Bitch, bitch, bitch, see updated driver at ABSE that controls Vrms to +/-3% as input changes from 9VDC to 15VDC. Different component values would control RMS power in a similar manner. What the OP needs may be a different story. Cheers, Harry
How badly is this design upset by a non zero source impedance? I ask because you have included no supply bypass.
-- Regards, John Popelish
I believed that the OP stated batteries. The circuit will correct for noise inside the 9V to 15V range on a pulse by pulse basis. If filtering is needed it would be wise to increase the 260Hz switching frequency for filter size reduction. Cheers, Harry
Good, a circuit which tries to do what is required.
"tries to do what is required" The OP wants to control RMS lamp voltage driven by a discharging battery. The circuit shown can be trimmed to +/-1% of a fixed RMS voltage for a battery input range of 15V to 9.0V. The +/-3% above was just a fast cut.What else do you want? Cheers, Harry
As the input voltage falls, the PWM needs to increase to keep the RMS voltage the same. I've tried regulating Average voltage. It doesn't work.
I'm a Verizon customer. We just lost access to binariy groups. Is there a resource on-line to get this pdf?
I'm trying to find this schematic. Is there an web based tool to download this schematic. F*cking Verizon.
Found it. Looking at things now.
This is one of the few applications where the average and rms voltage are identical.If you averaged the voltage before it hit the resistive load, there would be an issue. You're not doing this.
In both cases regulation is possible.
If you can't get the simpler arrangement working using an RC filter in the feedback path, adding an rms converter will not improve the situation.
RL
This is rubbish.
Average and RMS values are directly proportional to PWM duty cycle with the same supply voltage.
The PWM duty cycles required to maintain a constant average or constant RMS voltage with varying supply voltage are completely different.
20v with 25% duty and 10v with 50% duty give the same average voltage but a 2:1 difference in RMS voltage.
I think you are mistaken, if you are saying that the average voltage of a unipolar rectangular pulse is the same as the RMS voltage of that pulse. It is not. I think you are misremembering something else (a bipolar square wave, taking the average of the absolute value, for example). Please try running through the math.
For example, a 2 volt peak, 50% duty cycle, rectangular pulse has an average voltage of 1 volt, but and RMS value of
1.414 volts. It will produce a 2 watt dissipation with a 1 watt load (half of the 4 watt dissipation of a 100% duty cycle 2 volt 'pulse'), while 1VDC will produce a 1 watt dissipation, even though the average voltage across the resistor in each case is 1 volt.-- Regards, John Popelish
It's funny, considering the number of times I've gone through the procedure of explaining RMS and average relationships, over the years, to have to re-train myself on this matter. Sort of refreshing.
The formula Epk x rootD is so automatic that I seldom give it a second thought. Just set me down with a pencil and paper, however, and I seem to be able to prove to myself that the moon is made of green cheese.
Wonder why wasn't there wasn't more response, reacting to this (temporary....?) lunacy? I suspect that spending too much time typoing large databases must atrophy regular thought processes.
Sorry for the diversion.
RL
If this is the dumbest thing you have done in the last month, you should break out the champagne and congratulate yourself. ;-)
-- Regards,
John popelish
Well now is the time to bail out of Verizon and go somewhere else with out penalties. THey broke the contract of being a full internet provider, so now its up to you!.
This looks nice and simple. What is the switching frequency? It looks like 47kHz is as low as this thing can go. It looks like the UVLO would kick in at ~8.5V. Is there another IC with a lower UVLO threshold? I'd like to just design a single circuit that could operate from 6V to ~38-40V and just set the output voltage from ~0.6*Vin up to Vin. I would warn the user to avoid using battery voltages that exceed desired RMS output by too much. I could use an LDO between Vin and Vcc to limit Vcc voltage to something like
6V and adjust R5 to maintain voltage at the Comp pin to the desired range. A Zener from Comp to GND would limit voltage for safety purposes. The output (pin 6) I see is driving the gates of the power FET (Q1) , but also a P-ch FET Q3 which grounds VR through R7 when Pin 6 goes low. What is the reason to pull Vref low during Toff? Sorry if some of these questions seem naive, but this is quite new to me.
Sorry for the reply to myself... How about the UCC1803? Vcc-max = 12V UVLO at 4.1V Pin compatible with the UC3842/A. It has a lower Vcc-max, but if I use an LDO between the battery and Vcc, it should be OK at battery voltages higher than 12V.
Your design states a soft-start is on the "to-do" list. If I substitute the UCC1803/3803, how would a softstart be implemented? I need to start this thing slowly. 500mS is in the right range.
Many solutions, here is simplest; insert an 'Inrush Current Limiter" (#MS320R536 at DK) in series with the load, 0.50 ohms cold and 25mR running. Other more elegant solutions upon request. If you are going to operate down to 4.5VDC, select a power MOSFET which has low Ron @ 4.5Vgs. The operating frequency of 260Hz was stated on the schematic. Cheers, Harry
I thought perhaps it was a typo. Maybe 260 Kilohertz. But 260Hz is just fine. The voltage in use will be no lower than 6V. For the greatest majority of applications it will be over 10V. An FET driver like the TPS2819 can have a Max input of 40V with an internal regulator to reduce voltage to 14V max to the gate. Below 14V, the onboard regulator would just be in dropout. For battery voltages lower than 10V I would select an FET like the IRLR7843. Good Rds(on) at Vgs = 4.5V. The IRF2804 would be used for more powerful applications. I've seen folks use the kind of Inrush Current Limiter you've mentioned. Personally I'd rather do soft-starting on a time basis rather than a current limit based on series resistance. Also, at 30mm x 8mm, it's about the same size as the space I have for the entire circuit. Can I request one of the "more elegant solutions"? I'm still trying to wrap my brain around the workings of your schematic.
I see that with Rt=20K, the maximum duty cycle is limited to ~97%, would it be possible to increase the size of Rt to 100K and reduce Ct to 68n? That would keep the frequency roughly in the range you had (253Hz) Also, if the UCC2803 is substituted the frequency is determined by 1/RC, But specs recommend a Max Ct=1n and Rt=200K. That works out to 5kHz. Is a lower frequency just not possible? Or is increasing Ct to 22n possible even though it's way outside the recommended range?
I know this is a lot to throw at you, but you seem to have SPICE software and the brains to use it.
Thanks, Harry.
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