# 5v to 12v Boost converter question /w MC34063

#### johnson35762

Joined Nov 21, 2017
25
Hi everyone, I build a boost circuit using MC34063 as control IC,
The schematic I build is like this,

The datasheet can be found here: MC34063 datasheet
At first, I want to build a boost converter to drive the nixie tube clock I made, so I measure the current consumption of the project is about 81.9mA, when using 12v/2A AC-DC power adapter.

And then, I design the peripheral component of the MC34063 circuit as this website (Other website had the same result)
So I choose the component value in the schematic above.
The circuit works fine when I attaching small load such as EL-wire which only need 35mA of current,
but when I connect the nixie tube clock, the output of boost converter is suddenly dropped to 5 volts, and the current increase to 173mA.
As you can see in the picture, the output voltage hooked up to the oscilloscope's RMS value is 5.65 volt, and the meter shows the current is 175mA.

I'm wondering is the nixie clock needs a lot of current at startup to charge the input capacitance of the clock or something needs a high impulse current to drive?

So I use a 1-ohm resistor, connect it in series to the 12v/2A AC-DC adapter and the nixie clock, then hook CH1 to the adapter output, CH2 to the clock input, like the picture below, so I can measure the current by the voltage drop of the 1-ohm resistor.

The oscilloscope shows the result like this, the yellow is Ch1, which is the adapter voltage, the blue one is the nixie clock input voltage and the red one is CH1-CH2 the differential of two voltage, aka the resistor voltage.

As you can see, there is a very high voltage when the nixie clock attaches to the power source, convert the voltage into current, It's about 6A when I want to start the nixie clock.

The question is how can I solve this issue? Does something like "soft start" can apply in this situation?
or I need an external NPN transistor or MOSFET to make the circuit can handle more switching current?

Thanks for reading!! Hope you have a nice day

#### dl324

Joined Mar 30, 2015
12,265
Welcome to AAC!
I build a boost circuit using MC34063 as control IC,
The schematic I build is like this,
What type of inductor and diode are you using?

Your schematic would be easier to read if you mirrored the voltage regulator and drew it like most do. From the datasheet:

Note that there are no unnecessary wire crossings.

#### johnson35762

Joined Nov 21, 2017
25
The circuit you post is exactally which I reference from data sheet.
I use the toroid inductor which can handle 2A( if the seller didn't dcieve me) and the diode is FR107 fast recovery diode.

#### Bordodynov

Joined May 20, 2015
2,745
The circuit you post is exactally which I reference from data sheet.
I use the toroid inductor which can handle 2A( if the seller didn't dcieve me) and the diode is FR107 fast recovery diode.
It's a bad idea to use a ferrite toroidal core. You can only use powdered iron.

#### johnson35762

Joined Nov 21, 2017
25
It's a bad idea to use a ferrite toroidal core. You can only use powdered iron.
Hi, Bordo, but the inductor I use is like this

I used this inductor in 12v to 180v nixie tube drive circuit, and it works.
The only different is the nixie tube PSU use external MOSFET as a switch,
but this one use internal transistor in the IC as the switch.
Is this really a bad inductor?
And, how to recognize the inductor is iron powder core or ferrite core, can I determine it from outlook?
Thanks, best regard.

Moderators note : reduced image to its essentials

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#### Bordodynov

Joined May 20, 2015
2,745
Give me the actual core size. By color, I will determine the material and model it in LTspice. I'll see if the core will go into saturation.

#### Delta prime

Joined Nov 15, 2019
619
Hi there Most applications of iron powder cores are substitutions of inductors made of ferrite cores. These applications include DC/DC converter output filter inductors and power factor correction inductors.
In these applications you need the energy storage capability (proportional to B×H; all quantities are magnitudes) of the inductor core. Ferrite cores have a high permeability so you need to introduce an air gap to reduce this permeability, thus increasing the magnetic field H strength needed to magnetize the core to a flux density B. This air gap has a severe disadvantage: within the air gap the relative permeability is reduced to unity and this causes the flux to exit the core and enter the winding, leading to eddy current losses in the winding. The power loss density is concentrated around the air gap, so there is the risk of a hot spot.
Iron powder cores do not need the additional air gap since it is integrated into the material and, in consequence, spread within the complete core volume. This reduces the eddy current losses in the winding and the remaining eddy current losses are distributed throughout the winding length., energy storage is limited by the saturation flux density. In ferrite this saturation flux density is about 400 mT and decreases with temperature. In iron powder cores saturation flux densities of more than 1 T can be utilized, depending on the material

#### dl324

Joined Mar 30, 2015
12,265
If you're using a DVM to measure current, where in the circuit are you inserting it? Measuring current requires some thought to make sure it won't perturb the circuit too much.

