I was looking for a cheaper (than a relay), less space consuming way of active current inrush limiting. The circuit below will add minimally to the effective ESR of the main filter capacitor, the resistor in the cap's ground path limits the initial current, eventually the IRFP044's gate is charged and it shorts the resistor. I haven't drawn in the discharge path for the IRFP044 gate, but it is important. Most of this was discussed in On-Semi apnote 1542 with regard to SMPS converters, incandescent bulbs etc.
Here's the basics:
Here's the basics:
An externally hosted image should be here but it was not working when we last tested it.
Are you aware of that most of the current is for magnetizing the transformer core if we talk about toroids? The main problem is that the fuses goes in the house and the fuse in the amp will be unnecessarily large.
Both the rectifier diodes and the caps itself usually can take this inrush current. If you are talking about a switch mode power supply, it's a different thing though.
Both the rectifier diodes and the caps itself usually can take this inrush current. If you are talking about a switch mode power supply, it's a different thing though.
peranders said:
I have recently demonstrated to myself that my diodes can't load 36,000uF -- these are MBR20100CT's -- 20 amp, 100 V Schottky's, not MUR860's. Without the limiter the current in the first couple of pulses out of the rectifier is over 100 amps. Thus the project.
I've pointed out that in the 400 watt Sorensen supply the current is actively limited with a resistor/relay combination.
R8 -- the second current sense resistor is 0.001 Ohm, not 0.001k.
I tend to agree with peranders that at least also the transformer will draw a big inrush current (especially toroids). If you solve that problem first by inrush current limiting on the primary side, you might have solved the problem of popping diodes at the secondary side at the same time (for free).
Steven
Steven
peranders said:
the supply will deliver +/- 40VDC -- and it's not a torroidal transformer. in it's prior life the supply existed as shown below:
An externally hosted image should be here but it was not working when we last tested it.
That's a wicked supply. I like it.
For the benefit of everyone but jackinnj, yes even a small toroid can pop a 15A mains breaker. Think of the magnetic state of the toroid, and think of the state of the AC on the mains, and now imagine suddenly connecting the two domains 180° out-of-phase.
Now back to this circuit ... the discharge path of the gate seems crucial. The circuit needs to reset pretty much immediately after the supply is removed (this is also the reason a thermistor makes a lousy inrush damper). Got a plan for that? Maybe a diode in parallel with R7 & C3?
What's Q1?
Is this seriously going to cost less than a relay? Relays have a nice feature. If you power them off the DC supply, they will open up almost as soon as the power is removed. A 555 and a relay is pretty close to perfection as far as reliability of reset.
For the benefit of everyone but jackinnj, yes even a small toroid can pop a 15A mains breaker. Think of the magnetic state of the toroid, and think of the state of the AC on the mains, and now imagine suddenly connecting the two domains 180° out-of-phase.
Now back to this circuit ... the discharge path of the gate seems crucial. The circuit needs to reset pretty much immediately after the supply is removed (this is also the reason a thermistor makes a lousy inrush damper). Got a plan for that? Maybe a diode in parallel with R7 & C3?
What's Q1?
Is this seriously going to cost less than a relay? Relays have a nice feature. If you power them off the DC supply, they will open up almost as soon as the power is removed. A 555 and a relay is pretty close to perfection as far as reliability of reset.
A 600 VA transformer take 77 A (mine did) measured with a current probe (hall element 60 MHz bandwidth, with electonics, connected to an oscilloscope).Zero Cool said:
If you take a toroid without any load what so ever, it sucks this giant current. Test it yourself!
The very max current is the Peak voltage 325 V (in Europe)divided with the DC resistance of the primary winding.
I'm working in a phase-controlled approach circuit for SMPS and mains transformer applications. The SMPS version is based in active rectification by two thyristors and two diodes and the transformer version is based in triac switching
I use a PIC12C508 microcontroller to measure and check mains frequency and to start generating gate-trigger pulse trains at the rignt phase angle in order to produce a lineal voltage ramp at the rectified output. It uses periodic gate triggering to get rid of inductive loads
It also features shutdown input, on-off pushbutton switch, power good signal, brown-out check with restart and pwobably undervoltage and overvoltage protection [additional comparators required]
I use a PIC12C508 microcontroller to measure and check mains frequency and to start generating gate-trigger pulse trains at the rignt phase angle in order to produce a lineal voltage ramp at the rectified output. It uses periodic gate triggering to get rid of inductive loads
It also features shutdown input, on-off pushbutton switch, power good signal, brown-out check with restart and pwobably undervoltage and overvoltage protection [additional comparators required]
jwb said:
Q1 is to provide the discharge path for the gate of the IRFP044. I just hadn't drawn in the rest of the circuitry -- but basically when power is disconnected the time-constant cap discharges through Q1.
I don't have any problem blowing circuit breakers -- just diodes -- and these came to me from Avnet, so they are genuine.
Oh, the supply was from a NASA depot in Alabama -- I bought 3 and they were $1.00 each, but cost $80 to ship -- and it took two guys to move the pallet into my garage.
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