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Registered Member #236
Joined: Wed Feb 22 2006, 09:59PM
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Posts: 13
I was using a fuse at the negative output before the diode and this saved my boost converter a couple of times when I would forget to disconnect it from the capacitors. Usually when one fails, the results are anything but spectacular. This was using a 1750J bank. Most of the charge will go into the work coil. The biggest failure I ever had was a wisp of smoke. The diode is fairly good at holding back the pulse. If you're worried, use the fuse, but otherwise just diconnect it.
Banned Registered Member #110
Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
Do you have a schematic and have you actually built the booster? Also, the diode which is now to be added, is this the diode from the coil/switch node? Mainly, how were you boosting without a diode if this an existing circuit?
As for spectacular, the result of a cap bank dumping a "portion" of its energy back into the charge circuitry could be more spectacular than you think and would like to experience. It all depends on the degree of failure and the components used at time of failure. I have had boosters fail due to the IGBT's max switch voltage being lower that the voltage at charge time, and only the IGBT heated and did not work well thereafter. On the other hand I have had boosters fail due to the blocking diode being underated for either frequency, voltage, and current. The result has a magnificent and freightning explosion before my face because the diode failed "short" and sent the cap energy down the IGBT channel and up to the coil to the battery. Diode took the bulk.
Now think about this. If you use a diode and the diode did not do as intended because the rating is wrong, too high of a blow current, then the failure could have been avoided by using a properly rated component. Same is true for all the components in a booster. Rate them properly and failure is minimized. Boosters do not go boom for no reason. The reason usually is avoidable. I made tens of boosters of many voltage ratings and non have yet to fail since I learned to properly rate the components for the intended application.
Some ways to help: 1. Charging current is usually rather small, so the insertion of a current limiting resistor would help lower any potential surge currents leading back to the charger. 2. Use a Relay on the charger's output to electrically seperate the cap voltage from charger when not in use or during cap discharge. This does no help when charging, but will keep any negative voltages from going back into the charger if the cap bank were to go negative voltage. Negative voltage will sweep straight through a blocking doide and kill an IGBT if IGBT had not reverse recovery. 3. Use properly rated components, test, test, test. 4. Use your fuse which is above the charging current. The initial charge time consumes the highest current. In fact, the max charing current will be Vsupply/Rcoil, supply voltage divided by the inductors DC winding resistance.
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Boost Converter failure
