# Problem in designing the power supply dc to dc with ucx84x

#### qwwe

Joined Jul 18, 2017
31
Hello
I want to design an dc to dc converter with input voltage of 9 to 30 volts dc with cuk topology. For this I chose the uc2843 IC.
But in the design of the feedback part and the capacitor and the resistance of the compensator connected to the comp base and the resistance and the capacitor connected to the base of the vcc (which I specified in color), I have problems because I did not find any good guidance.

#### crutschow

Joined Mar 14, 2008
34,388
That's a somewhat complex circuit to use.
Read section 9 in the data sheet.

#### qwwe

Joined Jul 18, 2017
31
An example in Section 9 is Transformer

#### qwwe

Joined Jul 18, 2017
31
Yes, I have seen these but my problem is in calculating the feedback and compensator that is used in these pdfs for the case that is used with opto-coupler and trans

#### ericgibbs

Joined Jan 29, 2010
18,827
hi,
Could you post a circuit showing the proposed components, opto etc.?
E

#### qwwe

Joined Jul 18, 2017
31
I want to design a circuit like this, but with uc2843:

#### qwwe

Joined Jul 18, 2017
31
No one can guide

#### Aplicant

Joined Feb 23, 2017
32
Put 12v regulator (7812) to vcc if input voltage is greater than 20 or more
Put 100k and 100pF in parallel across FB and Comp
What voltage do you want at output
Feedback resistors are calculated according to the output voltage.

#### qwwe

Joined Jul 18, 2017
31
The input voltage varies from 9 to 28 volts.
Output voltage is 21 volts.
How do you calculate the resistance and capacitance associated with vfb?

#### ian field

Joined Oct 27, 2012
6,536
Hello
I want to design an dc to dc converter with input voltage of 9 to 30 volts dc with cuk topology. For this I chose the uc2843 IC.
But in the design of the feedback part and the capacitor and the resistance of the compensator connected to the comp base and the resistance and the capacitor connected to the base of the vcc (which I specified in color), I have problems because I did not find any good guidance.
There are 4 devices in that family with overlapping common specifications - 2 types for direct off line applications and 2 types for automotive. there are 2 different UVLO specifications, each with 50% & 100% duty factor output.

can't remember the UVLO lower thresholds, but you won't be going down to 9V unless you run the chip off an auxiliary booster.

#### Aplicant

Joined Feb 23, 2017
32
Put Rtop =50k
Put Rbottom = 6.8k
And i think your output will almost be 21v
Compensation resistor and capacitor are well calculated. It is a guess
They are usually good values (compensation resistor and capacitor)
Try it and write how it wokrs
Aplicant

#### Aplicant

Joined Feb 23, 2017
32
Sorry i was going to write :
Compensation resistor and capacitor are not very well calculated instead of compensation resistor ad capacitor are well calculated

#### ian field

Joined Oct 27, 2012
6,536
I want to design a circuit like this, but with uc2843:
Probably easier with an MC34063 - there's tons of examples online.

#### qwwe

Joined Jul 18, 2017
31
But I only have difficulty calculating the capacitor and compensating resistor, but it's an example in a datasheet that is calculated in terms of zero and amplifier poles and etc., which is complicated.

#### crutschow

Joined Mar 14, 2008
34,388
it's an example in a datasheet that is calculated in terms of zero and amplifier poles and etc., which is complicated.
Yes, designing proper feedback compensation for SMPS is complicated.
If you want simple, use one that already has the components designed for you.
Otherwise, the attached may help.

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#### ian field

Joined Oct 27, 2012
6,536
But I only have difficulty calculating the capacitor and compensating resistor, but it's an example in a datasheet that is calculated in terms of zero and amplifier poles and etc., which is complicated.
There are some calculator apps, an online one and one you download and install.

They give you the inductor value if you tell it frequency and how much current. The downside is it only applies to the bare chip - if you use an external transistor for higher current, you have to interpolate which requires more knowing what you're doing.