UC3854B: Abnormal output waveform
Part Number: UC3854B
I am using the UC3854B for PFC application ,but was not getting the results needed,the voltage level was lower that the input and the current waveform was badly distorted.
The input voltage level was around 60 VAC,is that enough or does it need to be higher?
Its an urgent matter ,so a prompt response would be highly appreciated.
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.
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In reply to John Griffin1:
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.
In reply to Russell Hammad:
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.
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.
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.
In reply to Peter Meaney:
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.
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
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.
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 270Vachttps://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 ?
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.
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