| mcintoda |
Greetings,
My group is going to be building a high power Class D amp for subwoofer apps ( <100Hz)
general questions:
1) Is there a problem with a full bridge desing without feedback?
2) Why do commercial applications that create Class D for subwoofer apps have switching freqs of 50-150Khz? Will 1Khz be acceptable (besides 2x for Nyquist criteria). Are the transistors more efficient if they are being switched rather than conducting? In other words are switching losses < conduction losses?
3) We would like to create a 3-state PWM. Most PWMs have 2 states, ON and OFF. We would like ON, OFF and Idle. This is because for 2-state PWM if the input is zero, then the duty cycle is 50% and the system is incurring losses. Anyone have any ways to implement this? Im sure it has to do with signal conditioning part of the circuit.
Attached is a block diagram |
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| BWRX |
| quote: | Originally posted by mcintoda
My group is going to be building a high power Class D amp for subwoofer apps ( <100Hz) |
Based on your questions you would be better off trying to build a low power class d amp - Tripath makes such chips, and there are solutions from other companies out there as well. The cost of such a project will be lower and it will be less complicated. |
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| mcintoda |
We would like to make it high power. We got all the parts as samples (for free).
here's the schematic so far |
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| Pafi |
mcintoda!
(What is "your group"? Your classmates?)
| quote: | | Is there a problem with a full bridge desing without feedback? |
No, if you don't mind high distortion, or you are such an expert like Texas Instrument's designer, who can achieve low distortion even without feedback.
| quote: | | Why do commercial applications that create Class D for subwoofer apps have switching freqs of 50-150Khz? |
Because
- inductor of filter below 5 kHz cutoff is huge
- < 18kHz is audible.
| quote: | | We would like to create a 3-state PWM. Most PWMs have 2 states, ON and OFF. We would like ON, OFF and Idle. This is because for 2-state PWM if the input is zero, then the duty cycle is 50% and the system is incurring losses. |
Actually it should be 3-level PWM, implemented by 4 states.
In 3-level PWM there is 4 state: Positive, Negative, ZeroHigh and ZeroLow! Idle state (high impedance) is unusable in audio amps. ZeroLow is implemented by turning on low side mosfets, ZeroHigh by turning on high side mosfets. During a cycle both Zeros are used alternatedly.
http://users.hszk.bme.hu/~sp215/ele...level%20PWM.gif
| quote: | | here's the schematic so far |
- 2524 is unable to produce 3 level PWM. For 3 level PWM you must have triangle wave instead of sawtooth, and 2 comparator.
- IRFP260N is bad choice. High Rds, high gate charge. Choose a lower voltage, lower resistance, lower gate charge, and faster one!
- Make dead time adjustment!
- Attach a filter!
Is 30V the voltage of the final design? |
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| mcintoda |
Thanks for your input!
We are 4 seniors in Electrical Engineering creating class D amp for a project.
Ok, I realize that the inductor size limitations dictate that the switching frequency must be sufficiently high. I will look around for a high-current rating inductor.
I understand your concept of Zero H and Zero L. It seems that, with our full bridge MOSFET controller, I just need to supply the right logic to turn the proper MOSFETS on or off.
The question I have is HOW to create the 4-state (or "3-state") signal from the audio. Do you need 2 comparators for:
1 for comparison w/ audio
1 for comparison w/ inverted audio signal
then some AND logic to get a 3 state signal?
I am having difficulty understanding the picture you attached.
The filter is not shown, you are correct. Furthermore the dead-time is built in with the SG2524 PWM controller or with the gate driver (HIP 4082).
Thanks for you help!
david mcintosh |
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| Tim__x |
Just the two comparators. You don't need any more logic.
You must remeber the output is taken as the difference of the two halfbridges. That means that it is indeed 3-state from the view of the load. Here's a table to show what I mean.
| code: | Out1 Out2 Difference (Out1-Out2)
0 0 0
1 0 1
0 1 -1
1 1 0 |
Four different inputs produce only three outputs. |
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