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Old 20th December 2005, 06:54 PM   #11
mzzj is offline mzzj  Finland
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Default Re: Delayed capacitor discharge in SMPS

Quote:
Originally posted by Vittorio


If the MOSFET switching the capacitor is synced to switch only during PWM switch OFF times ( zero current), then switching losses in this MOSFET should be negligible.

Adriano
I am not sure if i got this right, during low-line mosfet connects "boost" cap parallei to orginal filter cap or how because that would result amazing peak dissipation in mosfet switch?
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Old 20th December 2005, 07:11 PM   #12
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Quote:
3000 hours at 105C, 6000 at 95c, 12k at 85c etc...
How many hours is a MOSFET rated for?

How many other components on an SMPS board have a specified and expected degradation in specs?

If SMPS operates around the clock, 12,000 hours is just a year and a half.

My statement about reliability was RELATIVE -- compared to other components.
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Old 20th December 2005, 07:25 PM   #13
mzzj is offline mzzj  Finland
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Default Re: Re: Delayed capacitor discharge in SMPS

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Originally posted by mzzj


I am not sure if i got this right, during low-line mosfet connects "boost" cap parallei to orginal filter cap or how because that would result amazing peak dissipation in mosfet switch?

?
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Old 20th December 2005, 07:28 PM   #14
poobah is offline poobah  United States
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Hey Vector,

SMPS is nice because you can look at one cycle or part of a cycle and see things in a simpler way.

Let's talk about amp-seconds (coulombs). Say you want to put 1 Amp-second into a capacitor, forget about voltage for the moment, awssume the capacitor is large the voltage doesn't really change, and the capacitor has 1 Ohm of internal resistance.

Look at the power lost in the following:

1A * 1s, loss = I^2*R = 1 watt; over one second = 1 Joule

.5A * 2s, loss = .5^2*1 = .25 watt; over 2 second = 1/2 Joule

.25A * 4s, loss = .25^2*1 = 0.062 watt; 4 seconds = 1/4 Joule

In each case, the charge delivered is the same, however the faster you try to charge the cap the more energy is lost in heat.

In the case of a forward convertor (the same ole' 1 to 1 reset winding). you want your worst case low line DutyCycle to be as close to 50% as possible... so you are discharging the input caps at the slowest possible (most efficient rate).

Does this make sense?

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Old 20th December 2005, 07:36 PM   #15
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Quote:
during low-line mosfet connects "boost" cap parallei to orginal filter cap or how because that would result amazing peak dissipation in mosfet switch?
There is no original filter cap. The switched cap would be the only filter cap. It should be put onto the rails at a moment when the PWM switch is OFF.

An ORing diode upstream on the positive line should prevent any current from capacitor back into the source.

I have not thought about the exact details of switching, am just considering a concept for feasability.
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Old 20th December 2005, 07:38 PM   #16
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Vittorio


There is no original filter cap. The switched cap would be the only filter cap. It should be put onto the rails at a moment when the PWM switch is OFF.

An ORing diode upstream on the positive line should prevent any current from capacitor back into the source.

I have not thought about the exact details of switching, am just considering a concept for feasability.
Ok, this makes sense.
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Old 20th December 2005, 08:11 PM   #17
mzzj is offline mzzj  Finland
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I did some spice simulations.....
well, results are bit supprising, (or not) ripple voltage and required ripple current rating of filtering cap are 1:1 equal with Vittorio-method and traditional one.
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Old 20th December 2005, 08:15 PM   #18
poobah is offline poobah  United States
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You can't cheat the devil!
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Old 20th December 2005, 08:34 PM   #19
mzzj is offline mzzj  Finland
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Quote:
Originally posted by poobah
You can't cheat the devil!
I cheated with pspice some more and optimized mosfet switching timing to minimum ripple, apprx. 450w load and one 220uF cap.
Vittorio:
44v p-p ripple
traditional:
49v p-p ripple
Cap ripple current 2.77-2.8arms or sumething samesame...

Not a big difference, think more economical to increase capacitor size to get required max ripple voltage.
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Old 21st December 2005, 01:27 AM   #20
Eva is offline Eva  Spain
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I think that this approach is just not possible. Mains line has very high impedance at switching frequencies, which may even start to increase just above 100Hz. The filter capacitor is required at any time.
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