PSU Voltage Adjustment

Thread Starter

JulesP

Joined Dec 7, 2018
374
Hi all,

I have a PWM power supply that has a nominal output voltage of 12V and a current capacity of 25A continuous. While there is about 0.5V adjustment on the PSU itself, I need to be able to adjust this output to supply 6-12V.

Would the following arrangement work with the high power MOSFET acting as a sort of valve. I would need a heavy heat sink.

Thanks
PSU-PWM Voltage Adjustment.jpeg
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
Thanks. If I have a gate voltage of say 4V (via the pot), would that still work with a Drain supply of 12V?

I think you are saying that it will work but the FET would shut down as I dropped below say 3V.
 

ericgibbs

Joined Jan 29, 2010
12,884
hi J,
This is what LTS shows, for the MOS Vgate, power etc..

Update:
Used a 4410 as an equivalent for a 4110, similar spec,, in order to demonstrate to the TS the circuit would not work as he expects.
@jpanhalt

EAAA 1037 10.27.gif
 
Last edited:

jpanhalt

Joined Jan 18, 2008
11,088
Are you ordering the IRFB4410 (Eric) or IRFB4110?

Looking at the charts by Eric, if one considers the current at 7 Vgs of about 12A and Vo of about 3V, voltage drop from source is 9V. As an approximation, the effective total source resistance is 9V/12A = 0.75Ω and I^2 *R = 108W

Source: Wikipedia (https://en.wikipedia.org/wiki/TO-220)
When a TO-220 package is used without a heatsink, the package acts as its own heatsink, and the heatsink-to-ambient thermal resistance in air for a TO-220 package is approximately 70 °C/W.
Using a mosfet as a variable resistor (i.e, without being fully turned on) will generate a lot of heat.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
Are you ordering the IRFB4410 (Eric) or IRFB4110?

Looking at the charts by Eric, if one considers the current at 7 Vgs of about 12A and Vo of about 3V, voltage drop from source is 9V. As an approximation, the effective total source resistance is 9V/12A = 0.75Ω and I^2 *R = 108W



Using a mosfet as a variable resistor (i.e, without being fully turned on) will generate a lot of heat.
Perhaps there is a less wasteful way of tweeking my PSU output down? I thought the FET approach was like putting a twist valve on a water hose pipe - it restricted the flow and hence the pressure 'voltage' appearing at the output. From what is being said it doesn't work like that, or if it does then its only sensible for low power adjustments.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
I thought that was just the range over which you ran the simulation. So I cant get 6-12V with this? Would a change of FEt make a difference?
Yes I note that in Spice you use a 4410 whereas my suggestion is the 4110. Will that make a difference?
 

ericgibbs

Joined Jan 29, 2010
12,884
hi J,
Only a minor difference.
Your circuit is equivalent to a High side driver, ie: the Load is in the Source.
This means, it requires a Vgate with respect to the Source a 4V threshold conduction voltage.
So it will always require at least a Vg of 4V, so the maximum Vout will be Vin - 4V , ie ~8V.
E
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
hi J,
Only a minor difference.
Your circuit is equivalent to a High side driver, ie: the Load is in the Source.
This means, it requires a Vgate with respect to the Source a 4V threshold conduction voltage.
So it will always require at least a Vg of 4V, so the maximum Vout will be Vin - 4V , ie ~8V.
E
Ok so it's intrinsically not ideal. Would a different device e.g. an MJE3055, do better? Or instead a reconfiguration?
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
hi J,
As you may know the Imax for a 2N3055 is 10Amps, with a Gain of only 5.!!

So to get say 10Amp current out, it would require a 2Amp Base Current.
E

View attachment 227816
Eric, I will have a mooch around. Can you tell me the full expression you used in LTS it’s not all showing. Having learnt to use that simulator a bit I can test out my ideas. I’m familiar with .trans and .ac but not the (wp) one you used. Thanks
 

ericgibbs

Joined Jan 29, 2010
12,884
hi J,
I can post any LTS asc files I create, if that helps,

The 'wp' is simply 'wiper on the pot', its rotational position from 0.01 thru 0.99 ie: 1% to 99%

OK.?
E
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
hi J,
I can post any LTS asc files I create, if that helps,

The 'wp' is simply 'wiper on the pot', its rotational position from 0.01 thru 0.99 ie: 1% to 99%

OK.?
E
Thanks Eric. That’s a kind offer but I’m going to be trying a variety of BJTs and FETs to find one that has the right characteristics to give me something like 7.5-10v output, including the inevitable voltage drop under load. Perhaps if I post one that seems to work in due course and then you can confirm it?
 
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