Electronics > Beginners
step-up or boost with MC34063
Mechatrommer:
--- Quote from: gbg on February 23, 2019, 09:45:55 pm ---But how can i do?
--- End quote ---
there is no indication minimum voltage to the 34063 power pin, so i assume 3.7V is large enough. here's how (attached) from the datasheet...
Ian.M:
The problem with 2.png at low input voltages is the internal and external transistors form a Darlington pair and its Vce_sat at several amps Ic will be about a volt, which will push the already less than spectacular efficiency even lower, I estimate to around 50%. The resulting increase in input current, depressing Vbatt, to attempt to maintain 700mA @ 12V out, may even push it past the point where its efficiency decreases fast enough with decreasing Vin and its output voltage collapses.
If you are forced to use a MC34063 for this application, you *NEED* a 2S series LiPO battery to maintain an adequate input voltage and reduce the current, and also a low Vgs threshold MOSFET for the external switching transistor to minimise the losses there.
Refrigerator:
You don't necessarily have to use an IC. Some time ago I designed a two transistor boost converter to boost specifically from a single cell lithium battery.
R2 is not needed and to increase the voltage you just change out the zener diode. I've used this circuit to boost from 5.5V all the way to 400V (with suitable mosfets of course), including 9V and 12V.
The limit of my design is about 17W so you should be able to get at least 1A at 12V with efficiency of at least 85%, the circuit idles at about 50mA (with an output LED)
Also note that the circuit does not have a low voltage cutoff so make sure your lithium cell has a protection circuit.
gbg:
--- Quote from: Refrigerator on February 24, 2019, 10:06:50 am ---You don't necessarily have to use an IC. Some time ago I designed a two transistor boost converter to boost specifically from a single cell lithium battery.
R2 is not needed and to increase the voltage you just change out the zener diode. I've used this circuit to boost from 5.5V all the way to 400V (with suitable mosfets of course), including 9V and 12V.
The limit of my design is about 17W so you should be able to get at least 1A at 12V with efficiency of at least 85%, the circuit idles at about 50mA (with an output LED)
Also note that the circuit does not have a low voltage cutoff so make sure your lithium cell has a protection circuit.
--- End quote ---
I have a low voltage cutoff (dw01a)
What is amper of L1?
I can not find BSZ0920NS.
Ian.M:
Assuming Refrigerator's 85% efficiency figure is good, the average input current for 700mA @12V out would be 3.3A @3V - a reasonable cutoff voltage for a LiPO. The on time must be three times the off time as there's 3V across L1 during the on time and 9V the other way during the off time, so the average during the on time is 4.4A. The inductor current ramps up linearly during the on time (assuming Rds_on and R(L1) are insignificant compared to the inductance) so you need a peak current without saturation of double the average, so 8.8A. You'll need a bit of safety margin on top of that.
That's really pushing the limits for a 2200mAH high discharge current LiPO without protection, so you'll need a lot of low ESR bulk decoupling capacitance at the boost circuit input to have any chance of avoiding tripping the battery protection.
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