UC2843: the Error Amplifier Transconductance

user5084128

Prodigy 70 points

Part Number: UC2843

I need the Error Amplifier Transconductance of UC2843A-Q1.

over 7 years ago

0 Benjamin Lough over 7 years ago

TI__Mastermind 27135 points

Hi User5084128,

Thanks for your interest in UC2843. The error amplifier within UC2843 is not a transconductance amplifier. UC2843 uses a conventional operational amplifier which needs compensation connected from the amplifier output to the inverting input pin of the amplifier. For reference, take a look at this application note: www.ti.com/.../slva662.pdf.

Best Regards,
Ben Lough

0 user5084128 over 7 years ago in reply to Benjamin Lough

Prodigy 70 points

What's the difference between these two kinds of amplifiers?
What will be the impact on the design of the compensation loop for the power supply?(between FB pins and COMP pins )
The application note might have been removed, had its name changed, or is temporarily unavailable.I can't find it.

0 Benjamin Lough over 7 years ago in reply to user5084128

TI__Mastermind 27135 points

Here is the link to the application note:

The core difference is an operational amplifier is a voltage controlled voltage source. A transconductance amplifier is a voltage controlled current source. An op-amp is compensated by connected components between the amplifier output and the inverting input of the amplifier. A transconductance amplifier is compensated by connecting components between the output of the OTA to ground.

Best Regards,

Ben Lough

0 user5084128 over 7 years ago in reply to Benjamin Lough

Prodigy 70 points

Thank you very much for your answer, which has made me a great deal.

I have another question：Is there a ramp compensation inside UC2843？If yes,I need the detailed data.

Iooking forward to your reply.

0 user5084128 over 7 years ago in reply to Benjamin Lough

Prodigy 70 points

Does UC2843 have the output short circuit protection ?If yes, what is the reference voltage of short-circuit comparator?

0 Benjamin Lough over 7 years ago in reply to user5084128

TI__Mastermind 27135 points

Hi user5084128,

Slope compensation is achieved with UC2843 by adding a portion of the RT/CT waveform to the ISENSE waveform. Take a look at section 8.3.9 of the datasheet.

There is cycle by cycle current limit on UC2843 which will limit the peak current in the primary side MOSFET. The threshold is 1V. Take a look at section 8.3.2 and 8.3.3 of the datasheet for reference.

Best Regards,
Ben Lough

0 user5084128 over 7 years ago in reply to Benjamin Lough

Prodigy 70 points

But the cycle by cycle current limit can't protect when the output is short circuited.
Through parameter calculation, we can see that the original peak current is set to 0.28A. So the Rcs is:1V/0.28A=3.57Ω. Select Rcs 3Ω in the circuit. It is found in the test that when the output terminal is short circuited, the chip cannot stop the wave and cause the circuit damage.
Increase the Rcs resistance to 4.5Ω , the Short circuit protection is OK,but the Voltage Regulation is unsatisfactory. Vout=20.88V@Vin=200V,Vout=22.1V@Vin=250V. When Vin>250V,the Voltage Regulation is acceptable.
I want to know where the problem is. In the circuit, Rcsf=470Ω,Ccsf=470pF

0 Benjamin Lough over 7 years ago in reply to user5084128

TI__Mastermind 27135 points

Hi user5084128,

I would suggest installing two 7.15Ω in parallel for Rcs. Alternatively, you could attempt to increase the fast lane gain in your secondary side feedback.

Best Regards,
Ben Lough

0 user5084128 over 7 years ago in reply to Benjamin Lough

Prodigy 70 points

I have set the Rcs 4Ω, it still can't realize the short circuit protection.So two 7.15Ω in parallel is failed.
There is no optocoupler in the circuit . Directly feedback to the FB pin with two resistance,So I can't increase the fast lane gain