dc dc converter step-up output load issue

[fuji]

Yes the MC34063 is an old IC, as most people say, but this is the only dc dc converter that seems to impress me since I learn a lot from it, so I don't want to throw it out for now. I have one issue on the output voltage/current. The schematic is attached.

I am stepping up (boost) voltage from 9v to 11v at 7mA current. The problem is, when I put load on the output, the voltage drops 4 volts (becomes around 7.4v) bellow the main voltage output which is 11v at 7mA. I am still confused as what the reason is for this. Is it the axial inductor that I am using which is causing this huge voltage drop on the output? The Vf is a shottky diode 1n5819 ,which seems fine to me, unless I switch over to a 1n5817 becuase of the lower voltage drop??

The switching frequency is at 100Khz
Duty Cycle at 25%
Voltage ripple is at 0.1V
Iin at max is 0.019A
Iout is 0.007mA

Another question. Is it best to lower 9v battery to a smaller voltage battery to get an 11v output? I need to be sure that I use up the entire battery until it is literally dead, but used for a long time. As usual, lowering current is important in general for battery applications. That is why I chose 7mA as the output current.

 

[Tony Stewart]

Yes the MC34063 is an old IC, as most people say, but this is the only dc dc converter that seems to impress me since I learn a lot from it, so I don't want to throw it out for now. I have one issue on the output voltage/current. The schematic is attached.

I am stepping up (boost) voltage from 9v to 11v at 7mA current. The problem is, when I put load on the output, the voltage drops 4 volts (becomes around 7.4v) bellow the main voltage output which is 11v at 7mA. I am still confused as what the reason is for this. Is it the axial inductor that I am using which is causing this huge voltage drop on the output? The Vf is a shottky diode 1n5819 ,which seems fine to me, unless I switch over to a 1n5817 becuase of the lower voltage drop??

The switching frequency is at 100Khz
Duty Cycle at 25%
Voltage ripple is at 0.1V
Iin at max is 0.019A
Iout is 0.007mA

Another question. Is it best to lower 9v battery to a smaller voltage battery to get an 11v output? I need to be sure that I use up the entire battery until it is literally dead, but used for a long time. As usual, lowering current is important in general for battery applications. That is why I chose 7mA as the output current.

To debug, always report each pin voltage.
Pin 1 SW
Pin 2 Gnd
Etc
Pin 6 Vin

When applying load , specify R

When evaluating impedance of LC , check https://www.testecvw.com/carl/images/impedancenomograph.pdf
You have 50kΩ L and 0.2Ω C @100kHz
Keep in mind duty cycle of switch affects impedance of L as driver goes from open to closed Z =1/d *Rce for duty cycle , d

Now can U find the problem.?

 

[ronsimpson]

1uH seems like a very small coil. 100uH or more seems better.

 

[fuji]

1uH seems like a very small coil. 100uH or more seems better.

The inductor in the schematic is over 1.1mH

 

[ronsimpson]

The inductor in the schematic is over 1.1mH

Yes....my old eyes missed the (,) and (.).
-----edited------
I would try changing the current limit resistor to 2 ohms. 16 seems too large.

 

[fuji]

Yes....my old eyes missed the (,) and (.).
-----edited------
I would try changing the current limit resistor to 2 ohms. 16 seems too large.

Thanks for the reply. Usually when I lowered the current limit resistor in Ohms, the voltage dropped significantly on the output. I tried 2 ohms before and the output dropped to around 0.12v instead of 11v.

 

[Tony Stewart]

Without knowing SRF of L and ESR of C, my guess is Rsc is too big. The Vsc drop may be < 50 mV (?) so you can only expect Io=3mA

Choose a lower f or smaller L with higher SRF > 6x (?) fc
Recalc.

https://ukpyr.narod.ru/calc_mc34063.htm

For Io=7mA @ 11.1V Rload=1.6k thus impedance of L is too high or in other words boost current pulse too small.

