Boost Converter - Zener Minimum Load

[dknguyen]

I have a 200V boost converter here that is powering a pulsed load (ie. the actual load is much higher current than what the converter can supply so I'm charging capacitors during the off-time). The boost converter needs a minimum load though, but the current output is so low I don't really want to waste current through a minimum load resistor if it could be better used to charging the capacitors more quickly.

Can I use a 200V zener diode as the minimum load instead so that if the charging capacitors ever do reach max voltage then the zener turns on to stop the converter from surging? The zener can easily handle the maximum output current of the converter without the need for a resistor to drop the extra voltage across. Or is the zener not fast enough (or is the current is too low for the zener to effectively do it's job of clamping).

Zener - 1SMB5956BT3G (https://www.electro-tech-online.com/custompdfs/2007/12/1SMB5913BT3-DPDF.pdf)
Converter Voltage 200V, 6.25mA maximum, 10% load at all times

Also if the zener ever does turn on, it will completely drain the charge that has been built up on the capacitor right? THat's not such a good thing though I guess I could always add a diode to stop the capacitor from discharging through the zener. But the loss of the 10% load resistor might not have a very noticeable effect (but I am not sure) to warrant an extra zener and switching diode in place of a simple load resistor.

 

[eblc1388]

Also if the zener ever does turn on, it will completely drain the charge that has been built up on the capacitor right?

No.

The Zener diode will stop conducting once its terminal voltage falls below 200V.

 

[dknguyen]

Oh yeah, that's right!

 

[Hero999]

Some DC-DC converters behave irratically when they don't have a load, it might not be as simple as just connecting a zener on the output.

 

[kchriste]

I guess the only way you'll run into trouble is if the "boost converter" has a voltage regulated 200Vdc output that matches the zener diode voltage rating. If it has a regulated output, it would make more sense to use a 190V zener on the output with a series resistor to ensure the minimum current requirement is met at 200V output.
Minimum load resistors on switching regulators are typically used to ensure loop stability when there is no load attached.

 

[k7elp60]

I once had that very problem with a boost converter. It was a DC to DC booster from 12V to 18V. I used a LED and a current limiting resistor for the minimum current. The LED being illuminated told me the thing had power applied and was working.

 

[dknguyen]

It's unregulated. I guess the only thing I'm realy worried about now is if the zener can turn on fast enough to dissipate the extra energy in the transformers once the output capacitors are all charged up.

THe LED thing would probably work for the 15V regulators (though I don't think I'll use it. Either a zener or resistor for me), but I don't think it'd work for the 200V one. It's current output is already measely.

 

[dknguyen]

UPDATE: I had zeners in my circuit for a while, but I'm starting to think they may not switch fast enough or whatever to keep up with the switching of the boost converter in order to drain the excess energy away.

I was thinking of using Avalanche TVS diodes. Except...those are designed for transient pulses aren't they? So their steady state ratings aren't given at all. THey also have less accurate breakdown voltages than zeners. Does anyone know anything more?

NOTE: Remember that the load is a pulsed power load so the converter spends most of it's time charging these capacitors up to full voltage at which point no more current is drawn by the load. Only occasionally are the capacitors drained to provide power to the pulsed load. So when the caps are fully charged and the load hasn't drained them yet, the boosted energy from the converter's inductor has to go somewhere...