series pass transistor

I want to know how to calculate the series resistor for series pass transistor? What is the minimum zener current to be considered for series pass transistor to work? The mininmum zener current is not given in datasheet of 12V zener BZT52HB12.

I assumed 5 mA zener current + transistor base current, is it correct?

What is the max zener differential resistance? The datasheet is attached herewith for BZT52HB12.

Pls let me know my above queries.

Hi,

You did talk about the zener and it's datasheet.

But you need to start with your requirements:
What input voltage range, what output current range, what max allowed voltage error, what allowed drift...

Then comes a schematic
Then come the datasheets and the part values calculations.

Klaus

Hi,

abhishek.2138 said:

I want to know how to calculate the series resistor for series pass transistor? What is the minimum zener current to be considered for series pass transistor to work? The mininmum zener current is not given in datasheet of 12V zener BZT52HB12.

I assumed 5 mA zener current + transistor base current, is it correct?

...

Click to expand...

R(Izener+Ib)+Vzener=Vin
R = (Vin - Vzener)/(Izener + Ib)

Based on datasheet information, I would say use Izener = 5 mA.

Maximum zener differential resistance as provided in the datasheet is 10 ohm at Izener = 5 mA and 90 ohm at Izener = 1 mA, both at Tj = 25 degC.



Also, please show your particular circuit.

Well,

I think it would be more suitable to make Izener = 1mA at Vin(min).
Izener will increase as Vin increases. It can get to tens of mA.

KlausST said:

Hi,

You did talk about the zener and it's datasheet.

But you need to start with your requirements:
What input voltage range, what output current range, what max allowed voltage error, what allowed drift...

Then comes a schematic
Then come the datasheets and the part values calculations.

Klaus

Click to expand...

Please provide your requirements like Klaus had suggested.

Thanks Akamino & Klaus, following are the specs -

Input V range - 7 to 18V
Output load current - 0.4A
Power transistor - PHPT61003NY
Min gain of PHPT61003NY - 150
Zener V = 12V
Output V = 11.3V (12 - 0.7 = 11.3)

I agree if I keep zener current minimum, because at max input range zener current is large. So, is it feasuble to keep zener current = 1 mA at Vmin?

My typical input V = 13.5V. Also, can you please tell, what is max differential zener resistance?

Schematic & datasheet is attached. (in schematic consider transistor PHPT61003NY).

Hi,

With an input voltage down to 7V it simply is impossible to run any useful current through the zener.

And for sure you can't get an output voltage of 11.3V with an input voltage of 7V.

Decide to use a SEPIC or buck/boost switch mode converter.

Klaus

Hi,

You cannot boost the output with that power supply you intended to use. Just like Klaus has said, you need a type of switchmode power supply that can buck and boost at the same time. Flyback, SEPIC, non-inverting buckboost or zeta make a list of converters you can easily choose from.

I fully agree with you KlaussT & Akanimo that at 7V, I can not get 11.3V OUTPUT.
But, at input of 7V how much output can I get? Is it possible to drive some load at 7V input?

Also pls can you tell somthing about differential zener resistance?

Thanks & Regards,
Abhishek

design your own P-fet reg and 7V out is possible - this is known as an LDO or Low Dropout Regulator ... pulling the P fet gate to gnd gives just the on resistance as the series pass resistance .....

Hi,

I agree with Easy Peasy. Use an LDO.

Also pls can you tell somthing about differential zener resistance?

Click to expand...

A raw explanation could be found in each datasheet. But there are online descriptions, documents .... even video tutorials.

Klaus

Dear all,
Actuallly my application is of automotive background. There are certain automotive pulses such as load dump. This load dump pulse goes to around 37V. I tried to search LDO but most LDOs input is limited to 35V max. This gives limitation. The transistor which I have considered is of 100V VCE.

Also cost is major factor in automotive so we are not thinking of buck converter. Therefore, for low cost solution we are considering series pass transitor.
How much approx. output can I get with 7V input with zener diode of 12V? I am asking because at 7V, the zener will not go in breakdown.

Also, can you tell somthing about differential zener resistance?

Hi,

again modified requirement. In post#5 the input range was up to 18V .. now it`s up to 37V.

Dropout voltage fo your NPN: I assume almost 1V. The problem is that you need to satisfy BE voltage to get your NPN conductive.
(acc datasheet: V_BEsat is about 0.86V @ IC = 1A and IB = 50mA)
then you need to add the voltage drop at R_B. let´s say it is 0.1V @ 50mA. According Ohm´s law this means 2Ohms.
But then the iput voltage is limited by the zener max. power dissipation to about 12.1V.

Going back to the 18V input requirement:
This means you need R_B of about 120 Ohms.
Which then will increas the dropout voltage at 7V input to about 2V.

**
In the end all this is the cause why Easy Peasy recommended the P-Fet LDO circuit with a dropout in the millivolts range.

If there was a simple, rugged, cheap solution with low dropout ... then you surely could buy it.

****
So you need a more complex discrete solution..
* increased part count, increased assembling cost, increased stock cost, increased board space, increased failure rate .... I doubt this solution is (much) cheaper than a ready to buy automotive LDO.


Klaus