0
\$\begingroup\$

A circuit in my project uses rapidly switching MOSFETs to control current flow, and as a side effect, the current demand on the DC source varies rapidly.

enter image description here

The FET is switching at somewhere between 10kHz and 100kHz, and the current demand switches from 100A to about 150A every period at about that frequency. For example, this is a slightly less extreme example from LTSpice (ignore the small inductive spikes):

enter image description here

The DC source is a large 200V/1000A generator/rectifier from the 1950s, and likely can't handle any changes above ~60Hz. Is there a device or component I could add that could draw an essentially constant amount of current from the rectifier, but handle the rapid changes in load to maintain a constant voltage? A (very) large capacitor across the device could do it, but I'd prefer something a little more elegant and less likely to kill me (or at least advice on how to use something like that safely).

\$\endgroup\$
1
  • \$\begingroup\$ Working with a 200 kW DC source already sounds like there are plenty of things to kill you. Regardless of what ever solution you choose, you should be extremely cautious. \$\endgroup\$
    – Tyler
    Jun 23, 2017 at 15:06

1 Answer 1

Reset to default
2
\$\begingroup\$

Is there a device or component I could add that could draw an essentially constant amount of current from the rectifier, but handle the rapid changes in load to maintain a constant voltage?

It's called a large capacitor

A (very) large capacitor across the device could do it, but I'd prefer something a little more elegant and less likely to kill me (or at least advice on how to use something like that safely).

I'm still saying capacitor and to avoid getting harmed you should consider interlocks and/or a discharge relay that removes charge within several seconds when power is removed.

Whether you use a capacitor or not, it's likely that if you don't take care, the 200 volt supply will electrocute you.

You should also consider that if one of the coil's reverse diodes failed, the electric arc produced from 6 mH and 100 amps is about one-tenth of the energy delivered by a fairly standard heart defibrillator. How safe do you feel given that the catch-diodes you are using have a peak repetitive forward current of only 20 amps and a non-repetitive current of only 150 amps.

I'd be concerned about safety with or without a capacitor.

\$\endgroup\$
12
  • \$\begingroup\$ Andy's advice is well placed. Another good practice is to put a bleeder resistor across the capacitor that bleeds off the charge once power is removed. You waste a small amount of power for the sake of an additional safety measure. \$\endgroup\$
    – Glenn W9IQ
    Jun 23, 2017 at 15:18
  • \$\begingroup\$ @GlennW9IQ Thank you both for your advice. For reference, the diodes in the model are not the ones we're using (sorry for that). Right now, we are using STTH200W03TV1 diodes, but might try a power Schottky diode like the STPS200170TV1. For safety, I might also use two (or more) on each coil. We happen to have some large Aerovox 200uF, 130uF, and 2.5uF caps each rated for 50kV and 250kJ, 150kJ, and 3.5kJ respectively. \$\endgroup\$
    – JAustin
    Jun 23, 2017 at 15:31
  • \$\begingroup\$ I will have to check the specifications in detail, but do you think one of the larger ones directly across the supply with a fairly large (>100k) bleeder resistor and a discharge relay would be reasonably safe and effective? Spice thinks the 200uF capacitor would solve the problem. I can also bring in someone in the EE department to check the setup before we run anything. \$\endgroup\$
    – JAustin
    Jun 23, 2017 at 15:31
  • \$\begingroup\$ @JAustin those diodes are better of course. If LTSpice suggests 200 uF then why not get hold of some 200 uF capacitors rated for say 400 volts? 100k across the 200 uF would be a safe discharge in less than a minute. A relay and 10k would discharge in a matter of seconds of course. \$\endgroup\$
    – Andy aka
    Jun 23, 2017 at 15:38
  • \$\begingroup\$ Excellent. So for instance a capacitor like this across the supply like this plus a relay in parallel which I haven't added to the model. Do you have any recommendations for what kind of relay I should use? And do I need resistive components in series with the capacitor to handle the large current spikes every time the FETs switch since the internal resistance is so low? \$\endgroup\$
    – JAustin
    Jun 23, 2017 at 16:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.