 

[fuji]

Without knowing SRF of L and ESR of C, my guess is Rsc is too big. The Vsc drop may be < 50 mV (?) so you can only expect Io=3mA

Choose a lower f or smaller L with higher SRF > 6x (?) fc
Recalc.

https://ukpyr.narod.ru/calc_mc34063.htm

You may be correct. I punched in a few numbers in my calculator for this schematic and this is what I have:

Rsc(max): 15.8R
Rdriver: 1000R
Max ESR: 4.164R.

I'll take your advice and switch a few things around.

 

[ronsimpson]

If your output voltage is much lower than the input voltage then something is wrong!
Simply put, the 9V should run around and to the output even if the IC was removed from the board.
The load current is so large that you drop volts across Rsc.
The resistance of the L is so large that you drop volts across L.
The diode is in backwards.

 

[MrAl]

Hello there,

Check the series resistance of your inductor. It has to be low enough to allow the inductor to charge up in a certain amount of time. For an inductor of 1.1mH, it could be quite high unless you chose an inductor used for power supplies. If you have the spec's for the inductor handy we could take a look, or a part number so we can find the inductor. The series resistance of the inductor is a very important aspect of the inductor especially for boost circuits.

Also, check the loading factor of the battery or supply you are using. Load it with 20ma and make sure it can hold the voltage up. A fresh alkaline can do that but a heavy duty type may not work as well. If the voltage drops too much under load, the converter will have to boost more and that puts more drain on the battery.
At 9v 1000 ohms draws 9ma, so 500 ohms draws 18ma, so test with maybe a 400 ohm resistor right across the battery or supply you are using and measure the voltage and report that info here.

The more info we have the better informed we will be to know what is wrong.

 

[fuji]

Hello there,

Check the series resistance of your inductor. It has to be low enough to allow the inductor to charge up in a certain amount of time. For an inductor of 1.1mH, it could be quite high unless you chose an inductor used for power supplies. If you have the spec's for the inductor handy we could take a look, or a part number so we can find the inductor. The series resistance of the inductor is a very important aspect of the inductor especially for boost circuits.

Also, check the loading factor of the battery or supply you are using. Load it with 20ma and make sure it can hold the voltage up. A fresh alkaline can do that but a heavy duty type may not work as well. If the voltage drops too much under load, the converter will have to boost more and that puts more drain on the battery.
At 9v 1000 ohms draws 9ma, so 500 ohms draws 18ma, so test with maybe a 400 ohm resistor right across the battery or supply you are using and measure the voltage and report that info here.

The more info we have the better informed we will be to know what is wrong.

Thanks for the reply. I've read earlier posts from other forums that inductors play an important role for boosting converters. It is an axial inductor that I am using. For this 1000uH inductor in the schematic I have above, here are its specs which I have just found since the store I purchased from online had no datasheet link for my inductors.

1000uH +-10
______________________________
- Q min is 50
- Frequency of Q and L in Min: 0.796
- SRF (Mhz) min is 1.4
- DC resistance (max) - 17.40
- Rated DC current (mA) is 100

My schematic in my EAGLE Cad for this inductor will most likely be switched to an SMD inductor. I was also told that axial inductors for boosting converters are not the way to go. Not sure why.

My apologise for not replying sooner. Im trying to follow up with everyones posts. I'll keep you guys posted.

 

[MrAl]

Hi,

The axial inductors usually have higher resistance. In your case the output current is very low though so it may not matter as much. It would not hurt though to try a better inductor. You could also try to parallel two of them to try to get lower overall resistance, and help with the top end current rating too. If that works, you may want to look for an inductor with lower series resistance and better top end current.

Depending on how this chip handles start up the inductor may be banged with more than 100ma, so that's something to look at too.

The other problem is input voltage droop. This is just as important as the inductor series resistance. Measure that with a meter while connected no load and then full load.

Double check the frequency too because if the frequency drops too much it could go into discontinuous operation with load and that will degrade the output voltage.

If the device got damaged somehow during experimentation that could also cause a problem of course. Boost converters have the nasty habit of putting out a higher than normal voltage with no load, so always make sure there is at least some load even if small.