#### johnson35762

Joined Nov 21, 2017
25
Give me the actual core size. By color, I will determine the material and model it in LTspice. I'll see if the core will go into saturation.

The inductor color is yellow and white, but I think the white color is the base color of the core.
I disassembly the inductor, and get the inductor has 49 turns,
and the dimension of the core are:
inductance: 100uH
outer diameter: 102.9mm
inner diameter: 7.5mm
thickness: 5mm
and the diameter of the wire twine on the core is 0.6mm
Hope this parameter can help you simulate the component, I'm not good at simulating.

Best regard.

#### johnson35762

Joined Nov 21, 2017
25
Hi there Most applications of iron powder cores are substitutions of inductors made of ferrite cores. These applications include DC/DC converter output filter inductors and power factor correction inductors......
Thanks for your detailed explanation, now I know the difference between the ferrite core and iron powder core.

BTW, I also want to ask another question about where the color code inductor be used?
In most conditions, the color inductor always can not handle large current because the resistance of itself is bigger than other inductors, so I can't find out which application needs this kind of inductor.

And about the inductor purchase, I can only find the ferrite core inductor in the local electronic component store.
Moreover, there always no datasheet about the inductor in the electronic store.
Is there any method to get the iron powder core inductor? or I need to make an inductor by my self?
Thanks, best regard.

#### johnson35762

Joined Nov 21, 2017
25
If you're using a DVM to measure current, where in the circuit are you inserting it? Measuring current requires some thought to make sure it won't perturb the circuit too much.

In the beginning, I want to know the power consumption of the nixie clock, and I can design the boost converter's output capability.
So I connect the DVM between 12V/2A AC-DC adapter(which can successfully drive the nixie clock) and the nixie clock input socket, then I get the value is about 81.9mA (the stable current consumption).
This is the reason why I connect the DVM to measure the current.
Thanks reminding me that the internal resistance of DVM should not be neglect.

#### DickCappels

Joined Aug 21, 2008
6,819
While waiting for the true solution from Bordodynov, may I ask what the voltage is a pin 1 while the output transistors are conducting? I imagine it should be within a few hundred mv of ground, but if it is not, and particularly if it lifts up toward the end of the pulse, that could tell us why you are not getting the performance you expect.

Your inductor looks fine unless it is saturating, which is why the collector voltage at pin 1 would be interesting.

#### johnson35762

Joined Nov 21, 2017
25
While waiting for the true solution from Bordodynov, may I ask what the voltage is a pin 1 while the output transistors are conducting? I imagine it should be within a few hundred mv of ground, but if it is not, and particularly if it lifts up toward the end of the pulse, that could tell us why you are not getting the performance you expect.

Your inductor looks fine unless it is saturating, which is why the collector voltage at pin 1 would be interesting.

I measure the Pin1 and the output voltage in three situations: no-load, 50mA load, and 100mA load.
But the 100mA load can not reach the current I want because the Pin8 voltage drops to 4.66V(my input voltage is 5 volt).
And I use cursor function to measure the low state voltage of Pin1.

Output voltage: 12V, with output capacitor charge/discharge curve
CH1: Pin1 voltage
CH2: Output voltage @500mS/div

50mA current consumption measured by DVM, maybe there is a resistor error or DVM internal resistor
The output voltage barely maintains at 12V, but as you can see the RMS value of CH2 actually is drop to 10.8V,
and the positive duty cycle and the negative duty cycle is 39.3% and 61.7%.
From the principle of boost converter, the duty cycle should be $$\frac{Vo}{Vd} = \frac{1}{1-D}$$ in my case is 58.3%.
So I think this boost converter works fine, but still has a few problems.
CH1: Pin1 voltage
CH2: Output voltage @10uS/div

3. 110 ohm attach (84.1mA)
Here are the things go strange, I use two 220-ohm in parallel to be a 110-ohm resistor, I think the output current should stay at 100mA or a little drop because of the error of shunt resistor (Rsc).
But the output voltage decrease to 9 volts and the current is 84.1mA measured by DVM.
The negative duty cycle of Pin1 is 58.1%, which means that the IC tries to pull up the output voltage but didn't work (If I'm wrong, please tell me).
If the inductor is not saturated, the only reason I can think is the Rsc is too big that limiting the output current, so the voltage can not increase to the value I want.
CH1: Pin1 voltage
CH2: Output voltage @10uS/div

#### Bordodynov

Joined May 20, 2015
2,745
I have a few comments on the chart and the radio elements. The yellow and white ring is the number of the mixture -26. It's recommended to use up to 50 kHz. You've got two higher. This reduces the efficiency of the converter. Even using a 100 µHenry inductive linear core does not provide the required load capacity. Using the FR107 diode is also not the best solution. It is better to use the Schottky diode. For example 1N5817-1N5819. This will reduce ripples, increase efficiency and increase load capacity.
So far, I've calculated a circuit without using core non-linearity. I'll try to optimize the circuit a little later.

