UC3854B: uc3854b

Hello Russell,

I  dont know what your pfc design looks like but without a schematic it will be impossible to know what is happening.

A schematic, test waveforms and design requirements are all useful documents.

Regards

John

Hi

The design is based on the typical design from TI website for a 250 W power factor regulator utilizing the UC3854.,on which I see that the Vin range is 80-270 VAC ,is that the same input voltage range for the UC3854B.

Russell

Hi Russell,

There are significant differences between the UC3854 and the UC3854B.

You will need to make changes to your schematic depending on the part you are using

SLUA177A details these differences and also shows a reference design.

This might be a place to start.

Regards

John

Hi,

I simply need to know if there is a minimum input voltage(AC,RMS)that needs to be provided for the output to be around 40-75VDC,because on the typical schematic it is indicated the 80VAC is the minimum voltage.

Russell

Hi Russell,

The PFC is a boost converter, if the boost mosfet is not been switched then the output voltage will be the peak rectified version of the input voltage because the output is connected to the AC line voltage through the bridge rectifier and the boost diode.

The normal operating range for most commercial boost designs cover the universal AC input voltage range of 85Vac to 260Vac where the PFC output voltage is set at 380V to 400V dc.

Without more details such as the exact issue you are experiencing alnon with waveforms and schematic it will be difficult to give more guidance.

Regards

Peter

Hi Peter,

Thanks for the reply,but it did not address exactly what I am asking about.

I know that its a boost converter and that is actually my concern because it was not boosting the output voltage at all (,actually Vo was less than Vin ) and also my current waveform was badly distorted.

The switching Mosfet is off course being used and connected to the gate driving pin on the UC3854B,but the current rating that my college engineer used was 2 A which eventually caused it to burn out.

He also recommended to use a Vin AC of less than 60 v.

So my theory initially was may be the chip had to have a minimum Vinac applied for it to be able to boost that voltage,do you this that is the case,or may be its the low current rating of the mosfet that preventing the voltage boosting and cause the current waveform distortion.

Thank you Sir.

Russell

Russell

There is no minimum input voltage required on the AC input to the pfc

Have you checked that the enable voltage on ENA  (pin 10 ) has reached its threshold value of 2.65V ?

Also this part has a VCC(on) threshold of about 12V

John

Hi John,

What prompted me to ask about the minimum Vin is the fact that there is a voltage range for Vin OF 80-240 Vac  on the schematic for the typical design,can you explain that ?

I also want to know if the current rating of 2 A for the MOSFET can cause the outpiut voltage to be the same as the input and for the current waveform to be distorted.

Russell

Hi Russell,

Most pfc applications need to operate from an input range of 85Vac to 264Vac.
This is a "standard" range and it covers the full operating range for  both the 120Vac and 220Vac line applications.
It is sometimes called  "universal" input.

The application document for the UC3854B has an input range of 80Vac to 270Vac
https://www.ti.com/lit/an/slua177a/slua177a.pdf

The current rating of the MOSFET has no bearing on the output voltage value.

Do you have a scope plot of what the current waveform looks like ?

John

Hi John,

Unfortunately ,I don't a scope plot that I can send now,and because the MOSFET is burned out I cant recreate the output now.

In our case the input voltage needed to be less than 50 Vac so I am not sure if that would negatively affect the performance of the chip and consequently the output .

Do you have any other ideas on what may cause that issue.

Would for example the discrete passive components values affect tha?I can try to send you a photo of my schematics if that helps but its based out of the relevant documentation on TI website.

Russell 

Hi Russell,

We can only work with the tools we have and if you cannot add new information I will go ahead and close this post.

Please open a new post if you can add any new information.

Regards

John

You did not help me at all though, all your responses where general and not new.

You did not give any effort to help me.

Hi Russell,

Since we are working on the same topic on your new post, I am going to close this one.

Regards

Peter