[SOLVED] Converting NPN open collector transistor output to PNP open drain Output

[dhruv_electro]

Hi there,

We have old PLC which provide NPN Open Collector Transistor output. Internally it provides two pins,

Pin 10 of ULN2803 for connecting a VBAT of 12-24VDC and Pin No 18 to Pin 11 for Open collector output for various DC Loads. Now we needed to provide PNP type output for a requirement. So we have designed a ckt as shown below:



Load may be: DC relay, PNP Input (Internally Opto-Coupler), Solenoids, LEDs.

We have below queries:
0. Does any other ckt using other component or omitting opto-coupler could be suggested?
1. Does this ckt serve the needs?
2. How much load current we should recommend without heat-sink? MOSFET listed as Continuous Drain current -14A.
3. How much frequency of output we can recommend?, In PLC they have listed upto 80KHz for their NPN output, but as we have used PC817 how much frequency we could serve?
4. If we need more frequency upto 25KHz or upto 80KHz pulse train.?
5. We needed to provide the VBAT selected by user, how much voltage range we can recommend?



Please help us. We have very much poor experience with MOSFETs.

 

[Electro nS]

simply add a PNP transistor , with its base connected to open drain of NPN . its collector to the load and its emitter to supply voltage (12-24).
https://jeelabs.org/wp-content/uploads/2012/11/npn-pnp.png

if you need to do it with mosfets , check on "High side switches" in ST microelectronics website or IR or TI product line ..

these are P-mosfets instead of PNP with higher current ratings and with integrated drive and protection .

 

[spudboy488]

Wouldn't you want to add a pull-up resistor from the NPN collector to + to help with noise on the base of the PNP?

 

[dhruv_electro]

Hi there,

Thanks for your valuable reply.

these are P-mosfets instead of PNP
...

Is this ok?
Please comment about R7 and R8.

if you need to do it with mosfets ,
....

Out of curiocity:
1. What are the pros and cons of using MOSFET over PNP?

Wouldn't you want to add a pull-up resistor from the NPN collector to + to help with noise on the base of the PNP?

I have used R8 here, can you tell me more about noise in this ckt, because there may be possible of Inductive DC load here.

Adding:

Is there like P-type transistor or MOSFET array available like this ULN2803??

Sorry for my silly questions here. Thanking in advance.

- - - Updated - - -

Will you please comment about output frequency we can give and maximum load current we can promise here.?

 

[erikl]

View attachment 103886
Is this ok?
Please comment about R7 and R8.

I think it's ok. I'd suggest, however, to replace R8 by a 5-fold larger value than R7, in order not to reduce the gate driving voltage unnecessarily at 12V operation voltage. This also guarantees, that the max. Vgs of -20V won't be exceeded at 24V operation. For 28V operation - if necessary -, use a 12..18V Zener diode in parallel to R8.

What are the pros and cons of using MOSFET over PNP?

A MOSFET only needs drive current during switching, not during its "on" state. Saves driver power from the ULN array.

Will you please comment about output frequency we can give and maximum load current we can promise here.?

Your 80kHz requirement shouldn't give a problem; the max. load current depends on your heat transfer capability. Without additional heat sinking, see e.g. IRF9530's max. allowed drain current vs. its case temperature, Fig. 9 in International Rectifier's data sheet.

Also, a MOSFET array - if available - probably will fail because of the restricted heat transfer possibility.

 

[Electro nS]

yes this is OK
1-R7 good , R8 can be higher , 10K or something (as far as i know ) plz check in your isis simulator

2- the mosfets in general are better than bipolar transistors for higher current applications , they dissipate less heat ( beacause of lower resistance and voltage drop ) and approximatly zero current at base ( gate ) because they are voltage controlled devices . but for low current <1-2A i prefer bipolar since they are easier to drive and more robust to transients and safer for handling ESD .

hope that helps
regards

 

[dhruv_electro]

Thanks for your valuable time, you guys saving so many lives here.

Accumulating all your answers:

I think it's ok. I'd suggest, however, to replace R8 by a 5-fold larger value than R7, in order not to reduce the gate driving voltage unnecessarily at 12V operation voltage.

Sorry for adding to lately, but our superior have asked to maintain same specs as The PLC provide.
Here is Output specification from PLC manual:

Interior power Below DC5~30V
Max load:
Restance load 0.8A
Induce load 12W/DC24V
Lamp load 1.5W/DC24V

Now again comment for above detail regarding our circuit is requested.