#### Bordodynov

Joined May 20, 2015
2,745
I researched the choke. It quite allows the current to reach 1A. In your circuit, the current is 500 mA, I increased it to 900 mA. I also reduced the frequency to reduce core loss. I replaced the diode with a Schottky. Dropped a pin of the driver transistor resistor and reduced it. Added a 100 pF capacitor. Increased the output capacitor and added a ceramic capacitor. Look at this:

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#### DickCappels

Joined Aug 21, 2008
6,819
@Bordodynov , is it your opinion that R2, the peak current limiting resistor, was the reason the output of the power supply collapsed under load? That would suggest that the component value calculator under-estimated the peak current.

#### johnson35762

Joined Nov 21, 2017
25
I have a few comments on the chart and the radio elements. The yellow and white ring is the number of the mixture -26. It's recommended to use up to 50 kHz. You've got two higher. This reduces the efficiency of the converter. Even using a 100 µHenry inductive linear core does not provide the required load capacity. Using the FR107 diode is also not the best solution. It is better to use the Schottky diode. For example 1N5817-1N5819. This will reduce ripples, increase efficiency, and increase load capacity.
So far, I've calculated a circuit without using core non-linearity. I'll try to optimize the circuit a little later.
Hi, Bordo, very thanks for your help, especially the inductor core information and the simulation result.
As you said, the core material is mixture-26, and I found this manufacturer product may be my inductor core:
KDM Toroidal cores and the part number is KT50-26.
I have a few questions: How could you know the inductor core type by knowing the color and dimension, is there any website to search?

#### johnson35762

Joined Nov 21, 2017
25
I researched the choke. It quite allows the current to reach 1A. In your circuit, the current is 500 mA, I increased it to 900 mA. I also reduced the frequency to reduce core loss. I replaced the diode with a Schottky. Dropped a pin of the driver transistor resistor and reduced it. Added a 100 pF capacitor. Increased the output capacitor and added a ceramic capacitor.
Very thanks again, I did a small experiment this morning, I change the current limiting resistor to 0.25ohm, and measure the current when the timing capacitance is 250pF, 470pF and change the diode to 1N4148 Small Signal Fast Switching Diodes (This diode is the only another switching diode I can get in my room, I'll buy the 1N5817 tomorrow)
The result is like below
(Load = 110 ohm, current all increase to 95mA because of the smaller current limiting resistor):
1. Rsc = 0.25 ohm, Ct = 250pF

2. Rsc = 0.25, Ct = 470pF

3. Rsc = 0.25ohm, Ct = 470pF, D = 1N4148

I think the biggest change is the current limiting resistor, the timing capacitor has some effect on the output, but I think the inductor maybe not saturate yet (?

#### Bordodynov

Joined May 20, 2015
2,745
Note that I have reduced the 180 Ohm resistor and even connected it in the wrong way for more effect. It is possible not to change the connection method, but also to reduce the resistor to 100 Ohm. Don't be afraid, at current through the choke 1 A it is quite linear and will not go into saturation. I had to look for the core material on the Internet more than once. And it's never been difficult to find what kind of material. The truth is, I did ask GooGle in Russian. Using Schottky's diode will further increase the output voltage by at least 0.5V. In this scheme, using 1N4148 is not successful. On it, with such a high current for it, a lot of voltage drops (1.5 - 2 V). The diode you use causes high losses because it is slow.

#### Bordodynov

Joined May 20, 2015
2,745
@Bordodynov , is it your opinion that R2, the peak current limiting resistor, was the reason the output of the power supply collapsed under load? That would suggest that the component value calculator under-estimated the peak current.
No, not only that. Bad choice of rectifier diode also contributed to it. High frequency also made it worse. This material is recommended for use at frequencies below 50 kHz. I also saw the effect of increasing the output transistor base current. The collector current increase was rounded and I decided to reduce the resistor that sets the output transistor base current.