Help with high current PS?

Thread Starter

TSOMB

Joined Jul 31, 2014
25
Hi folks, im looking to make a 50A 4vdc supply in which I can control the current via trimmer. I plan on using a hand wound transformer and the only thing im stuck on is the current regulation. I typically have used the NPN BD437 but that is only good to 4A. I went in my parts drawer and found some steel case transistors but they are all too high of a working voltage.

So my question is this, which transistor would best suit my application, as well I need the ability to parallel the outputs. Im still new to electronics so please be gentle lol.

Thanks!

*edit I just came across 4 IRF130 rated 14A each, will these work?
 
Last edited:

b1u3sf4n09

Joined May 23, 2014
113
You can use something like AO4576. You would want at least 4 in parallel, so I don't know how realistic that is for your available space. Also, if you go this route, make sure to use low resistance, high wattage resistors on the outputs to ensure equal current loading on each transistor.

You may be able to get away with using something like PSMN3R0-30YL,115. That is a lot of current for 1 transistor to pass, so I would say tread with caution if this is the path you choose to take. Dedicated heat sink for the power transistor would be a given.
 

Thread Starter

TSOMB

Joined Jul 31, 2014
25
Thanks for the suggestions, though I have one other question... The load will be running between 2 ~ 4vdc when at full current, do the transistors posted above have any issue with minimum voltage out? Also what kind of voltage drop would be expected (if any) I looked at the datasheets but couldnt find that info which im sure is there. I plan on winding the transformer so that there is not too much surplus voltage to keep the heat as low as possible.
 
Thanks for the suggestions, though I have one other question... The load will be running between 2 ~ 4vdc when at full current, do the transistors posted above have any issue with minimum voltage out? Also what kind of voltage drop would be expected (if any) I looked at the datasheets but couldnt find that info which im sure is there. I plan on winding the transformer so that there is not too much surplus voltage to keep the heat as low as possible.

As for your question on operating range:

The IRFP064 would not work optimally at voltages below about 2 volts, but would still be able to provide about 100 amps or so at 2 volts.
See Figure 8 of the datasheet here: http://www.irf.com/product-info/datasheets/data/irfp064n.pdf

You SHOULD be able to get away with those voltages, here's the deal though. Alot of these high amperage single IC units are going to be FETs of one type or another:
Most of them will want a considerable gate voltage to turn them on. The IRFP064 for example needs a minimum of 2 volts before it bothers to transconduct, but no more than 4 volts. (See Vgs(th) on above datasheet.)

And Voltage Drop for a MOSFET is explained really well by a post sourced from HERE:
Mosfets have 3 regions of operation that we will concern ourselves:

The first is cut-off mode, when the mosfet does not conduct.

The second is the saturation mode, when the mosfet is switched on and is conducting with a very low drain to source resistance.

The third region is in between cut-off and saturation, and is known as the triode region. This is the region where the mosfet begins to conduct, but has a high channel resistance.

There are 3 key voltages that will determine what mode the mosfet is operating in. These are:

VGS, the voltage from the gate to the source, taken with the gate as positive and source as negative

VDS, the voltage from the drain to the source, taken with the drain as positive and the source as negative

Vt, the mosfet’s threshold voltage. This value is a device parameter, and is roughly given in the device’s data sheets. (Base on the Datasheet of the STP36NF06 MOSFET VGS is 10V to get saturation mode VDS is 25V and Vt is 2V)

For an N-Mos device:

Cutoff when VGS < Vt
Triode when VGS > Vt, and VDS < (VGS – Vt)
Saturation when VGS > Vt, and VDS > (VGS – Vt)

For you case here, you are using 9V supply so the MOSFET is in the triode region therefore have high channel resistance which the voltage drop in the Drain and Source is high.

Read this article for more detailed info on MOSFET
http://roko.ca/robotics/h-bridge-fundamentals/4
Datasheet of the STP36NF06
http://www.st.com/stonline/products/literature/ds/9619.pdf
 

Thread Starter

TSOMB

Joined Jul 31, 2014
25
As for your question on operating range:
The IRFP064 would not work optimally at voltages below about 2 volts, but would still be able to provide about 100 amps or so at 2 volts.
See Figure 8 of the datasheet here: http://www.irf.com/product-info/datasheets/data/irfp064n.pdf

You SHOULD be able to get away with those voltages, here's the deal though. Alot of these high amperage single IC units are going to be FETs of one type or another:
Most of them will want a considerable gate voltage to turn them on. The IRFP064 for example needs a minimum of 2 volts before it bothers to transconduct, but no more than 4 volts. (See Vgs(th) on above datasheet.)

And Voltage Drop for a MOSFET is explained really well by a post sourced from HERE:
Thanks for the info. So do you think it would be wise to use lower output components with paralleled outputs? Or do the smaller output components
also have the same issue? The thing is that as the current is lowered, the vf also drops. So as example, when running at say 10amp which is approx
threshold, the vf may be as low as 1v or just over. I am aiming to have full control right down to the threshold. Which would be the better route?



I believe a 200W laser is well outside the allowed range of items for discussion here.
I did not mention anything about power output, and lets not forget that DPSS systems are very inefficient. Also 50A is not going to net you "200W"
with any laser medium I have encountered. Though if you know of one, please do share :)
 
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