UC3854 PFC problems

on

Boost diode back to front?

DNA

Shouldn't there be a lot of smoke?

Graham Holloway

use

work

Boost diode is ok polarised.

I think inductor is the problem.

What kind of ferite corres are suggested for this type of application? Torus, EE,...?

Faulty/wrong capacitor?

Not Low ESR type?

Yes I tryed with varyous and without air gap, but always the same :-(

For protection I use 10 ohm resistor in serie with input AC voltage.

When I connect UC3854 to 18V, and disconnect input voltage of power part, duty cycle is about 90%

Then when I connect 60V to input, without a load, duty cycle is about 70%, and I get 380V at output, that is ok.

When I connect load ( 5.6k ohm resistor) on output, duty cycle is about 90%, output voltage drops to cca 100V and mosfet comes very hot.

In the moment when I disconnect load, duty cycle is about 5% for the moment, and then goes to 80% without load.

I tyed with one coil, with another coil, and with 2 coils in serie, and always the same thing.

I really don't know where can be the problem...:-(

I agree you need a scope and a current probe to even begin thinking about troubleshooting these.

Also lok at the layout re grounding,,, a small amount of ground coupling from the power circuits back into the control circuits can casue unpredicable behavior. Use single point grounding for the power and another single point ground for the low level stuff.

Mark

"Spirit"

....... Phil

Without any load, and a DC input voltage, after the start up transient the output voltage should be correct, and the duty cycle zero. Because of some losses (resistor divider for measuring the output voltage, ...), the duty cycle will be very low.

Did you made any calculations on the value of the coil?? saturation current?? Duty cycles that should be normal?? Continiuous/discontinous boost operation.

PFC's are generally not the most easy things to design, if you want to go on with the project, some guidelines (for these, an oscilloscope is needed. Without oscilloscope, I give you very little chance to make it work correctly)

- start from scratch with verifying basic UC3854 operation on your board : 1. oscillator running correct frequency

1. try a circuit that you can generate variable duty cycles at the output with a variable resistor --> you can use the current error amplifier with a simple feedback resistor in a basic opamp amplifier. This sets the level where the saw tooth from the oscillator is compared, and generates a certain duty cycle at the output.

- when you can generate veriable duty cycles, you can start playing around with the power stage. Use only DC input at the moment.

1. apply small input voltage (from a current limited power supply), DO NOT FORGET TO APPLY A LOAD RESISTOR TO THE OUTPUT!! Calculate the output voltage with a certain duty cycle (use also this duty cycle to start testing), the output voltage will be equal or less than the calculated voltage (it will be less if the boost converter works in discontinous mode).
2. Monitor the solenoid current!! Put a small ressitor in series with the soleoid (for example 1 Ohm for first low power tests, later on 0.1 Ohm or less). The resistor must have LOW PARASITIC IMPEDANCE otherwise, the voltage measured on it will not be equal to the actual current in the coil (it will show steps).
3. play around a bit with the duty cycle, load resistor and input voltage.
4. evaluate the inducatnce and saturation of the inductor used. The inductance can be calculated measuring the slope of the current and the voltage across the inductor. Saturation is the point where the current starts to rise faster than linear. The soleoid should always be used below saturation!
5. At the point where you reached the full input voltage and load conditions ( = output voltage set correctly with the potentiometer, and full load applied), evaluate output ripple, thermal behaviour of the MOSFET, solenoid, ... Also some early measurements about efficiency can be done.

When the tests above are finished with success, a first milestone is reached. Remember you are still working in a DC environment, not with AC input, so the final result will be different (according to efficiency, heating, output ripple ...)

- now it is time for the control loop. This is not a piece of cake. The control loop settings depends highly on the inductor value and output capacitance. This chip uses current mode control, wich is a big advantage for boost converters. Some interesting documentation is written by THE man of switching power supply control loops : Lloyd Dixon (the unitrode days).

There should be a document available for this control loop type used in PFC devices, but I can not find it right away.

Have fun.

moment,

Indeed, grounding can be very imortand. Also when doing measurements, don't be supprised to see a lot of switching noise. I'm always wondering how they generate the nice and clean waveforms in datasheets or application notes, ...