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Old 12th February 2011, 06:33 PM   #1
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Default SMPS offline, HV and LV Design

I am working on an smps design and am interested in community suggestions.

SMPS specs
type: forward, active-clamp (probably)
frequency : 250 kHz (not critical, anything from 100 kHz < freq < 400 kHz should be good)
power : 300 W
Input : universal AC. I think 110/220 switch with capacitor doubler instead of PFC.
Outputs :
+40 V : high current, supply to class-D amp
-40 V : high current, supply to class-D amp
+7 V with linear post-regulator for analog supply
-7 V with linear post-regulator for analog supply
+5V with linear post-regulator for uP and other digital stuff. (maybe just produce +3.3V directly, no post-regulator)

My initial design question is regarding partitioning - is it better separate this into 2 supplies, one for the +/-40V, and another for the low voltage stuff. Or, simply wind a single transformer, use a single pwm etc. Although the real estate is a problem with multiple sections, I think it'll fit - seat of the pants guess. I'm worried about regulating it, i.e. which winding to use.

gene
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Old 12th February 2011, 06:47 PM   #2
tomchr is offline tomchr  United States
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I built a project like that a good ten years ago. Except I used a flyback topology -- NOT the right topology if you want to transfer 300 W of power... Flyback does give you the advantage that you can generate multiple output voltages easily without having to regulate every one of them.

A forward converter sounds fine. You may find that 250 kHz will lead to too high switching losses. Anything above 20 kHz should be fine but significantly above is obviously better.

From 40 V to 7 V can be done with one of National Semiconductor's SimpleSwitchers. LMZ14203 for example. I think there are also versions that accept even higher input voltages if you're worried that you're pushing it too close to the 42 V max set by the LMZ. I used an LMZ12002 in a filament heater for a vacuum tube amp and got really good results. 10 mV ripple at 500+ kHz at 1.5 A load... I don't think there's any reason to use a linear post-regulator with performance like that.

+5 digital supply. Use another switcher. Again, I recommend the LMZ-series... They're a bit spendy in low quantity but only a few bucks more than a DIY solution.

I'm sure there are other buck converters than the LMZ-series, that's just what I've used and liked.

~Tom
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Old 13th February 2011, 11:32 AM   #3
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So you are suggesting to partition design with +/-40V, then post regulate +40 down to the lower stuff.
PROS
1) only the +/-40V needs to be safety isolated.
2) I think the post regulation will be easy since it's DC-DC with Vin = 40. Turns ratio will be low, transformer may be smaller and easier to make.
3) Always some load on the +40V leg (all the low-voltage stuff), so the regulator will never fall out of regulation.

CONS
1) synchronizing a primary side PWM and an isolated PWM more complicated. I want to sync to keep frequency mixed byproducts out of audio range.
2) efficiency - 90% for mains-to-40V, 90% for 40-to-low_voltage, that's 80% and an additional loss in any linear post regulation.
3) I am concerned about using the +/-40V (amplifier supply rails) for non-amp use. But, I could add an additional winding for (maybe 40, maybe something else) post-regulation. It adds complexity for sure - another wrap, another filter coil.


Hmm, as I think about this, maybe this *is* a good way to go -
offline section:
Vin : 110, 220 VAC
Vout : +/40V
Isolation : full safety barrier

DC-DC section:
Vin = +40V
Vo = +7
-7
+5
Isolation : none.

Post DC-DC section:
Optional. Maybe drop DC-DC section to +/- 5V (analog) and +3.3 (digital)
else use linear regulators.

Although more lossy this way, the lower voltage stuff probably doesn't use that much power. Percentage-wise, it's not good, but absolute power loss won't be that much.
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