This also guarantees, that the max. Vgs of -20V won't be exceeded at 24V operation. For 28V operation - if necessary -, use a 12..18V Zener diode in parallel to R8.

Please see the placement of 12V Zener.

the max. load current depends on your heat transfer capability. Without additional heat sinking, see e.g. IRF9530's max. allowed drain current vs. its case temperature, Fig. 9 in International Rectifier's data sheet.

Can we recommand 2A load current without heat sink or more?

Also, a MOSFET array - if available - probably will fail because of the restricted heat transfer possibility.

Thanks for the info, but not even for lower load current like 500mA..??

1-R7 good , R8 can be higher , 10K or something (as far as i know ) plz check in your isis simulator

I have gone for 680 ohms for R8, is it fine considering operation for 5-30VDC.

Again in ISIS simulator it gives the 30mA branch current for branch R8. So how much wattage rating required here for both R7 and R8.? Do i need to go for R8 = 10K considering 5-30VDC? We have in our inventory normally 1/4 watt (1206 smd or leaded) resisters.

2- the mosfets in general are better than bipolar transistors for higher current applications , they dissipate less heat ( beacause of lower resistance and voltage drop ) and approximatly zero current at base ( gate ) because they are voltage controlled devices . but for low current <1-2A i prefer bipolar since they are easier to drive and more robust to transients and safer for handling ESD .

hope that helps
regards

first Noise and Now ESD. Sorry for poor knowledge for this terms, but how much problem we are facing here, we generally provide free wheeling diode for inductive DC loads.

 

[Electro nS]

the circuit is fine now , and keep the zener it is important .and regarding R8 man use very simple ohm's law: i=30ma , r=680 then P=IxIxR= 30*30*680/1000000=0.6W !! not good ? toomuch heat for nothing and 1/4 watt resistors will not do the job you neeed 1W for this purpose , so use higher values . and for your information this resistor is used to turn off the mosfet so it can be high , no porblem use 5k or 10k .

and ESD is electrostatic discharge , it shouldnot be a problem if you handle the mosfet with care , which means donot touch the leads with your fingers if your table and your body is not grounded . it has nothing to do with inductive load , the diode should be enough , but make sure the P-mosfet voltage rating is aproximatly double the supply voltage , example for 24v system use 50v (vds rating)

 

[erikl]

View attachment 103907

Interior power Below DC5~30V

Please see the placement of 12V Zener.

I'd suggest to use a 15V Z-Diode. Even better gate drive voltage for your IRF9530.

Can we recommand 2A load current without heat sink or more?

Yes, but this is the absolute (w.c.) max. current without additional cooling. Results in a junction temperature of 100..125°C.

Thanks for the [array] info, but not even for lower load current like 500mA..??

For octal high side drivers I only know bipolar ones (but so what). E.g. MICREL MIC2981/2982 , up to 350mA.

I have gone for 680 ohms for R8, is it fine considering operation for 5-30VDC.

With the Z-Diode solution, the 30V capability, and considering the ¼W limitation, you'd have to choose new resistor values for R7 & R8. Pls. see below!

Again in ISIS simulator it gives the 30mA branch current for branch R8. So how much wattage rating required here for both R7 and R8.? Do i need to go for R8 = 10K considering 5-30VDC? We have in our inventory normally 1/4 watt (1206 smd or leaded) resisters.

For the 30V application, you need at least R7≧1kΩ (with a 15V Z-diode). With the 12V Z-diode, R7≧1.3kΩ (because of the ¼W limitation). Use R8≧10*R7 .

Check by simulation, if this still works well for your 80kHz requirement. If not, you have to use lower value resistors with higher wattage capability (or two of them in series connection).

 

[dhruv_electro]

Hi there,

Sorry for late reply. We were busy with other software developments.

Finally we have a solution, Thanks to you people. You have saved a lot of pain.

 

[erikl]

For > 10kHz operation you should use an appropriate Schottky diode instead of D2 (1N4007), because the latter one isn't fast enough to properly cut down the switch-off voltages from an inductive load.

 

[dhruv_electro]

For > 10kHz operation you should use an appropriate Schottky diode instead of D2 (1N4007), because the latter one isn't fast enough to properly cut down the switch-off voltages from an inductive load.

Can you suggest any part number.??

 

[erikl]

There are thousands ... and I don't know which one is available in your country, and which package you need. Any one with blocking voltage rating > your max. supply voltage, and current rating > your max. load current is appropriate.

E.g. 15MQ040N from Int.Rectifier, or 1N5822 from many companies.