di/dt spikes during ramp up

One more thought on this.

I had run that same configuration using a generic coupled inductor model simply using K statements. (perfect BCM waveforms)

Once i switched to the WE models things got a lot slower and this above mentioned di/dt effect appeared. (This is a 312uH 150W  inductor)

.SUBCKT PFC_PQ38_760806302 1 2 3 4
X1 1 2 3 4 WE_PFC PARAMS:
+ Lp=.00031264
+ C=.00000000000722
+ Rparp=2450
+ Rserp=.14904
+ Cparp=.00000000001391
+ Rsers=.24629
+ Ls=.0000045
+ Cpars=.0000000011
+ Rpars=1762
+ K=.932417
.ends

Folks,

Here is the latest waveform that clearly shows the std. BCM waveform with a set of arbitrary pulses intertwined. It appears the PWM terminates the cycle and immediately after restarts the cycle only for a brief period. Conditions 120VAC input 600Hz, 150W=Pout, 380V, NTC=10 ohm. Rload=1000. L1=312uH L2=4.5uH. Rsense=25m

Top is VdrainQ1 (This corresponds 1:1 with GateQ1)

Mid is L1 main inductor current 

Bottom wave is output current through power rectifier.

Please let me know what you believe is the cause of this? (This is running the original WE high current L=312uH model in LTSPICE)

I originally thought it may be related to Isat=5.2A limit or Rsense =25m as it happens right at the peak (At the point of turn off of the Gate in Q1). 

But this also happens at the tail end of the 60/600Hz wave where current are lower and T is longer, which leads me to believe it is something related to how the VC and PWM controller 

manages the cycle. It shows how Q1 conduction terminates, then Output Diode 2 conducts, followed by the MOSFET turning on again for a brief period.

Clearly the frequency shifts dramatically between the crest and bottom of the wave as the controller maintains energy balance in BCM across this wide Vin range. 

So all that is as expected. 

I am in the process of final layout and would like to get your input as to possible causes. 

Thank you 

Hello All,

Some more thoughts on this. There are obviously multiple things at play. Allow me to dissect this better.

1. Calibrating the spikes I am seeing

I had created a model which leaves the controller out entirely to investigate fundamental behavior of the selected components in such BOOST configuration.  This was also to speed up simulation etc. 

I set D to .56 (Vout=Vin/1-D) and the load at std 150W. Cout=224uF. As expected i get a 380V output at PSS. Obviously heavy inrush and start up current but the circuit settles into equilibrium (PSS) at around 380V and 140W using D=.56 value. Rload=1000 (T=D+(1-D) fsw=100kHz nominal. The circuit reacts to changes in D as expected. (T=D+(1-D))

As you can see from the waveforms, the parasitic capacitances both in the MOSFET and the Diode (fast output diode) and the resulting overlap in conduction produces the expected spikes prior and after conduction. (time and charge it takes for the depletion  zone to be established). And to assure no excessive ringing and powerloss, I added an additional ultra fast diode to the MOSFET. This is also to mimic the forward behavior of the output diode. Essentially both going from depletion to conduction with similar timing. Switch off time for the MOSFET  is now 22ns. You may also notice the turn on spike in the output diode as it goes from depletion into conduction. 

I have no good remedy to the turn on spikes.

As the MOSFET goes from depletion to full conduction and as the diode retreats into depletion mode this causes massive spikes. so i wanted to calibrate what i was seeing when using the controller. Mind you in this model i use a std. PWM rise time of 1ns. which is ideal behavior of the push pull driver. but it is to see the effects of both the MOSFET and the diode overlapping.

Conclusion 1: turn off behavior at peak current is as expected. This is true for all  modes. BCM / CCM .

Conclusion 2: turn on behavior from the 0 floor results in an initial spike as expected. (in BCM)  Turn on in CCM causes a massive spike. 

Consulsion 3: turn on in CCM causes massive spikes in the inductor the diode and the MOSFET. (Hence CCM is to be avoided if frequency bandwidth is available)

Below: Turn off 22ns cycle w added body diode

CCM mode high crest switching 

Quasi BCM mode no controller 100KHz .056=D limited spikes

see images 

2. True simulation w LT8312 chip inserted. Pout=150W  Vin=170VDC peak Rsen=15m Cout=112uF (for simulation time) 60Hz Vac L=312uH inductor. 

Observation is that, there is a CCM mode interspersed with BCM mode

If one looks closely at the various waveforms it becomes obvious that the controllers law switches between CCM and BCM in an somewhat arbitrary way. See the waveform below. I would like to know why the controller does this switching of modes, as the CCM type switching causes massive current spikes throughout.  I did perform an RMS analysis on this and at the end of the day the average current is not much above 1-2A. However the spikes pose a massive EMC and component lifetime issue. LMK why the controller swaps modes arbitrarily and will not alter frequency and what other can be gleaned from this?

And if something or nothing should be done about this. Kindly 

see images Start up 

Detail negative spike in output diode 

Modes interspersed CCM and BCM 

Spikes as the input Vin 60Hz dictates operation