| Electronics > Beginners |
| how does blackdog's PSU work? |
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| exe:
Concerning discussion "how fast bench PSU should be", I'd say I want a faster one because I use them to supply, say, esp32 which has built-in WIFI. So, with a poor power supply it will reboot while transmitting. To some extend this can be fixed with more output capacitance. Although, I didn't do any measurements, so, may be, just putting a smaller cap closer to the load could help. Another topic interesting to me is how to parallel caps. Say, my power supply has output cap, and my load has an input cap (they may resonate). So, may be low-esr output caps is a bad idea? But I'm too far from understanding these matters. |
| blackdog:
Hi, My opinion 1e A LAB power supply must be nice to his load, realy taking care of it. ;) 2e This implies, among other things, that he has the smalest possible capacitor at the output and has a rapidly responding current limitation. 3e No power On/Off and enable switch abberations. 4e Low Ri and low noise. 5e Stable with a wide range of loads. But, the design i'am showing here is not for large inductive loads, a 2 of 4 quadrant powersupply should be used then. 6e There is no "one size fits all" powersupply. 7e As stable as posible with high capacitve loads, normally not a problem because the wiring resistance to the load. Just an example of which I think the designer has not understood how to do it right. My Rigol DP832 power supply, 2x30V at 3A and 1x 5V 3A. On the output banana jacks there is 470uF! That tells me that they have not had good control over the loop stability during development. In fault situation the energie in the capacitors can kill your load... I handle this rule of tumb, use about 50uF for every Ampere output current (for linear LAB power supply's) Something else with this power supply, the sense wires were bundled together with the live wires, so stupid ... A simple different twist fixt the problem, see this topic: https://www.circuitsonline.net/forum/view/130740#highlight=dp832 The better you design the loop control(phase margin), the lower the output capacitor can be. (within a certain range) My design has two capacitors and resistors on the output so this wound not ring by itself (ofcouse i tested this :-) ) The serie resistors will almost certainly kill the Q of these components. Kind regards, Bram |
| technogeeky:
--- Quote from: exe on March 23, 2018, 10:28:05 am ---Concerning discussion "how fast bench PSU should be", I'd say I want a faster one because I use them to supply, say, esp32 which has built-in WIFI. So, with a poor power supply it will reboot while transmitting. To some extend this can be fixed with more output capacitance. Although, I didn't do any measurements, so, may be, just putting a smaller cap closer to the load could help. Another topic interesting to me is how to parallel caps. Say, my power supply has output cap, and my load has an input cap (they may resonate). So, may be low-esr output caps is a bad idea? But I'm too far from understanding these matters. --- End quote --- This is where "high speed" power supplies like the Keithley 2304 come into play. There are HP models for these as well. These are also known as "mobile communications" power supplies. There are also similar models that are known as "battery simulators". Dave has done a teardown of one of these[1]. The primary difference between a high speed power supply and a battery simulator is that the battery simulator has circuitry to control the output impedance to a selected value. These power supplies usually have very little output capacitance. I am not 100% sure, but I believe in general these power supplies have a dual push-pull topology. They are essentially the positive voltage half of a SMU (source measure unit). One way you can determine if the supply you are looking at is a high speed supply or not, even without reading the detailed specification, is looking for transient response in the specification. In the end (page 111) of the Keithley 2304a manual, you can see that they specify the transient response time to be less than 50 (resp. 40) microseconds. This may or may not seem fast to you, but keep in mind for these high speed power supplies this is a 1000% change in load. Most supplies will only specify for a 100% change in load. The numbers listed in the manual are the worst case. In practice, I've measured times as low as 5 to 10 microseconds for a 1000% change in load. In any case, the overall point of these power supplies is to be able to simulate/emulate the battery or power source for a mobile communications device (anything with a transmitter, really). It can source current (normal usage) and sink current (emulating a battery being charged). In such situations, you often use a small amount of power normally until the wireless radio transmits. At this point, the current usage can increase by a huge amount, but only for a small amount of time. So the overall picture to the power supply is a small quiescent current with a pulse train superimposed on it. All of this could be done using an oscilloscope, current probe, and a battery bank. Note that these are not linear power supplies. They have extremely good noise characteristics for switching power supplies (almost as good as linear, which is an impressive feat). [1]: battery simulator . |
