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| Half-Bridge 0-30V, 0-20A Feedback loop problem |
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| paladyn:
Hello, this is my first post, I am writing this post because I hope that you will help me solve my problem. Designs adjustable Half-Bridge converters with 0-30V and 0-20A parameters. The power block itself is able to give over 800W at maximum PWM and it is not a problem. The problem is the feedback loop, which works unstable. I thought it might be a problem with the low choke value, originally 47uH, but then I used the 250uH inductor and it's even harder to stabilize the converter. In general, the whole works, it is possible to regulate the voltage and current, but there are considerable squeaks of the transformer at low voltages, ie 10V and quite large currents of 10A. There are also significant noises at the output as well as sinusoidal oscillations. Generally, these are noises with an amplitude up to 1V when measuring AC on an oscilloscope The loop is created on the external LM324 operational amplifier, while the internal SG3525 amplifier works as a voltage follower. The switching frequency of the transformer is 40kHz. Using the book Switching Power Supplies fromA to Z - Sanjaya Maniktala I used and calculated coefficients for the type 3 compensator for the following data: fESR = 6392 Hz fLC = 424Hz for 47uH inductor and 184Hz for 250uH inductor fz1 = fz2 = 424Hz fp0 = 260Hz fp1 = fESR = 6392Hz fp2 = 1/2 fsw = 20kHz frcoss = 1/10 * fsw = 4kHz Vramp = 2.8V Vin = 43V (output voltage after diodes) fsw = 40kHz Cout = 3 x 1000uF Total ESR = 8.3mohm I calculated the following element values for the 47uH inductor C1 = 30.6nF -> I used 33nF C2 = 17.5nF -> I used 18nF R2 = 12.3KOhm -> I used 12kohm C3 = 678pF -> I used 680pF R3 = 1.4kohm -> I used 1.5kohm For inductor 250uH, the coefficients are: C1 = 30.6nF -> I used 33nF C2 = 42nF -> I used 42nF R2 = 28,3kohm -> I used 27kohm C3 = 297.6pF -> I used 300pF R3 = 600ohm -> I used 560ohm Despite many attempts, I have no idea where the problem is. It also adds inverter schemes. The diagrams are two because the inverter consists of 2 PCBs 1. PCB is a power part 2. PCB is the control part Thank you for your help |
| xavier60:
I can't help directly, I don't understand the theory well. I mainly what to show how I did the fast current limiting via the soft start pin, which you are likely to need also. It is temperature dependent but this can be useful. For voltage regulation, I used the internal error amp in transconductance mode. The response is very good. It doesn't sense the full PSU's output terminal voltage, it regulates to maintain 3V drop across the series pass MOSFET of the proceeding linear post regulator. |
| xavier60:
It looks to be too much gain for the voltage loop. Try increasing R1 from 1.5K to 20K. |
| MagicSmoker:
Did you notice the regulated output voltage is one of the parameters needed to calculate the compensator component values? As a result, if you want to make the output voltage adjustable over a wide range (NB - smoothly going down to 0V isn't an option, practically speaking) then you will have to grossly overcompensate the loop (that is, set a really low crossover frequency - perhaps a few 10s of Hz). Going with current mode control (CMC) will improve matters by eliminating the pole from the output LC filter, but CMC doesn't play well with the half-bridge unless measures are taken to ensure equal volt-seconds are applied each half cycle of operation (the full bridge is more amenable to CMC as long as the DC blocking capacitor that is usually in series with the primary is deleted). The usual approach to making a wide output voltage range switcher is to cascade it with a linear regulator and design the switcher/regulator control loop such that the former maintains a few volts of headroom for the latter (that is, the switcher maintains a nominally constant difference between the input and output voltage of the linear regulator). The slower the transient response of the switcher (ie - the lower the crossover frequency), the more headroom is required for the linear regulator (along with a much larger filter capacitor on the output of the switcher; way more than is required to meet ripple specifications). |
| paladyn:
Thank you for the suggestions, I will try to change the R1 resistor to something bigger and I will try to change the fcross to a significant one, eg 500Hz. From what I see, designing such a flexible converter will not be easy, but it will try. I still have the question of calculating the coefficients, the transformer ratio is somehow included in the calculations? |
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