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| DIY Injection Transformer for Power Supply Control Loop Response Measurements |
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| rbola35618:
The designer of the Cleverscope is Bart Schroder. I approached Bart several years ago about implementing the FRA option. He redesigned the function generator so that the grounds on the function generator would be isolated from the scope grounds. He used an opto-isolator to do the isolation. You can float 300Vrms on the function generator. The isolation ideal was originally proposed to Bart by a colleague of mine Robert Raulk. Regarding the loop measurement, if you are doing PFC then you need to go down to 1 to 5Hz. In my case, I do not do PFC since my design are only DC to DC space environment. So when I do a loop plot, I do them from 100Hz to 100KHz. My crossover frequency are around 1KHz to 10KHz. So I do not really need that high of a frequency. The Venable injection transformer state a bandwidth from 10Hz to 10MHz. So 10MHz is well above the frequency that I am interested. In addition to the FRA, I use the Cleverscope as a LCR. Bart implemented a LCR option. I built a jig with a current sense resistor and I am able to characterize my magnetics, and capacitors. You need very high frequencies, (The Cleverscope goes up to 65MHz) to characterize the magnetics and capacitance. When characterizing the magnetics, I am interested in the parasitic such as the leakage inductance, ESR, the parasitic capacitance (self resonance). The parasitic usually can only be seen at very high frequency where they become dominate. I can see where Keysight can also implement the LCR option into their 3000T series. This would great if they could do that. I have the regular 3000 series and it can not do the loop analysis. I asked Keysight if they could add the loop analysis to the regular 3000 but they said the FPGA was not powerful enough. But that is OK since I have the Cleverscope. |
| rbola35618:
Hi Bud, The input do not need to be isolated. The ground of both channel 1 and 2 are connected to the power supply ground while the positive probes are connected to an injection resistor. Keep in mind that the resistor is 10 ohms to 100ohms and if you are monitoring a 5V output, then both end of the injection resistor will be close to 5V. You then need to connect a signal source across the resistor which as I mentioned is riding on 5V. So if you connect a non-isolated function generator across the resistor, you will end up shorting your 5V output to ground thru the function generator's ground. Therefore you have to use an isolation transformer. I hope this make sense. Robert Bolanos |
| Bud:
Thanks Robert. Basically you are saying transformer is a DC block. I interpreted In the context of this thread that the purpose of transformer is galvanic isolation from the power supply for safety reason. |
| Kleinstein:
The transformer is there to add the signal to the 5 V output with all its higher frequency components. So it is not just to block a clean 5 V, it is there to inject a differential signal. The links to the electronic replacements could help to understand the purpose: They have an amplifier that adds the driving signal and outputs it to the other side. For a low frequency circuit this could work quite well. But with a faster circuit the amplifier needs to be pretty fast, not add a significant phase shift that might upset the DUT. This might even apply to frequencies higher than you are interested in. So for slow circuits this electronic way might be an alternative to the injection transformer. The amplifier way also has another limitation: it does only work at low voltages, so the transformer can provide this level including DC. |
| rbola35618:
The only problem that I see with the Keysight is , if I am not mistaken, the resolution of the ADC is only 8 bits. The dynamic range and the ability to get the signal out from the noise is not as good as a regular FRA or the Cleverscope. My Cleverscope has a 10bit ADC and I can upgrade it to a 14 bit ADC. So theoretically, the Cleverscope should be more sensitive. I have done some test where I can a good loop plot using only 1mV on input stimulus signal. I then tried doing that on out 30K Venable and it was not able to get a good plot. I think some of the higher end Keysight may have a higher resolution ADC. I would assume that those with higher ADC would work much better. RB |
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