| Electronics > Projects, Designs, and Technical Stuff |
| Is 550uF too big for a power supply that has CC limit? |
| << < (5/10) > >> |
| bloguetronica:
--- Quote from: T3sl4co1l on January 29, 2019, 03:45:12 pm ---The remark about unity gain is not about stability of the amp itself, but about its place in the loop. Namely, even if the amp is severely overcompensated (say by putting 1000uF from out to -in), it still has gain of 1 from +in to out, which means the loop gain has not dropped to an ideal dominant pole as you might've been expecting with such a large cap. The solution is to apply filtering to +in so it drops at the same rate. Compare this, which has G = 1 at high frequency, with this, which has G --> 0 at HF. For an error amp, R2 and R4 --> infty, so that DC error is as nearly zero as possible. For a pole-zero compensation of course, a resistor is placed in series with C2 and C4. The zero itself means HF gain levels off; the point of doing this, versus leaving it alone, is you have control of the gain and frequency where this happens, rather than being stuck with wherever G=1 rolls it off at. Tim --- End quote --- Op-amp stability is a science in itself. I confess that it is hard for me to understand. --- Quote from: blackdog on January 29, 2019, 05:09:05 pm ---Hi bloguetronica, For example, assume that IC7 is set as a 1x amplifier and has 60 degrees of phase space. Of these 60 degrees little remains, this because of the following components in de current loop: IC6, R7+C12, Gain Q1 and Q4 your power stage, which will eat all phase margin. 45 degrees is the absolute minimum you need for a good en stable power supply, this under all conditions. That at a certain capacitor value at the output connector, and the power supply does not generate, does not mean that it is stable... You will have to test this circuit at various output voltages and also at various currents. Test it dynamically with short pulses, pay special attention to the transition area when your power supply changes from CV to CC. Kind regards, Bram --- End quote --- Testing is planned for sure. Lets hope I don't fry another board. I'll have to test this using a 150uF output capacitor, see if it is stable, and do some improvements. I hope there is no instability due to the interaction and feedback with the DC-DC pre-regulator, and for that R15 and C16 will have to serve (those were added too). --- Quote from: xavier60 on January 29, 2019, 05:46:27 pm ---Have a close look at a typical LM723 implementation. Although the output stage is voltage follower in CV mode, it becomes transconductance in current limiting mode. --- End quote --- Thanks! The LM723 implementation is no different that the typical implementation that employs a series sensing resistor that will pull the BE junction of a transistor, which will in turn pull down the output of the error amplifier. I've used a 2.7K here to avoid shorting the amplifier output. Their approach is simple and effective, but only works if you want a fixed current limiting (something that I may have to do if this doesn't work). --- Quote from: floobydust on January 29, 2019, 07:34:52 pm ---550uF is too much. Prove it by setting any PSU to say 15V at 10mA and testing some LED's, as a real world example. Energy is stored in the output capacitor which will be charged at 15V which dumps into the LED load down to 3V. You thought the LED was 12V but it's a 3V part. The CC mode transition takes some time as well. So the LED might just look like a strobe light (bright) flash happened, or the LED might be dead now due to the impulse. Some lab power supplies I can't even test LED's, they surge so bad. This is my criterion, a decent PSU is at most ~100uF but if you work with discrete semi's and small parts, 550uF is too big. --- End quote --- Yup, even 270uF is enough to damage a LED in CC mode. Tested that yesterday. I'll have to consider the minimum value of 150uF just to keep the supply stable. Kind regards, Samuel Lourenço |
| bloguetronica:
Hi, After seeing a few videos, it seems that I'll have to test the transient response of IC7 to see if there is any ringing. I'll try to test for both transients on the inverting and non-inverting input. I'll also check the stability of IC4B. Kind regards, Samuel Lourenço |
| David Hess:
10 to 100 microfarads per amp is good for a simple but well designed constant voltage and constant current power supplies. Anything higher indicates problems with the frequency compensation. Usually these problems come about because of additional phase lag from multiple cascaded stages within the control loops or excessive or uncontrolled voltage gain in a level shifter. High performance constant voltage and constant current power supplies can do much better than this with only say 0.022 microfarads per amp but use faster output transistors and have provisions to prevent integrator windup in the error amplifiers. They also may use a class-ab output stage with some active pull-down capability. |
| bloguetronica:
--- Quote from: David Hess on January 30, 2019, 02:24:01 am ---10 to 100 microfarads per amp is good for a simple but well designed constant voltage and constant current power supplies. Anything higher indicates problems with the frequency compensation. Usually these problems come about because of additional phase lag from multiple cascaded stages within the control loops or excessive or uncontrolled voltage gain in a level shifter. High performance constant voltage and constant current power supplies can do much better than this with only say 0.022 microfarads per amp but use faster output transistors and have provisions to prevent integrator windup in the error amplifiers. They also may use a class-ab output stage with some active pull-down capability. --- End quote --- I think that this other power supply, that has the corresponding schematic attached, might have stability issues too. I ran into oscillations caused by a misplaced ground connection (www.eevblog.com/forum/projects/usb-controlled-precision-power-supply-(or-voltage-reference)/msg2007782/#msg2007782). Normally, this supply is stable, but the constant current load that that you see is being supplied via a SMPS and causes some oscillation, as long as it is not grounded to earth. I'll have to do some tests tomorrow. Never had such issue with the FAU200 model, that uses a simple buffer to control the pass transistor. Kind regards, Samuel Lourenço |
| David Hess:
--- Quote from: bloguetronica on January 30, 2019, 03:09:05 am ---I think that this other power supply, that has the corresponding schematic attached, might have stability issues too. I ran into oscillations caused by a misplaced ground connection (www.eevblog.com/forum/projects/usb-controlled-precision-power-supply-(or-voltage-reference)/msg2007782/#msg2007782). Normally, this supply is stable, but the constant current load that that you see is being supplied via a SMPS and causes some oscillation, as long as it is not grounded to earth. --- End quote --- Maybe I am reading the schematic wrong but it looks to me like the remote sense to the voltage feedback loop is backwards, but it reminds me of a circuit I was studying a couple weeks ago from the LT1010 datasheet shown below. The OPA703 is slow enough that it should be able to control the output through a TIP31 emitter follower without problems. The first example shown below is a good example of what I was talking about. D2 clamps the current control operational amplifier to prevent integrator windup. C1 and to a lessor extend C2 maintain stability despite the current control loop being in series with the voltage control loop with two relatively fast operational amplifiers. This design requires no output capacitance but has what is effectively a fast full class-ab output stage in the LT1010. Note that the 2 ohm resistor in series with the output of the power stage aids stability when a capacitive load is present and this is an advantage of having the current shunt in series with the output. The second example is from National Semiconductor is the low output capacitance design I was thinking of. D2 takes advantage of the external compensation feature of the LM301A current control operational amplifier to clamp it preventing integrator windup preserving fast response. Q2 makes the output stage more like class-ab with the ability to pull the output down. The third example uses 100 microfarads of output capacitance for a 1/2 amp output current to control transient response because no clamping of the operational amplifiers is used. In practice I think that capacitance could be 22 microfarads without problems but they wanted extra stability. |
| Navigation |
| Message Index |
| Next page |
| Previous page |