If you have a scope you could also check the input current surge during start up with full load.

 

[ronsimpson]

With the inductor and Rsc you have about 30 ohms. What is the load? Higher than 1k?

 

[MrAl]

Hi,

He said 7ma earlier.

 

[ronsimpson]

He said 7ma earlier.

Yes and the voltage dropped too much.
So the resistance of Rsc+RofL is more than 30 ohms or the load is not a simple resistor of 7mA. (or some thing different is wrong)

 

[ronsimpson]

I did not make a circuit like your but....I did make one in spice. (simulation)
I could not get anything to work until the current limit resistor was 0.5 ohms or less.
Also I removed a resistor and connected DRC and SWC together.

Well it works ok. There is about 1v p-p ripple on the output voltage.

 

[MrAl]

I did not make a circuit like your but....I did make one in spice. (simulation)
I could not get anything to work until the current limit resistor was 0.5 ohms or less.
Also I removed a resistor and connected DRC and SWC together.

Well it works ok. There is about 1v p-p ripple on the output voltage.

Hi Ron,

Well could you show your circuit too? My calculations tell me about 150ma peak during start up but dropping down significantly later. I am using 36 ohms total series resistance and shooting for 11ma output at 11 volts (1k load), with capacitance.

I guess at some point we could try simulating the more exact circuit. The feedback mechanism of this chip is a little different than most switchers but should work to some degree.
One guess would be that it cant start properly with load so it never gets up to the full operating conditions.

 

[fuji]

I did not make a circuit like your but....I did make one in spice. (simulation)
I could not get anything to work until the current limit resistor was 0.5 ohms or less.
Also I removed a resistor and connected DRC and SWC together.

Well it works ok. There is about 1v p-p ripple on the output voltage.

When you placed that current limite resistor, what was the voltage and current at start up?
Did you try to put load on the output to see if a voltage drop occurred?

According to the datasheet of the mc34063, the diode seems to be in the correct position for boost converter.

 

[ronsimpson]

The MC34064.lib need to be down load from the internet. Sorry I can't up load it. Try here
https://www.eevblog.com/forum/beginners/simulate-mc34063-on-ltspice/

The MC34064 is not a good choice for light loads. It cycle skips. When I used them it works very hard until the voltage get above the mark them it does nothing until the voltage gets below the desired limit. In this case (with 470pF timing cap) it turns on for one cycle and loads the inductor with a large amount of power. Slams the power to the output. The voltage goes up 1 volt. Then it skips many many cycles. With 100pf cap it turns on for several cycles then off for many many cycles.

Because it wants to turn on for a full cycle (or several full cycles) the inductor current gets very high. This is probably why it does not want to run with Rsc values above 0.5 ohms. (in spice)

 

[fuji]

The MC34064.lib need to be down load from the internet. Sorry I can't up load it. Try here
https://www.eevblog.com/forum/beginners/simulate-mc34063-on-ltspice/

The MC34064 is not a good choice for light loads. It cycle skips. When I used them it works very hard until the voltage get above the mark them it does nothing until the voltage gets below the desired limit. In this case (with 470pF timing cap) it turns on for one cycle and loads the inductor with a large amount of power. Slams the power to the output. The voltage goes up 1 volt. Then it skips many many cycles. With 100pf cap it turns on for several cycles then off for many many cycles.

Because it wants to turn on for a full cycle (or several full cycles) the inductor current gets very high. This is probably why it does not want to run with Rsc values above 0.5 ohms. (in spice)

So for this reason, this probably explains why car-phone chargers and portable tablet chargers have good stability with the mc34063 on higher loads. The mc34063 seems to work well on higher loads around 50mA to 150mA or more. When I did step-down with this IC, I also had current issues around 7mA as it kept fluctuating. So overall it seems this chip only works well on higher loads.

What do you recommend better than the mc34063 for boosting? Not too expensive, but can run 11V with a very low current output (5mA-10mA) at least and with a fairly accurate (2% or 3%) output reference. My goal here is to run a higher voltage but with very low current to conserve battery life.