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# UC1901: UC1901 problem

Part Number: UC1901
Other Parts Discussed in Thread: UC3845

Hi Team

good day!

Customer have some question:

There are three questions when uc1901 is used in isolate feedback:
1. What is the relationship between driver output drivA / B and comp or inv of internal op amp? What’s the relationship between driveA/B and inverting of internal op amp in amplitude and fluctuation of amplitude?

2. The driving output becomes DC after being rectified through the drive transformer. What is the relationship between the DC amplitude and the inv (voltage output feedback end) of the op amp? Is the higher the output voltage, the greater the rectifier value?

3. When uc1901 and uc3845 are used together, how to design the comp compensation circuit separately? If the rectified DC of the transformer is taken as the feedback input of uc3845 to design the loop compensation, is the loop compensation design of uc1901 still necessary?

Regards

Aosker

• Hi Aosker!

Thank you for reaching out and for your interest in the UC1901 & the UC3845 devices! Let me do a little bit of research on this and I will get you an update early next week.

Thanks again, I look forward to talking more soon!

Aidan Davidson

• Hi Aosker!

1. What is the relationship between driver output drivA / B and comp or inv of internal op amp? What’s the relationship between driveA/B and inverting of internal op amp in amplitude and fluctuation of amplitude?

The waveform at the driver outputs is a squarewave with an amplitude that is proportional to the error amplifier input signal. There is a fixed 12dB of gain from the error amplifier compensation pin to the modulator driver outputs.

2. The driving output becomes DC after being rectified through the drive transformer. What is the relationship between the DC amplitude and the inv (voltage output feedback end) of the op amp? Is the higher the output voltage, the greater the rectifier value?

Correct.

3. When uc1901 and uc3845 are used together, how to design the comp compensation circuit separately? If the rectified DC of the transformer is taken as the feedback input of uc3845 to design the loop compensation, is the loop compensation design of uc1901 still necessary?

For this question I recommend checking out Section 7 of this application note. It gives a design example with the UC1840, a similar device to the UC3845.

Thanks again for getting in touch, I hope this information helps!

Aidan Davidson