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Changing viltage for SMPS By DAC

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Paul GRIM

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Hello all,

I would like to vary the output voltage for my buck converter by a DAC.
V_DAC vary between 0 to 3.3V
V_HV vary between 0 to 100V
Can you help me how calculate the relation between V_DAC and V_HV :bang:
How i calculate R1, R2, R3 :?:

buck.PNG

Thank you
 

KlausST

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Hi,

is the OPAMP the feedback amplifier of your buck converter?


If so:
Voltage across R2 is constant 2.5V. No change in voltage no change in current, independent of output voltage.

The voltage across R1 should change 100V while the voltage across R3 changes 3.3V. No other voltage/current changes.
--> R1/R3 = 100V/3.3V --> R3 = R1 * 3.3V / 100V = 0.033 * R1

Output voltage will rise with falling DAC voltage.
U_out = 100V - (100/3.3) * U_DAC

Imagine U_DAC = 2.5V, then voltage across R3 is 0V, therfore current is 0mA.
The output voltage = 100V - (100/33) * 2.5V = 24.24V
Then current through R1 = current through R2

U across R2 = 2.5V,
U across R1 = 24.24V - 2.5V = 21.74.

Now R2 = 2.5V *R1 / 21,74V = 0.115 * R1

Klaus

***
Edit: Calculation error of R2
 

Paul GRIM

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Hi Klaus,

Thank you for your answere :grin:
yes, the OPAMP is the feedback amplifier of my buck .

But if i simulate my circuit, i havn't a good value
for ex: R1=20K; R2=2.3K; R3=660

V_DAC=0 => Vout=100V
if V_DAC=3.3V => Vout=45V :?:
Have you an idea for this error ?

Thank's

Paul
 

BradtheRad

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For comparison, here is my simulation of a regulated buck converter.



The op amp acts as a comparator. Hysteresis is added via positive feedback.

The op amp turns on when the load voltage drops. It shuts off when load voltage rises to the reference V at the input. A buck converter is an easy type to operate with this simple feedback.
 

KlausST

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Hi,

Maybe i have a calculation error.

For a real DAC the 600 Ohms may be a bit low.
Try twice the values.

I'll check my calculations in 12 hours.

Klaus
 

Paul GRIM

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Hi,

I am checked your formulation and they are correct.
my output voltage vary if the V_dac vary :grin:
this relation is approuved in simulation (Uout=100V-(100/3.3)*Udac)

in our case if Udac=0 Uout=100V, is it possible to invert this relation (If Udac=0 Uout=0 and if Udac=3.3 Uout=100V) :thinker:

Thank you so much

Paul
 

KlausST

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Hi,

in our case if Udac=0 Uout=100V, is it possible to invert this relation (If Udac=0 Uout=0 and if Udac=3.3 Uout=100V)

.. with an inverting OPAMP at DAC output to change 0..3.3V --> 3.3V ... 0V

Klaus
 

Vbase

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Can you replace 2.5V ref with the DAC output? You can't otherwise do 0-3.3->0-100
 

Paul GRIM

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Good remarque, but i havn't access to 2.5V ref voltage, it's internal of the chip
 

KlausST

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Hi,

Can you replace 2.5V ref with the DAC output? You can't otherwise do 0-3.3->0-100
Why not? Because of ghe supply rails?
Then just give the feedback resistor R1 a higher value to gain some headroom.

Klaus
 

Vbase

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Hi,


Why not? Because of ghe supply rails?
Then just give the feedback resistor R1 a higher value to gain some headroom.

Klaus

I meant he can't do it with his circuit. It looks like Paul got my point.
If Paul discloses which DAC maybe we can advise him how to save adding an inverter.
Is ghe a German swear word?
 

KlausST

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Hi,

A typo: "ghe" --> "the".

Klaus
 

Paul GRIM

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Hi,

please see this figure
Capture.PNG

My PWM is controlled by an analogic controller,
The DAC generate juste a reference voltage.

Paul
 

Vbase

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It looks like it's possible to modify Vref from pin 11 but it is a risky business. Pin 11 is provided for C3 to balance C4. There are no guarantees that the error amp can take the common mode error and will response in a linear fashion. If you are making many of these circuits then you may run into troubles.

If you still want to try, measure the DC at pin 11 that it is 2.5V.
Select R8 R7 to give you 100V output.
Inject 1.25V to pin 11 or put resistor to ground that will take the voltage to 1.25V.
See if the output goes down to 50V.

BTW, if the 100V is sourcing low current it could be easier to do it with a linear amp instead of PWM.
 
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