PSU capacitor selection in smps fed half-bridge class d amplifier - Page 2

poobah · 27th January 2006, 02:38 PM

Ah ha...

hmmmmm.

So is this caused by assymmetry in bass signals... or just bass in general?

Eva is on the opposite side of world I think... probably sleeping still?

zilog · 27th January 2006, 02:43 PM

Quote:

Originally posted by poobah
Ah ha...

hmmmmm.

So is this caused by assymmetry in bass signals... or just bass in general?

Eva is on the opposite side of world I think... probably sleeping still?

Rail pumping occurs as soon as you have output voltage != 0, the lower the frequency of the output wave, the greater the pumping. Eva should be awake, I think she's in my time zone

subwo1 · 29th January 2006, 08:12 AM

I guess I'll throw in a thought. The main pumping mechanism in a class D amp would occur when a power supply rail passes enough net current through the output filter inductor for a sufficient length of time.

When a MOSFET turns off, the voltage on the inductor flies back to the opposing power rail momentarily adding charge to its filter capacitor either through the body diode if the MOSFET there has not turned on yet, or through the channel if it has already turned on. If the net voltage at the output load is zero, the cycle-by-cycle voltage increases on the power supply filter capacitors keep transferring back and forth evenly, with neither capacitor building up extra net voltage. Net current passing through output inductor for any appreciable length of time will pump up one of the power supply rails, the greater the current, the greater the pump-up. For this reason, DC output voltage offset into a low resistance produces the serious pumping.

Eva · 29th January 2006, 09:13 AM

I'm awake

I have not experimented with class-D at all, but if we analyse it as a simple buck converter, then the so called "rail pumping" effect becomes a logical consequence of output filter resonance (peaking at HF).

Note that in any buck converter suffering from output filter peaking you may actually get AC output voltages whose peak value is higher than the input voltage, provided the system is excited at the right frequency.

Eva · 29th January 2006, 10:28 AM

I'm sure that it was invented long ago, but a simple voltage balancer like this should avoid any supply pumping issue:

The switches should be operated in a complementary way but with some dead time to allow the transformer to self-balance (with the help of strong RC snubbers that would produce rise/fall times proportional to magnetizing currents).

zilog · 29th January 2006, 06:25 PM

Eva,

It looks like a solution, but wont efficiency become suffering if I cant manage to get my smps transformer turns ratio equal?

Would it be possible to use this configuration together with a single voltage smps to create a dual output voltage smps by using the balancer to create a fake ground?

Eva · 29th January 2006, 08:24 PM

That sounds interesting... Yes, you can use such a balancer to keep a virtual ground centered between both rails, but don't expect this to cause so much complexity reduction in the main SMPS. On the other hand, if the main SMPS lacked a ground output, then the balancer would have to work harder.

zilog · 29th January 2006, 09:58 PM

Eva,

You dont have any web reference to this voltage balancer? I feel like I need some guidance as I dont really understand how to design snubbers etc. I also wonder how I should incorporate this circuit into my smps, I suspect that I need to make extraordinary things to the voltage mode feedback loop in order to make this work.

zilog · 29th January 2006, 10:44 PM

Another thing I am uncertain of is how to decide on things like switching frequency, duty cycle (I guess as large as snubber reset allows), magnetic core size (how does core loss depend on pumped current/voltage?). I guess a too low switching frequency in the voltage balancer will cause excessive ripple voltage on my rails.

zilog · 7th March 2006, 07:55 AM

I still wonder if this balancer is feasible in a practical circuit - wont the voltage difference between my two secondary windings be forced to be equal - and thus forcing the offset voltage to dissipate through the copper resistance of the transformer secondaries?

I am thinking about using this circuit but with a hysteresis scheme that prevents if from operating with less than say 1V voltage difference. Also I guess the balancer will need to switch synchronous with the primary switches to avoid unneccesary beating effetcs.

29th January 2006, 08:12 AM	#13
subwo1 diyAudio Member Join Date: Jan 2002 Location: North American Continent	I guess I'll throw in a thought. The main pumping mechanism in a class D amp would occur when a power supply rail passes enough net current through the output filter inductor for a sufficient length of time. When a MOSFET turns off, the voltage on the inductor flies back to the opposing power rail momentarily adding charge to its filter capacitor either through the body diode if the MOSFET there has not turned on yet, or through the channel if it has already turned on. If the net voltage at the output load is zero, the cycle-by-cycle voltage increases on the power supply filter capacitors keep transferring back and forth evenly, with neither capacitor building up extra net voltage. Net current passing through output inductor for any appreciable length of time will pump up one of the power supply rails, the greater the current, the greater the pump-up. For this reason, DC output voltage offset into a low resistance produces the serious pumping. __________________ USMPS http://groups.yahoo.com/group/switchmode/

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27th January 2006, 02:38 PM	#11
poobah diyAudio Member Join Date: Nov 2005	Ah ha... hmmmmm. So is this caused by assymmetry in bass signals... or just bass in general? Eva is on the opposite side of world I think... probably sleeping still?

29th January 2006, 09:13 AM	#14
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	I'm awake I have not experimented with class-D at all, but if we analyse it as a simple buck converter, then the so called "rail pumping" effect becomes a logical consequence of output filter resonance (peaking at HF). Note that in any buck converter suffering from output filter peaking you may actually get AC output voltages whose peak value is higher than the input voltage, provided the system is excited at the right frequency.

29th January 2006, 10:28 AM	#15
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	I'm sure that it was invented long ago, but a simple voltage balancer like this should avoid any supply pumping issue: The switches should be operated in a complementary way but with some dead time to allow the transformer to self-balance (with the help of strong RC snubbers that would produce rise/fall times proportional to magnetizing currents).

29th January 2006, 06:25 PM	#16
zilog diyAudio Member Join Date: Oct 2005	Eva, It looks like a solution, but wont efficiency become suffering if I cant manage to get my smps transformer turns ratio equal? Would it be possible to use this configuration together with a single voltage smps to create a dual output voltage smps by using the balancer to create a fake ground?

29th January 2006, 08:24 PM	#17
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	That sounds interesting... Yes, you can use such a balancer to keep a virtual ground centered between both rails, but don't expect this to cause so much complexity reduction in the main SMPS. On the other hand, if the main SMPS lacked a ground output, then the balancer would have to work harder.

29th January 2006, 09:58 PM	#18
zilog diyAudio Member Join Date: Oct 2005	Eva, You dont have any web reference to this voltage balancer? I feel like I need some guidance as I dont really understand how to design snubbers etc. I also wonder how I should incorporate this circuit into my smps, I suspect that I need to make extraordinary things to the voltage mode feedback loop in order to make this work.

29th January 2006, 10:44 PM	#19
zilog diyAudio Member Join Date: Oct 2005	Another thing I am uncertain of is how to decide on things like switching frequency, duty cycle (I guess as large as snubber reset allows), magnetic core size (how does core loss depend on pumped current/voltage?). I guess a too low switching frequency in the voltage balancer will cause excessive ripple voltage on my rails.

7th March 2006, 07:55 AM	#20
zilog diyAudio Member Join Date: Oct 2005	I still wonder if this balancer is feasible in a practical circuit - wont the voltage difference between my two secondary windings be forced to be equal - and thus forcing the offset voltage to dissipate through the copper resistance of the transformer secondaries? I am thinking about using this circuit but with a hysteresis scheme that prevents if from operating with less than say 1V voltage difference. Also I guess the balancer will need to switch synchronous with the primary switches to avoid unneccesary beating effetcs.