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Class D Switching Power Amplifiers and Power D/A conversion

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Old 19th March 2008, 05:52 PM   #21
mag is offline mag  Switzerland
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Hi Munky,
you are not an idiot, you are just learning.

If you think that my implementation with 2 comparators will be too
complicated for you, you can use the method that Pafi is suggesting.

Use the resistor to charge the mosfet gate (turn-on) and the diode to discharge (turn-off). This will make the turn off much more faster that turn on introducing some dead time.

The amount of dead time is set by the resistor value.

Don't put capacitors on mosfet gates, you will only slow down something that is already too slow for 200KHz operation.

An IRF540 has around 2nF of gate capacitance, charge it trough the resistor and discharge it trough the diode. In this way the discharge time is set only by the maximum current that your gate
driver can sink (or source on the high-side).

Change your output filter and your load to 4ohm resistor
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Old 21st March 2008, 03:24 AM   #22
munky is offline munky  Canada
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Hey, i have changed the load resistor back to four, and changed the lpf. Changing the lpf made a huge improvement, since there is barely any cross over. The signal response is still pretty slow, I tried to put the diodes into the circuit, but CircuitMaker keep running into errors. One problem is, when I put the pwm circuit with the hbridge circuit, nothing is working properly, I believe it is because the impedance doesn't match. What is the best way to do impedance matching? Thankyou for all the suggestions.
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Old 22nd March 2008, 09:23 AM   #23
Pafi is offline Pafi  Hungary
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Again:

Quote:
I also don't know what do you want with these measurements. Where do you suppose to see the existence of dead time (or "cross-over")? And what do you measure actually? Y Axis is scaled in volts, and it seems to be real, but you have written I2_2 and I1_1 (currents) next to the curves.

Dead time is invisible with these measurements! Dead time means that Q1 have to switch off before Q2 switch on. You have to watch drain voltage of Q1 and gate voltage of Q2, or drain currents! Time relation between A and C is not really important!
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Old 22nd March 2008, 05:30 PM   #24
munky is offline munky  Canada
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Hey, sorry about that. Here's an attachment of the probes on the drain of Q1 and gate of Q2 to measure the voltage. I'm not too sure if this is even right, since there are random voltage spikes on the gate of Q2 when the drain of Q1 is on.
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Old 22nd March 2008, 08:45 PM   #25
Pafi is offline Pafi  Hungary
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With this value of R4,R9 (and R10,R13) Q2 (and Q7) never really turns on. They need -6 V of Vgs at least! (Now they get only 8V-12V=-4V)

But if you increased Vgs, they would cross conducted.

1st step: achieve fast gate charge and discharge. This needs high current (0,2 A minimum, but more is better). (You've got some ideas, take a pick!)
2nd: make charge a little slower, or delay it!
3rd: check if there is appropriate dead time! (Check drain currents and gate voltages too! This is why simulators are soo good: you can measure whatever you want!)

Note: discharging of a pMOS means making its potencial more positive, and vica versa.
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Old 22nd March 2008, 11:47 PM   #26
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This is a very interesting thread for me, but I wish I had found it a few months ago!

I'm working on a project for university designing a class-d amp on chip in a 0.35um cmos process, so its interesting to see how people are doing it with discrete components. Anyways, for my bridge, I changed the W/L of the drivers to change the rise/fall time. Rise/Fall time is equal to 0,69*Ron*CL, where Ron is your average on resistance and CL is your load capacitance.

I'm pretty sure these values would not need to be calculated in discrete components (since they are in the data sheets). I guess this isnt as relevant since you are using BJT drivers, but I thought I'd mention it anyways!
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Old 23rd March 2008, 12:20 PM   #27
mag is offline mag  Switzerland
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Hi Munky
Pafi is right, if you don't measure the drain current you can not
see if there is cross conduction or not.

I don't know why circuit maker is not able to measure the current on mosfet, but you can add a resistor in series of 0.000x ohm just to check the current (see attached schematics).

Your circuit is actually too slow for 200kHz and having a VGS on mosfets of only 4V means that their RDS_on is high and they are not fully switched on.

Change your driver structure to the one Pafi is suggesting in his link, or check the one attached, very similar to Pafi's one BTW.

For simulations (at least to see if there is some dead time) you can even remove the low pass filter. Check the current on the node connecting the P_mos drain to the N_mos drain. If there is no dead time you will see lots of current on that node.
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File Type: pdf discrete_driver.pdf (12.1 KB, 61 views)
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Old 23rd March 2008, 08:59 PM   #28
munky is offline munky  Canada
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Hello, I've tried to implement the circuit for faster switching, so I used the totem pole and the diode idea. If I used something too close to pafi n maq's idea, I think my proff will kill me. I got a quick question though, when measuring the drain voltage, when the voltage is at -6V, does that mean that MOSFET is on? and what if it's pass -6V, is the MOSFET off at that point? Attached is the implemented circuit, do you fink the switching is ok? (Note: The dotted lines shows where -6V is at). THANKS
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Old 23rd March 2008, 09:12 PM   #29
munky is offline munky  Canada
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Hello, I've got rid of the LPF to see how much current goes through the drain of the MOSFETs. In the simulations it is giving me 3A of current, but when I had the LPF, the current dropped to like 0.5A for some odd reason. Attached is the current check coming out of the drain of the MOSFETs without the LPF.
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Old 23rd March 2008, 09:22 PM   #30
munky is offline munky  Canada
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This is the one with the LPF. The current drops alot, is it because of the inductor? If so, I thought inductors are suppose to be "lossless".
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