As a DIY audio newbie, I got a STA 508 with snubbing network on its data sheet. the input PWM has a frequency of 3 MHz. when the amplifier is unloaded, the output is fine, but when it's loaded(8 ohm), the amplitude just drops to 85% of its original value??? what is going wrong here??? Note: I did't add any analog feedback in this application
3Mhz PWM sounds a little bit fast, normally these things work in the 100's of kHz.
What inductors are you using in your output filter? Could be they're too lossy. Show us a schematic.
What inductors are you using in your output filter? Could be they're too lossy. Show us a schematic.
The filtering network is exactly the same the sta508 data sheet
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00003546.pdf
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00003546.pdf
No schematic then? And no power supply details? It could be your PSU is drooping.
So what's the reason you're using 3MHz PWM? The datasheet filtering isn't designed for that - note the example freq in one of the tables is 384kHz.
So what's the reason you're using 3MHz PWM? The datasheet filtering isn't designed for that - note the example freq in one of the tables is 384kHz.
Can't be PSU's dropping since I have measured it. the pSU is a tench power supply which is good. The schematic is exactly the same as the example in the data sheet.............
the modulator's output is at 3MHz..........I have observed the output of STA508, the output looks normal.......... just as the input PWM except with higher voltage....... is there any thing that can cause this kind of drop?
the modulator's output is at 3MHz..........I have observed the output of STA508, the output looks normal.......... just as the input PWM except with higher voltage....... is there any thing that can cause this kind of drop?
I'd check on my scope where the voltage drop is occurring. A 15% drop will be easily visible on a scope - first place to look would be across the inductors. Look at the waveforms on both sides.
I guess the signal freq is significantly higher than 1 kHz. At about 10 kHz such drop is normal due to inductive reactance.
Increased dead-time distortion due to high fpwm is the other possible explanation, at low signal amplitude.
Increased dead-time distortion due to high fpwm is the other possible explanation, at low signal amplitude.
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the input signal frequency to the class d amp is 1k......
could you please have a more detailed explanation of the second point?
could you please have a more detailed explanation of the second point?
the voltage before going through inductor is 10V p-p PWM signal, which is correct............... but after going through the inductor, the waveform change as a sine wave....... I don't think these two are comparable...............
During transition of switching node the mosfets are in a much higher impedance state. This causes typically 2 regions symmetrically to 0 output current in DC output transfer function where gain is decreased. Drop in gain is increased with higher relative dead-time.
Sorry sir, I didn't get your point, could you please explain with more details or example?
appreciate...........
appreciate...........
To understand this you have have to know the basic PWM waveform, inductor current, and a simple modell of dead time, and then combine them. If all of them are unknown, then I don't have time to tell everything, but if you know almost everything mentioned, then maybe I (or somebody else) can help.
But if you are not familiar with these kind of circuits and methods of analization, then forget all, and just reduce PWM frequency!
10V? Is this the power supply rail? Not too much...
What you have to seek for is
1: drop in p-p PWM signal when you apply the load.
2: shift in the discrete levels of PWM proportional to modulating signal.
1: is due to not perfect PSU, 2 is due to channel resistance.
Channel resistance directly appears on output as additive resistance, relative PSU drop is a modulation component. Equivalent Series Resistance of inductor also causes some drop.
If your load is inductive, then inductive drop of output inductor can also lead to loss of amplitude.
Measuring a full bridge can be tricky. Hou do you measure it? And at what amplitude?
so you are saying that the power supply of sta508 may drop, in this sense, the drop on PSU will cause the drop on the output?
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