PSU Using The LM338

[MrClamperSir]

Linear regulators (like the one you are using) are limited by the difference between Input and output voltage, especially under heavy current.Their heat dissipation is defined by this voltage difference times current.
Assuming you want to stick to the transformer you have, a best approach is to use a switching mode regulator like LTC3115,which will have noт just way better efficiency.but also the ability to produce output voltage lower or higher than the available input voltage. Go to www.linear.com for details and LTSpice illustration.
This Buck Boost regulator can tolerate up to 45V at the input and can produce up to 40V at the output with max load of 2A.
You can regard this regulator as DC transformer. The product of output voltage and current (output power) will almost match the respective number at the input (minus around 15% losses). This implies that at lower than input voltages you can get higher than input current (as step down transformer implies) As other voltage regulators, the output voltage is defined by a resistive divider, scaling down the output voltage for comparison with internal reference voltage. This implies волтаге control range above the reference as the formula is Vref*(1+R1/R2). If you need more than 2A have a look at LT8705 - again Buck Boost circuit but with external MOSFETs. The LT demo board for LT8705 illustrates up to 200W at input voltage as high as 80V.

Thanks for the info. What's the output ripple look like on a switch mode compared to linear? Also How long will a switch mode power supply hold up over everyday continued use compared to a linear?

 

[davenn]

What's the output ripple look like on a switch mode compared to linear?

it's just much higher frequency. This has the advantage of making it so much easier to filter

Also How long will a switch mode power supply hold up over everyday continued use compared to a linear?

ohhh gosh ... how long is a piece of string ? ( an open ended question )

just about everything in consumer electronics these days is powered by a switchmode PSU
Their reliability, in general, is still determined by the circuit's weakest link ... the good ol' electrolytic capacitor.

I would estimate that some 80 - 90% of their failures ( going by the number I have looked at over the years)
is due to electro cap failure. Now, as a rule of thumb, this isn't due to crappy caps. it's more of a overall housing design issue of the SMPS

take your average inline power pack for a laptop or a wallwart plugpack.



have you ever noticed 1) they are VERY compact and 2) they have absolutely no ventilation to get rid of heat
They run hot, the electro's dry out and the PSU fails

The SMPS's that last for yrs and yrs are the ones that are in well ventilated cases ... desktop and mini tower PSU units are quite reliable
and if not abused will last many years


There's some general thoughts for you

Ohhh the other 10 - 20% of failures ( and it applies to linear or SMPS units .... high voltage power surges on the mains input
eg from lightning strikes etc


Dave

 

[MrClamperSir]

it's just much higher frequency. This has the advantage of making it so much easier to filter



ohhh gosh ... how long is a piece of string ? ( an open ended question )

just about everything in consumer electronics these days is powered by a switchmode PSU
Their reliability, in general, is still determined by the circuit's weakest link ... the good ol' electrolytic capacitor.

I would estimate that some 80 - 90% of their failures ( going by the number I have looked at over the years)
is due to electro cap failure. Now, as a rule of thumb, this isn't due to crappy caps. it's more of a overall housing design issue of the SMPS

take your average inline power pack for a laptop or a wallwart plugpack.

View attachment 25399

have you ever noticed 1) they are VERY compact and 2) they have absolutely no ventilation to get rid of heat
They run hot, the electro's dry out and the PSU fails

The SMPS's that last for yrs and yrs are the ones that are in well ventilated cases ... desktop and mini tower PSU units are quite reliable
and if not abused will last many years


There's some general thoughts for you

Ohhh the other 10 - 20% of failures ( and it applies to linear or SMPS units .... high voltage power surges on the mains input
eg from lightning strikes etc


Dave

Thank you!! So the ripple on a well designed smps is less than or equivalent to a well designed linear supply?

 

[davenn]

The heat wasted in a linear regulator is:

(Vout - Vin) * Iout

If you need only 9V out, using a 24V transformer will produce 5 times the heat of using a 12V transformer.

Bob

he did comment earlier on ... that tho his current project required that lower voltage
he wanted a variable supply that was capable of putting out up to 20 - 25V for other uses

So. no, he doesn't want to reduce the transformer secondary voltage rating


Dave