UC3843: Asking about components in UC3843 circuit

Tri Dinh

Part Number: UC3843
Other Parts Discussed in Thread: UC2842, UCC28C42, UC2843

Hi, I am Tri from Vietnam.

I am doing the DC-DC 230VAC to 12VDC application. And UC3843 is used in this case. But I get stuck to some problems. Please help me to solve it!

Question 1: I'd like to ask about two diodes Zener between VCC & OUTPUT and OUTPUT & GND pins, I don't see any recommendations or equations to calculate this one. Could you please tell me how to determine it? ( I chose diode Zener 18V-1W between VCC - OUTPUT, 12V-1W between OUTPUT - GND by myself, is it right?)

Question 2: I chose Css = 10 nF and Rss = 10k ohm in Solf Start function, is it ok?

Question 3: Because I see Design Example Schematic on page 22 use UC2842. So if I use UC3843, do I need to change any components or parameters?

Note: Fsw is selected to be 110KHz same as example schematic.

Sorry if I got any mistakes because my English is limited.

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello Tri Dinh,

For your application I suggest you use the UCC28C42

http://www.ti.com/lit/ds/symlink/ucc28c42.pdf

You can see that Figure 31 of this data sheet is very similar to the drawing you attached in your post.
However the data sheet for the UCC28C42 has a detailed application section and it will guide you through the design process used in selecting the components.

The only difference between the UC2843 and the UC3843 is the wider temperature range of the UC2843.

A 10nF and 10k on Css and Rss should give you a soft start time of about 0.1mS

Regards

John

0 Tri Dinh over 5 years ago in reply to John Griffin1

Prodigy 40 points

Hi John,

Thank for your helping. Your information is very useful to me.

Best regards,

Dinh Cao Tri.

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UC3843: Asking about components in UC3843 circuit