LT4320 CLoad Selection

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dantwomey

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Joined 2002
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#1
Just for fun and my own personal education I thought I'd build my own LT4320 based rectifier for my test bench. Looking at the LT4320 manual it looks pretty straight forward. Just make sure you choose MOSFETs with a low RDS on and you're good. The only thing that has puzzled me a bit is the calculation of C Load. The manual seems to talk about C Load in thousands of uF when I've seen much lower values in other designs here. As a non-EE what am I missing?

Regards,
Dan :idea:

CLOAD SelectionA 1μF ceramic and a 10μF minimum electrolytic capacitor must be placed across the OUTP and OUTN pins with the 1μF ceramic placed as close to the LT4320 as possible. Downstream power needs and voltage ripple tolerance determine how much additional capacitance between OUTP and OUTN is required. CLOAD in the hundreds to thousands of microfarads is common.A good starting point is selecting CLOAD such that:CLOAD ≥ IAVG/(VRIPPLE • 2 • Freq)where IAVG is the average output load current, VRIPPLE is the maximum tolerable output ripple voltage, and Freq is the frequency of the input AC source. For example, in a 60Hz, 24VAC application where the load current is 1A and the tolerable ripple is 15V, choose CLOAD ≥ 1A/(15V • 2 • 60Hz) = 556μF.CLOAD must also be selected so that the rectified output voltage, OUTP-OUTN, must be within the LT4320/LT4320-1 specified OUTP voltage range.
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#2
The small capacitors on the board prevent the 4320 from exploding if no external capacitance is connected. No real limit on the maximum capacitance you can have, so use the same considerations as sizing the capacitance bank on any other PS.
 
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#3
analog_sa said:
The small capacitors on the board prevent the 4320 from exploding if no external capacitance is connected. No real limit on the maximum capacitance you can have, so use the same considerations as sizing the capacitance bank on any other PS.
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Avery good piece of info for me because I was going to put my scope on them 'unloaded' to see what the output looked like.

Regards,
Dan :wave:
 
#8
It also helps not to think in "will use it for device X" but look at it from a technical point of view so a specific rectifier (with way lower Vf than diodes have) that feeds a filter cap bank and a load. What voltage/current/power is needed for the load? What is the maximum voltage the load endures? Is the specified transformer not too high in voltage considering the now higher filtered voltage? What does one allow for maximum ripple voltage or, what does the load allow to be maximum ripple voltage?

With certain higher loads it may be necessary to use either CRC or CLC filtering. Maybe the builder wants even lower ripple voltage (than being accepted as OK or good enough) as a device is as good as its PSU... In general the allowable power on surge current with ideal rectifiers is higher than with the specified diodes so larger value filter caps and higher rated transformers are now possible.

OK now power on and off effects may occur so maybe "device X" that has very low ripple voltage now has some quirks as heavy plops and woofers going everywhere and even "farting" behavior when powered off. Then maybe speaker protection or just simple and cheap delayed speaker switching with fast switching at power off (often omitted in better audio) may be be a necessity.

Just some remarks for better understanding impact from a different angle...
 
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#9
Great information! PCBs are pretty straightforward and cheap so experimenting is going to be fun. I'm using the DiyStore's universal power supply as a base minus the rectifier section. I will be able swap different rectifiers in and out to explore the results.

Many thanks,
Dan
 
#12
#13
Gate charge 69 nC and gate input capacitance 4200 pF from the specs, I would definitely have skipped this one... Avoid oversizing the MOSFET the LT4320 datasheet says. May work OK for a while and in some cases but it certainly does not meet the criteria as described and maybe it is wise to keep to datasheet wisdom for guaranteed operation.

This one meets a few criteria and it is isolated: FDPF085N10A
 
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#15
jean-paul said:
Gate charge 69 nC and gate input capacitance 4200 pF from the specs, I would definitely have skipped this one... Avoid oversizing the MOSFET the LT4320 datasheet says. May work OK for a while and in some cases but it certainly does not meet the criteria as described and maybe it is wise to keep to datasheet wisdom for guaranteed operation.

This one meets a few criteria and it is isolated: FDPF085N10A
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This is a ballpark calculation:

Using capacitor i-v relation, with 5 nF gate capacitance and 1 mA LT4320 output current capacity, MOSFET gate will change voltage by 3V in 15 uS. That’s fast enough.
 
#16
The targets I'm aiming for are less than 10mOhm RDSon, a VGS th between 2-3V, and a total gate charge as low as possible.

That's why I posted the IRF60B217. RDS 7.3mOhms, VGS 2.1V, and gate charge of 44nC. On top of that it was 50% cheaper than most of the other suitable candidates.

The nice part is that there doesn't seem to be any shortage of suitable MOSFETs.

Regards,
Dan 🙂
 
#17
That MOSFET is a good choice. It is both cheap and adequate. I just asked as you maybe have a preference for isolated versions (TO-220F). The ideal rectifier is not so ideal with regards to buildings costs 🙂

Yes, the electric car, photovoltaic stuff etc. have stimulated MOSFET development.
 
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#18
jean-paul said:
That MOSFET is a good choice. It is both cheap and adequate. I just asked as you maybe have a preference for isolated versions (TO-220F). The ideal rectifier is not so ideal with regards to buildings costs 🙂

Yes, the electric car, photovoltaic stuff etc. have stimulated MOSFET development.
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I'm just going to do a small amount of tinkering for the fun of it and to see if I can find an ideal combo for my Firstwatt projects.

Regards,
Dan 🙂
 
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