# Boost circuit

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#### TAKYMOUNIR

Joined Jun 23, 2008
352
Can please some one help me with this circuit the input voltage is dc voltage and vary from 25vdc to 200 vdc and the output should be 120vdc(fixed) i use tiny 380 for switching and tl431 as a reference it is very low power less than 20 w OUT PUT VOLTAGE IS ACROSS C2 I TRY IT BUT THE VOLTAGE DOES NOT STOP IT GO TO MORE THAN 400 VDC SO HOW CAN I REGULATE THE OUT PUT AND MAKE IT 120 VDC

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#### praondevou

Joined Jul 9, 2011
2,942
At most I would expect the output voltage to go to 200VDC, since the TL431 will conduct if it's reference voltage is higher than 2.5V thus drawing more than the threshold current out of the EN pin of the TNY380.

Did you physically build this circuit with the component values shown in the schematic?

Thread Starter

#### TAKYMOUNIR

Joined Jun 23, 2008
352
How can i make the out put 120 vdc and stable i mean it does not change when input change
at most i would expect the output voltage to go to 200vdc, since the tl431 will conduct if it's reference voltage is higher than 2.5v thus drawing more than the threshold current out of the en pin of the tny380.

Did you physically build this circuit with the component values shown in the schematic?

#### praondevou

Joined Jul 9, 2011
2,942
That's essentially the same question as in your first post. You want to know why the circuit doesn't work.
So my first questions are:

1. Why did you choose 197.5k and 2.5k as a voltage divider?
2. Did you build the circuit or are you just working with the simulation?

#### praondevou

Joined Jul 9, 2011
2,942
I just remembered that the TL431 is supposed to work with at LEAST 1mA cathode current. The easiest would be to put a pull-up resistor of 24k from cathode to the input voltage (25V to 200V) and a zener diode of anything between 8.2V and 3.3V in parallel with the TL431 to avoid exceeding the max voltage rating on the EN pin. I cannot provide a drawing right now.

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