Lv 6
Pyrus asked in Science & MathematicsEngineering · 4 weeks ago

How to rectify very high voltage to get minimum ripple voltage?

I'm designing a circuit to drive a 5 Ampere 320 V BLDC motor. I had already asked the question before, and I just needed to be sure of the circuit. I'm using the circuit below.

Naturally, I'm going to use power diodes to protect the circuit from feedback at the output of the circuit (the input of the motor). Since the minimum voltage required at the gate of the power MOSFETs isn't enough coming from the Arduino Uno Microcontroller, I'm going to use 2N2222 transistors as switches too for the gates that will provide them a 12 Vdc voltage. I ran the simulation and it is actually working fine (ran it on Proteus). 

However, there are some problems in real life, naturally. I need to rectify the high voltage as I need to convert it from AC to DC. I have a power bridge that can support 25 Amps and 800V, so that's not really a problem. The problem is in the smoothing area. I calculated the ripple voltage desired (which is around 5 V), and saw that I would require a whopping 12000 uF 500V capacitor, which I don't even know where to get honestly. The closest thing I have to that is a much smaller 10 uF capacitors, which are nowhere nearly good enough. 

There is another issue. The voltage I need is 320 Vdc. Would it be enough to get a 1:1 transformer and feed it 220 VAC, and take the peak of that voltage (at the capacitor level) and reach the nearly desired voltage (~314 Vdc)?


So yea, I have these problems, and honestly I'm more of a control guy than a "power" guy, so I'm still learning as I go. Would I get a voltage drop after I convert the 220VAC to 314 VDC by "ideally" minimizing the ripple? (Because 220 sqrt(2) = ~314). Any help would be appreciated. Also, what are the capacitors that you would suggest for me? I don't mind using multiple capacitors in parallel. 

Update 2:

@qrk Thank you for the quick reply! I didn't consider the current requirement for the capacitor. So, what you're saying is that the capacitor is bound by both voltage as well as current? That would mean that even if I calculated a minimal capacitance requirement of (for example) 1 mF, I would actually need 1 mF per amp, making it 5 mF? Naturally the voltage rating of the capacitor will be high as well (500 V). So, it would be safe to consider a minimum of 10 mF? 

Update 3:

@qrk I ran the simulation on Proteus. When you say Spice, do you mean PSpice simulator? Because that's the other one that I have, but I didn't test it on it... at least not yet. 

Update 4:

@qrk And yes, the rectifier that I am using is indeed a full wave bridge rectifier. I'm more concerned about the ripple voltage though. V_ripple = I / (2*f*C). For me, the frequency is 50 Hz, as that's the electricity in my country. To make sure of this, I looked online as well, but there are lot of conflicting information. Some say that the ripple is between 220 V and 314V (the peak after 220*sqrt(2)). Others say it's between 314 and 0. I believe it is the latter, hence how I got 10000 uF. 

Update 5:


 Wrong. You failed to understand what I'm trying to do. The ripple voltage has to be as low as possible because there is a PWM coming from the Arduino microcontroller to the circuit I linked above to control it. There is NO WAY that the high ripple is frequency is going to be well synchronized with the frequency of the Arduino. It will cause a lot of issues especially during the start up of the motor, all because you want to skip the inclusion of a smoothing capacitor?? 

2 Answers

  • qrk
    Lv 7
    4 weeks ago
    Favorite Answer

    Use a full wave bridge rectifier. Rule of thumb is 1000uF per amp, thus, you need around 5000uF minimum. Rectifying 220VAC will get you about 310VDC which is close enough to 320VDC. You can run this through a SPICE simulator and see if the ripple voltage is OK for your needs.

    If you have access to 3-phase power, the capacitor requirements are very small.

  • 4 weeks ago

    DC motor able to run on high ripple DC and there is no need to put any filter capacitor.

    It needs a 350Vac out step up transformer following a fullwave rectifier, output becomes close to 320Vdc. 

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