Electronics > Power/Renewable Energy/EV's

need help to compute Irms of this topology

**iso14000**:

Hi Folks

I would like to compute rms current flowing into R1 according duty ratio that drive the Mosfet.

the goal is to modulate power given to a heater in such a way my EV still charge without making my contract gives up .

I'm living in France , so mains rms is 230V , my contrat is for 30A , and these two loads are two much burden together for that figure.

So I imagined that simple buck with a large inductance to allow CCM. I don't want to be a bad guy for EMC.

I tried to wrote "something" with duty as input and current as output for an equation (I believed it was close to a PFC one)...

and I failled!

if any of you have an idea....

thanks

**jwet**:

Since you've gotten no responses- I will try. First let's nail down the problem-

If I understand correctly- you have a circuit in your garage that can deliver 230V at 30A and you want to power a heater and an EV Charger. The total is more than 30 amps so you want to decrease the heater output when you're charging to stay under the total of 30 amps.

In your SPICE model, it looks like you're rectifying and filtering the line input to make "DC" and then applying a PWM to drive your "Heater"- the 30 ohm resistor shown in the circuit.

What you're after finally is a closed form solution (equation) for Irms vs duty cycle of the FET drive to the low side of your load. You've included inductors to smooth out the transitions for EMI.

Is this correct?

First- let's eliminate the inductors- they may be required but they make analysis harder at the beginning.

Second- Calculate the RMS of a pulsed waveforms- go back to RMS definition - square root of sum of squares over a period- square roof (Ipk^2 x Duty + 0 * (1-duty)). This give you just I sqr(D)

Example is you have 10 amps peak through your load (this would be a Vin of 300VDC divided by your 30 ohms) and drove the FET at 10% Duty Irms would be 3.16 amps. At 50% duty, you'd have 7.07 amps, at 1%, you'd have 1 amp.

I hope this helps- thank me with the thank button if so.

**T3sl4co1l**:

Rectifying mains is somewhat cutting off your nose to spite your face: the power factor is typically around 0.5. Power factor is bad because it's only drawing current on the peaks, which about doubles the RMS current draw at any given setting (actually a bit worse at light load, and a bit better at heavy load; it depends).

I presume you want to maximize capacity under the constraints (given Irms input).

If you had a PFC front-end, that would be fine, but that's a lot more complexity.

Note that phase control (TRIAC stuff) and pulse skipping (integral cycle control) also have poor power factor. Cycle control is a funny case, because the power factor is 1 during a cycle, but considering many cycles, it has the same on-off behavior as any other switching scheme (PF = sqrt(D) or something like that).

The other way is to PWM the AC directly, which is also complicated but for slightly different reasons.

For ease of use, I might suggest getting a VARIAC and... put a servo on it or something? Next best thing maybe: get a switching supply of adequate total capacity, and PWM the output from that.

Not sure offhand if there's a high PF, continuous heater controller out there already.

Tim

**Siwastaja**:

How is your contract being monitored? Does it involve actual monitoring or is it just the main fuse size? Here it's just the fuse, so you can utilize the fact that the gG type fuse does not blow below 1.25*In or so, and even at 1.5*In it takes ages (tens of minutes, hours) to blow. Therefore, a simple on/off relay switching once every 5 minutes or so would do the job without wearing out too quickly.

**jwet**:

Tim- I figured we'd get to this DC business soon enough- this is clearly not the right approach but I tend to play these balls where I find them. It does make for easy analysis.

I was going to leave it as a exercise to the student to derive the RMS given a Triac driven by a PWM.

Regards

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