Can we use such PWM duty cycle as a DC to DC cellphone charger/converter?

[crutschow]

The schematic looks okay.

Don't know why you're not getting 5V out. What's the voltage across the zener?

 

[Willen]

Hi, I experimented with few series resistor of zener and got different voltages. For 5.1 ZD, I used 2.2k and got 4.8V reference and output voltages. With 1K, got 4.9V and with 330 ohms, got 5.01V but this time 330 ohms got hot!

So I used 5.6V ZD and used high value resistor- 15K to get lower voltage than 5.6V. And yes, I got 5.3V output and charged cellphone with 450mA current. But N-MOSFET need a little bulky heatsink (getting hot with 3x2x1.5cm heatsink), so boaring!
Wish to get MC34063 so portable charger IC!

 

[rumpfy]

Willen,
If you get a datasheet for the zener you are using, you will see that the zener voltage is dependent on the current. For the BZY88 type, the zener voltage at 5 mA is between 4.8 and 5.4 volts. Your voltmeter might be say 0.1 or 0.2 volts out so you could see an apparent difference in the zener voltage from its nominal value.
If you want to try for a 34063, please get the Application note AN920 from ONsemi. Get the datasheet too; if havent already done so.
hope this helps.

 

[Willen]

Willen,
If you get a datasheet for the zener you are using, you will see that the zener voltage is dependent on the current. For the BZY88 type, the zener voltage at 5 mA is between 4.8 and 5.4 volts. Your voltmeter might be say 0.1 or 0.2 volts out so you could see an apparent difference in the zener voltage from its nominal value.

Really, I knew new thing. I used to think that regulated voltage of zener is exactly as its rated voltage. I never taught its output is depended to its current.

If you want to try for a 34063, please get the Application note AN920 from ONsemi. Get the datasheet too; if havent already done so. hope this helps.

I have datasheet already, but I didn't get the chip in market. I just saw a charger device of my uncle with the chip used.

 

[audioguru]

Hi Willen,
First of all, most zener diodes have a voltage that is plus or minus 5% of their rating at their rated current. If the current is higher than their rating then the voltage is a little higher and if the current is lower than their rating then the voltage is a little lower. It is because a zener diode has a "dynamic resistance" which is called the "maximum impedance" on the datasheet at the rated current.

Second, the voltage rating of a zener diode determine how its voltage changes when its temperature changes which might be caused by current through it heating it:
1) A voltage rating of about 5.6V will not change with temperature changes.
2) A voltage rating of less than 5.6V will drop when it is heated.
3) A voltage rating of more than 5.6V will increase when it is heated.
Which is why two 5.6V zener diodes can be used in series for 11.2V with no change caused by temperature instead of using a single 11V zener diode whose voltage increases a little when it is heated.

A zener diode rated at less than about 4.7V regulates poorly.

Here is a pile of zener diode data mostly from Motorola/ON Semi but they show that their 4.7V zener diodes have zero temperature coefficient:

 

[Willen]

Hi guru,
Again feeling amazing to hear that 5.6V is very stable to temparature. Why not upper and lower! Seems some nice internal chemistry.

TL431 shunt reg chip also works like a zener diode, do it have such current depended regulation? I guessed not because it's a complicated integrated circuit (IC).

I thought it has VERY high price rate as other ICs but few days ago I luckily found it to a shop and they sold me just like a TO-92 transistor price. Bought some pieces, and again planning to buy tens of these. Probably it's VERY useful for further...

 

[crutschow]

...............................
TL431 shunt reg chip also works like a zener diode, do it have such current depended regulation? I guessed not because it's a complicated integrated circuit (IC)....................

Yes, it has a complex internal design to provide a tight initial tolerance, and be very stable with temperature and current variations, sort of a "super zener".

 

[Willen]

We discussed and I build a DC to DC convertor circuit here in the thread. This post I posted from my cellphone which was charged by the circuit- https://www.electro-tech-online.com/attachments/dc-to-dc-convertor_built-schematic-gif.88880/ which was originally designed here on the thread by 'crutschow'. Thank you lot for the useful stuff!

Today I am thinking about its failure aspect. Lets say if Source and Drain of the N-MOSFET shorted, then will it make 12V output (same as 12V input)? Or if oscillator/comparator failed? I don't want destroy 5V devices connected to it.

 

[crutschow]

We discussed and I build a DC to DC convertor circuit here in the thread. This post I posted from my cellphone which was charged by the circuit- https://www.electro-tech-online.com/attachments/dc-to-dc-convertor_built-schematic-gif.88880/ which was originally designed here on the thread by 'crutschow'. Thank you lot for the useful stuff!

Today I am thinking about its failure aspect. Lets say if Source and Drain of the N-MOSFET shorted, then will it make 12V output (same as 12V input)? Or if oscillator/comparator failed? I don't want destroy 5V devices connected to it.

Glad to know the circuit works in practice, not just in simulation.

If you want to protect against failure then you can add an SCR overvoltage crowbar circuit to the output such as this, which is commonly used in power supplies for that purpose.
That circuit turns on the SCR to clamp (short) the output to ground if the voltage goes higher than about (Vz + 0.7V) where Vz is the Zener voltage.
The circuit is reset when power is removed.
Of course, since the output is now shorted to ground, you need something to disconnect the circuit to avoid frying something else, such as the fuse shown in series with the input of the regulator.

 

[Willen]

Glad to know the circuit works in practice, not just in simulation.

If you want to protect against failure then you can add an SCR overvoltage crowbar circuit to the output such as this, which is commonly used in power supplies for that purpose.
That circuit turns on the SCR to clamp (short) the output to ground if the voltage goes higher than about (Vz + 0.7V) where Vz is the Zener voltage.
The circuit is reset when power is removed.
Of course, since the output is now shorted to ground, you need something to disconnect the circuit to avoid frying something else, such as the fuse shown in series with the input of the regulator.

Hi again,
And now I am trying to experiment by modifying your LM339 based 12V to 5V supply. I want to use a simple blocking oscillator to generate frequency. And the frequency amplitude will be regulated by series resistor and a 5.1V zener to Gnd. And want to feed Gate of the mosfets. Finished! Are there any possibility?

Actually I am confusing how your device is working! There's 12V peak frequency exist in the gate of N-MOSFET however its output is just 5V comparatively to positive. I found the signal fed to gate is just almost 30% ON and 70% OFF. So it's PWM creating the 5V output, comparatively to positive 12V?

 

[analog1]

be carefull be very carefull if the batteries are ion-lithium. The overcharging, temperature can destroy those batteries in no time. otherwise i suggest forget about the voltage just simply put a current generator in series [ of the right current of course] LM type of regulator can be used for a simple current source on resitor that is all

 

[Willen]

Below is a simulation of the circuit with the changes you suggested (as I understand them).

My simulation showed that the comparator was fast enough to operate with a 20μH inductor but I increased its value to the minimum you suggested.

I initially didn't include any additional hysteresis because the circuit simulation operates fine without it. But in the actual circuit some added hysteresis may be needed to curb oscillations from any stray feedback between the comparator input and output. That's generally a layout and decoupling issue.

With the added hysteresis and smaller output capacitor, the output ripple is about 450mVpp, which seems rather high as compared to about 50mV for my original circuit. Thus it's not clear that your component values are that well chosen either. I particularly don't understand why you want a smaller output capacitor (?).

R4 and C4 are to damp high frequency oscillations from the inductor and any stray capacitance that can occur when the MOSFET switches off.

View attachment 88588

Hi again,
I followed your design and tried to make the circuit simple. How it looks? It has no feedback so I could use it as variable regulator, output voltage monitoring is not added though.