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DC-DC Step-Up Converter

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TimoTimo 07/30/2014 at 19:350 Comments

I will need 20V for one of the ink cartridges (HP C6602A). So I need to step up my voltage from 12V to 20V. Here I use a step-up dc-dc converter around MC34063 (from UTC in this case).

Multiple datasheets for MC34063 are available, e.g.:
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF
http://www.ti.com/lit/ds/symlink/mc33063a.pdf

Even some in depth discussion:
http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF
http://www.ti.com/lit/an/slva252b/slva252b.pdf

Calculators and diagrams:
http://dics.voicecontrol.ro/tutorials/mc34063/
http://www.nomad.ee/micros/mc34063a/

Video-tutorial by David L. Jones (funny and informative):
http://www.eevblog.com/2010/09/10/eevblog-110-lets-design-a-dc-to-dc-switchmode-converter/

Diagram with missing values (calculated below):


Here are my calculating steps for the missing parts. The following parameters are chosen initially:
Vin = 12V - 10% = 10.8V     (10% safety margin)
Vout = 20V       (needed for one of the ink cartridges)
Vf = 0.4V          (forward dropping voltage of diode by datasheet)
Vsat = 0.5V      (switch saturation voltage by datasheet)
f = 100kHz       (maximum frequency)
Iout = 50mA     (starting low, have to see, what's really needed)

Begin with calculations (just plug in values one-by-one):
ton/toff = (Vout + Vf - Vin) / (Vin - Vsat) = 0.9320388
ton+toff = 1/f = 10µs
toff = (ton + toff) / (ton / toff + 1) = 5.17588µs
ton = (ton + toff) - toff = 4.82412µs
Ct = 4 * 10^(-5) * ton = 193pF         (used 2 * 82pF measured as 200pF)
Ipk = 2 * Iout * (ton/toff + 1) = 0.1932A
Rsc = 0.3 / Ipk = 1.55Ω                    (1Ω in series to 2 * 1Ω in parallel = 1.5Ω)
Lmin = (Vin - Vsat) / Ipk * ton = 257µH           (used 330µH)

Instead of choosing Vripple, I used a 470µF low ESR cap and therefore (this part of) the ripple is very low.
Co = 9 * Iout * ton / Vripple          (used 470µF, so this part of the ripple is 4.6188mV)

Vout = 1.25 * (1 + R2 / R1)
=> (Vout / 1.25 - 1) * R1 = R2
here: 15 * R1 = R2

From AN920-D.PDF (see links above): The current through R1 + R2 shall be 500µA.

=> R1 + R2 = 16 * R1 = 20V / 500µA
=> R1 = 2.5kΩ => R2 = 37.5kΩ

I don't have those values, so I use (measured values):
R1 = 2386Ω              (5473Ω and 4230Ω in parallel)
R2 = 35820Ω            (976Ω and 34844Ω in series)
R2 / R1 = 15.0126    (quite good value)

The driver collector got 123.9Ω + 61.8Ω = 185.7Ω.

The input filter capacitor is a 100µF/16V low ESR cap.

Finished circuit:

Measuring:

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