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

 

hi J,
This clip is from the d/s.
Note the Vgate turn On voltage.
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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.

 

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

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hi J,
This is what LTS shows, for the MOS Vgate, power etc..
EView attachment 227738

Thanks, that’s very helpful. I shall order me bits.

 

hi,
Have you noted that Vout is only 0V to 7v.?
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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.

 

hi,
Have you noted that Vout is only 0V to 7v.?
E

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?

 

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.

 

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?

 

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.
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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?

 

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.
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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

 

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.?
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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?

 

Hi J,
Pleased to help.
Find a suitable equivalent for the power P MOSFET, under F2> nmos..
Also consider P MOSFET's
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Hi J,
Pleased to help.
Find a suitable equivalent for the power P MOSFET, under F2> nmos..
Also consider P MOSFET's
E

Sorry I’m being a bit slow here but F2 is?

 

hi,
The keyboard F2 is the LTS key for calling up the Component menu.
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hi,
The keyboard F2 is the LTS key for calling up the Component menu.
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Makes perfect sense. I tend to use the slower menu route