So should I increase the inductance of the primary or secondary side?

[unsigned_char_array]

What does your simulation say?

[Richard Li]

I didn’t use this chip, I work for DC-DC switcher, in DC-DC , there are two kinds of operation, one is normal PWM, change pulse width, for heavy load, for light load, the efficiency drop, then keep pulse width, cut the pulse, call skip mode as @unsigned_char_array   noticed, discontinuous mode, I guess this chip use same,  Most chip test at 60 Hz, so the resistor , capacitor inductor use 60 hz,  You should adjust R C , try fit your usage condition. For my experience, first you need find the PWM to skip mode change point loading, then easy fig out.

[TRUNG_EMC]

Thanks

[Richard Li]

Do you have equipment can test switch feedback phase? if you want make big change like change frequency, you need measure “Bode Plot”, switch feedback <30 degree is very dangerous, easy cause system crush. Online a lot of tool can make simulation, we don’t trust it, use hardware equipment measure.

[TRUNG_EMC]

I only have an oscilloscope.

[Richard Li]

My suggestion is don’t make big change except you have enough knowledge for switch circuit. As @unsigned_char_array noticed, at least need got simulation result.

As your title, your design is flyback, Check Design equations of high-power-factor flyback converters based on the L6561

you need change fig 15 R7,R8, C2 maybe R5.

Before you change, couple questions you need ask yourself, these like trade-offs, the light load low PF really need change (if it only very small percentage, no need.) , is your product want use different voltage- frequency (50-60?), how high cost component you can use, how long this product you want keep lifetime.

We try find one flyback design bug use longer than 2 months, more than 4 engineers work, I found it is control chip deign bug. It is very painful.