I needed to build a 5V to 48V DC to DC converter. After scouring the web for sample circuits, I found one that was based on the SG2534N. Well, the circuit had problems and I spent entirely too much time trying to get it to work. I guess the SG2534N has been around quite a while, but the amount of hobbyist understandable information is very small.
But, while flailing with with the SG2534N, I found quite a few references to the MC34063 IC. After a week of little luck with the SG2534N, I ordered the MC34063 from mouser.com.
While waiting for the parts, I spent time just experimenting with SMPS in general.
Besides being less expensive ($0.50 vs. $2.00) I was able to get this circuit up and running without too much trouble.
First, here are some references:
- High Voltage Ringer – This is the original post I stumbled across that gives a rough picture of the IC and a decent schematic of its use.
- Datasheet – This, the TI datasheet, is the best as it contains all of the necessary calculations for the passives. The ON datasheet found elsewhere is missing these calculations.
- Calculator – it does all of the math for you, EXCEPT the capacitance for C0 seems to be bogus.
- Theory and Applications – quite useful for understanding all the various ways the IC can be used.
- How to Build a Switch Mode Power Supply Circuit with an MC34063 – Great tutorial and really helped me calculate the passive values by hand though his examples were vague in a few places.
- Building a SMPS based on the MC34063 – Another good tutorial.
Determining the Overall Circuit Layout
The way the passive components are connected to the MC34063 depend on what type of power supply you are building: boost, buck, boost-buck, or inverting. I am building a boost circuit, so the overall circuit layout will look like this:
There are a few components that have static values:
- Cin – This capacitor is next to Vin and should be 100uF.
- R – This resistor should be 180 ohms.
- D – This is a schottky diode of sufficient power rating. I used an MBR1100RLG.
I need to calculate L, Rsc, Cr, C0, R1, and R2. This is where the calculator comes in. To use the calculator, you are going to need to know
- Vin – this is the minimum input voltage. 5V for my calculations.
- Vout – this is the required output voltage. 48V.
- Iout – this is the output current. I want 25mA (note that the input current will be much higher than the output current when stepping up this far – in my final tests, it takes 220mA @5V to generate 25mA @ 48V).
- Vripple – the voltage ripple in the output. I’ve been using 1mV for this.
- Fmin – the minimum frequency. The chip runs at a max of 100MHz, so that is what I use.
Using these requirements, according to the calculator, the passive values should be:
Ct=366 pF Ipk=593 mA Rsc=0.506 Ohm Lmin=62 uH Co=2060 uF R=180 Ohm R1=1.5k R2=56k (47.92V)
The value for Co is really big. When I do the calculations by hand, I get 1/100th of that. I don’t know if that is exactly how far it is off by, but if you just use the calculator, I’d question the Co value if it is large.
Using the worksheet found in the TI datasheet, here are my calculations. Note that I take the Vf and Vsat values from the “How to Build a Switch Mode Power Supply Circuit with an MC34063” tutorial.
This gives me:
- Ct – 362pf. Closest I have is 330pf.
- Ipk – the peak input current should be 527mA. So my input power supply and inductor need to be able to support that much current.
- Rsc is .569 ohm. I have a .5 ohm, so I’ll use that (reducing resistance increases current).
- L(min) is 72uH. The next size inductor up I have is 100uH.
- Co is 20uF. The only 100V capacitors I happen to have are 47uF, so that is what I will use (I believe C0 is a minimum value).
I’m just missing R1 and R2. I arbitrarily decide to use a 100K ohm resistor for R2 so I just need to do some algebra to determine what R1 must be:
So if I set R2 to 100K, R1 needs to be 2674 Ohms, or 2.7K which I have.
This worked amazingly well. I got about 48.5V and could draw 23mA. Here is my final circuit:
Jan 22 2016 update:
I found this post the MC34063 on EEVBlog. Wish I had found this before. It explains everything, better than I can.
Oct 2016 Update:
One ‘tip’ I discovered later but forgot to document here: keep all of your wires as short as possible.