# PWM dc boost

Discussion in 'General Electronics Chat' started by anaskoara, Apr 18, 2013.

Mar 25, 2008
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hi
can any one explain ho we can use PWMto boost dc volt to higher level like
converting 12v dc to 20v dc

I know we can use PMW less volatge level but .
I don't want circuits schematics I want just basic explanation

regards

Apr 24, 2011
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3. ### TheComet Member

Mar 11, 2013
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^Click the link, basically what I said.

Let's say you have a coil with current flowing through it. If you disconnect the coil from the current source, the magnetic field will collapse and create a voltage spike of a few hundred volts.

That's exactly what step-up converters do. Every boost converter charges a coil, disconnects it, and transfers the voltage spike into a capacitor through a diode, then repeats the process.

A PWM signal controls how long the coil charges, and how long it discharges, which in return is used to regulate the output voltage. The more load you have on the output, the larger the duty cycle becomes to compensate.

TheComet

4. ### Gibson486 Member

Jul 20, 2012
199
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For an inductor

V = L di/dt

With PWM, you switch the cycle on and off. So, when di/dt increases (your PWM goes faster), V becomes bigger to compensate.

5. ### crutschow Expert

Mar 14, 2008
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It's the duty cycle not the frequency that determines the output voltage.

ErnieM likes this.
6. ### JDT Well-Known Member

Feb 12, 2009
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Every time the transistor switches off, the energy stored in the inductor is "pumped" into the capacitor - increasing the voltage on the capacitor.

During the transistor ON time, the inductor builds up the energy in its magnetic field. This takes time. In fact the energy stored is proportional to time. This is where the PWM comes in!

In any boost converter the transistor ON time is longer than the OFF time. This is because it takes longer to store the energy in the inductor than it does to release it. This is because the voltage when storing is less than the voltage when discharging.

Mar 25, 2008
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thank you all for your response
but how the PWM control the process
I want to know more about current mod control and voltage mode control

8. ### cabraham Member

Oct 29, 2011
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The ON time may be less than, equal to, or greater than the OFF time. A boost converter functions with PWM duty cycles from 0% to 100%. When the input voltage is only slightly less than the output voltage, i.e. 12V input w/ 15V output, the duty cycle is well under 50%. The ON time is less than the OFF time.

For cases where the input voltage is way below the output voltage, the duty cycle exceeds 50% so that the ON time is greater than the OFF time. For 12 to 15 volt, the duty factor is given by

Vout = Vin/(1-D), where D is duty cycle, so D would equal 0.20 in this case, a little more to cover losses, say 25%. The ON time is 25% the switching period, with the OFF time at 75%.

The buck-boost converter OTOH, has a duty cycle greater than 50% when Vout exceeds Vin. If Vin=12V, Vout=12V, then ideally D=50%, but in reality D is maybe 55% or so to cover losses. Anyway I just thought I should mention this.

Claude