Boost converter using 3843

[Freeza]

im trying to make a boost converter Vin 15 to Vo 30 v

is my circuit efficient to be a boost converter ?
why is that if i add a load parallel to capacitor out i get a negative Vo ?

thanks

 

[dougy83]

You're missing the ground reference on the negative side of the 15V supply

 

[Freeza]

i just put the ground and it does not the same graph again its only 15 v now

why is this ?

 

[dougy83]

Because you don't have a voltage divider to feed the feedback pin

 

[Freeza]

i just made another circuit kind same as this i get 30~32 v Vout if there is no load connected in output,
but when i put a certain load the voltage drops depends on the resistor .Is there a way that i can get a Vout 30~31 v even it has a load in output ?

 

[ronsimpson]

Please send new circuit with your changes.

 

[Freeza]

i make another circuit using the layout given to us .
the connection of UC3843
Vin = 15 V
Vo = 30V
Pccm=30w
Pbcm=10w

just calculated the duty cycle w/c is .5 Vo=Vs/(1-D)
L=75mH
CT and RT based on datasheet
Rout P=V^2/R

why is that my circuit wont amplify voltage when i tap in the Rout a probe ?
is my calculations and values wrong ?

 

[ronsimpson]

You have many problems. Some were said in posts above.
Power to IC is wrong. In real life this will kill the IC. In spice it will not work.
No ground connection for input voltage.
No voltage divider on Vout to Vf.
....The IC has one job. To keep Vfb at 2.5V. Because you are sending 15 to 30 volts into Vfb the IC thinks the output voltage is too high so it stops working. In real life this kills the IC. You must divide 30V down to 2.5V. The IC will then egulate to keep 2.5v at Vfb.

 

[Freeza]

thanks for the corrections
i modified it it runs in simetrix approx 30-32v but at some point error occured

like this



any suggestion about the error same as in multisim

 

[ronsimpson]

Time stamp error: I don't know but look it up under help. There is some "time setting" this is set too small.

This is not how the IC was designed to be used but it is mostly working.
Normally SENSE looks at a resistor in the source of the MOSFET. (with a small RC).
The OSC cap is very large. 1.67uF?? The IC must be working very slow.
The error amp is strange. Move C4 to where R2 is. Remove R2. Make R3 10k or 33k.

HAVE YOU READ THE DATE SHEET FOR THE IC? LOOK AT APPLICATION CIRCUITS.

 

[Freeza]

we are ask to have a F = 500 hz based on these AS3843 instead of D= .5 i adjust to .77

 

[ronsimpson]

OK.....500hz. That is very slow. I use it at 50khz to 200khz. This makes the inductor much smaller.
Sounds like a school project.

 

[Freeza]

its a school project

any suggestion on how to select the proper diode i consider Ioutmax

 

[ronsimpson]

Diode voltage: The diode will see 30V. Get a 50, 60 or maybe a 100V diode.
Diode Current: Average current is IoutMax. Peak current is about 2xIouts maybe 3xIout.
Because you are not using Isenc pin there is no current limit! The current during start up is very large.
You are using the IC in voltage mode. It should be used in current mode to make it more stable and have current limit.

 

[Freeza]

oh thanks for that