| David Hess:
--- Quote from: blackdog on March 23, 2018, 08:31:36 am ---I am aware that with a power supply without external sensing, I have little control over what happens after the wiring at the load. --- End quote --- Whether external sensing is used only matters at low frequencies. The contradiction is designing for fast loop response to provide low output impedance at high frequencies when the inductance in the external leads prevent the control loop from even seeing the load at high frequencies. --- Quote ---Furthermore, it is not too difficult to bring out the sense wires with some extra security components to get better control at the load for short 4-wiring applications. --- End quote --- I have done this before down to the microvolt level at load currents of amps and it works fine but capacitive decoupling at the load is what lowers the impedance at high frequencies. The high frequency performance of the power supply becomes less relevant at the leads become longer. When using remote sense, be sure to limit the difference between the force and sense lines at the power supply with diodes or resistors to prevent the output from running away damaging the load if a sense line becomes disconnected. --- Quote from: exe on March 23, 2018, 10:28:05 am ---Concerning discussion "how fast bench PSU should be", I'd say I want a faster one because I use them to supply, say, esp32 which has built-in WIFI. So, with a poor power supply it will reboot while transmitting. To some extend this can be fixed with more output capacitance. Although, I didn't do any measurements, so, may be, just putting a smaller cap closer to the load could help. --- End quote --- Usually fast closed loop designs with high frequency transistors are used to provide a low AC output impedance while using only a minimum of output capacitance improving the constant current performance. A standard power supply might have 47uF per amp of output capacitance to provide a low AC impedance while a high frequency design can get by with less than 0.1uF and a carefully controlled ESR to preserve stability. --- Quote ---Another topic interesting to me is how to parallel caps. Say, my power supply has output cap, and my load has an input cap (they may resonate). So, may be low-esr output caps is a bad idea? But I'm too far from understanding these matters. --- End quote --- The power supply's output capacitance forms part of the frequency compensation and if it has an ESR which is too low, excessive phase shift can lower phase margin compromising transient response or even causing oscillation. So the output capacitor may have a very controlled ESR to prevent problems. The bypass capacitors at the load are much smaller and located remotely where the series resistance is greater so low ESR does not matter for them. This can be seen in the application circuits for regulators like the 78xx and 79xx series where a small low ESR ceramic bypass capacitor is shown at the input if the input bulk capacitor is located remotely but the output shows *only* a small high ESR tantalum or aluminum electrolytic capacitor. These regulators are well behaved and will not care about a 0.1uF ceramic capacitor at their output but this capacitor does nothing useful except perhaps keep RF out. For power supply bypassing purposes, it needs to be placed at the load instead. |
| blackdog:
Hi David, I talkt about remote sensing on the Dutch forum, and explaned that if you want a fast reacting power supply, you need to build it like a HF amplifier :-DD Twisting high current cables, twisting your sense wires, keep the buffer capacitor close to the power stage, the output connectors close to the power transistors etc. And of course the development of a fast power supply means that the wiring to your load is becoming increasingly important to keep the Ri low. I did some testing with my HP 6632B with a 1-meter long connection cable with remote sensing. Of course, capacitors were needed at the terminals for some good dynamic behavior, but you cant go to high. There is a lot of usefull information about sense wires in the PDF of the KeySight 6632B. Long cables together with sense wiring can almost never result in a fast power supply due to the induction of the wiring. You cant win :) Actually, we should have a separate section on this forum, which contains all the basics for designing a power supply for various applications where we can put our joint knowledge together. There is so much misunderstanding about the design of power supplies, look at the many compensated to dead designs on the Internet... Kind regards, Bram |
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