current control (tricky one)

[bmachining]

Ive come up with .001 ohms for R2.
R2 sets the off time of the mosfet driver , so i guess this isnt as critical? too large an off time would mean more ripple??

 

[Ubergeek63]

No, that would be the bottom of R2-the low side of the big cap is circuit ground. Remember, it is a current regulator running in peak current mode. All it needs is the current sense voltage.

I get 0.007 for R2 for 35Apk. What did you use for your "nominal frequency"? Lower Toff makes the max frequency higher (with the associated troubles) and the inductor smaller.

Just remember that the duty cycle is from equation 2: Toff/Tperiod=1-Vout/Vin and when you set the off time (Toff) you set the minimum duty cycle since the blanking period is around 0.45uS. That sets your low end drive current. If you get the other app note I suspect it will work fine for lower output voltages just switching the R1 to ground.

Constant frequency peak current regulation does not like running over 50% duty cycle (if you read up on in you see the term "subharmonic oscillation") which prevents it from regulating well when the output voltage is greater the 50% of the input.

Constant off time peak current regulation has the off time and blanking period limiting the duty cycle which limits how low a voltage you can generate from a given input.

On the bright side, outside of those limitations, they are easy to stabilize.

 

[bmachining]

My Bad!! R2 is currents sense,
R1 is frequency.

0.007 for R2! i got 0.008 @30A, and rounded it up hehe.

R1..... Would it be a good idea to use a trimpot to adjust this?

 

[Ubergeek63]

is the input 40VDC or 50VDC? 40VAC would give you 48-65VDC peak (the valley depends on the load). Don't use a pot, pots are horrendous. 178K would give you 8uS off time and 5% ripple for 10V@30A.

In equation (4) the 0.3 is "Assuming a 30% peak to peak ripple in the output current"

The easy way is to plug the formulas into a spread sheet and play with the numbers, unless you actually have a full blown math package that you understand

 

[bmachining]

I will bump the input up to 50DC. Maths isnt one of my favourites, so yeah some of those formulas are a little daunting hehe.

Does the rest of the circuit look like it will be ok?

 

[Ubergeek63]

Seems workable other than what i mentioned above

 

[bmachining]

Thankyou very very much!!! i will do an order and build! Much appreciated! I hope u can make use of it also!

Will let u know how i go with it

 

[bmachining]

https://www.electro-tech-online.com/custompdfs/2008/05/irfp4110pbf.pdf

How would this Moset go? the Rds on resistance is different...

 

[Ubergeek63]

Quite good actually. I would estimate less than 37W dissipation and one of the best junction to case thermals I have seen. (as long as the case to sink is there...heat sink grease or silicone pad)

a quick worst case is:

40A*40A*0.004R=6.4W + (conduction losses, actually multiplied by duty cycle)
(88ns*2+67ns*2)*100KHz*50V*40A*0.5=31W + (switching losses)
43nC*2*100KHz*50V =0.2W + (drain to gate charge)
210nC*100KHz*12=0.25W (gate charge actually in the driver)

 

[bmachining]

Kewl! there available locally so if i smoke 1 or 2 during testing i can easily get more!

How critical is the inductor size? in uH?

 

[Ubergeek63]

no very, higher lowers the ripple current and lower changes the frequency. Either way you run into a limit that you can tweak. At least with peak current control you do not have to worry about instability

 

[bmachining]

I picked up a couple of toroidal cores, 40mm OD 24.1mm ID 14.5mm L, and with an inductor calculator i found, using 8 gauge wire they should be acceptable. will have 2 of them.

 

[anon86]

Have a comparator drive a pass transistor, and feed a current sensor output into the comparator. Adjust the comparator threshold using a pot or something. Whenever the current gets too high, the comparator's output changes such that the pass transistor turns off disconnecting all the current flow.

Hi,

Im trying to design a similar circuit - that is a current limiting circuit which has the ability to be varied (with the intention that it operated as a variable resistor, to simulate the V-I characteristic of a PV cell).

I have limited knowlege of this type of circuit - can it be modified to allow for the adjustments to be made via microcontroller? Perhaps including a gain stage to up the output voltage from the micro??

 

[prabrahim]

hi

No other ideas from anyone? Maybe a schematic?

Did u get any Ideas, please let me know.