coulomb
Well-known member
It depends if the Power Factor Correction circuit has a lower limit. From earlier posts, it sounds like that limit is around 29.9 A (perhaps aiming for 30 A).I would think at 208V it would be able to consume about 31amps?
Yes, although see below.PF is the ratio of watts/VA
You're allowed to say ϕ now, so cos(ϕ) = Power Factor.Reactive components like coils and capacitors introduce a phase shift between the voltage and current. Some electricians (outside the US, I think) are more used to 'phi' -- the angular shift. Cos(phi) = PF
But that's for circuits with only linear elements (resistors, capacitors, and inductors). When non-linear elements (diodes, high frequency switching devices) are present, the power factor gets impacted (real power < apparent power), but more importantly, the current waveform can become highly non-sinusoidal. If you just rectified the mains and put a big capacitor after it, all the current would be drawn just at the peaks of the voltage waveform, and the peak current could be ten or a hundred times the average current. That's just not acceptable for relatively high power devices like EV on-board chargers. In Europe, I believe that anything using more than 400 W has to be power factor corrected.
So the most important job of the Power Factor Correction circuit is to cause the current to be largely (often 98-99%) proportional to the voltage, so that the EV looks like a resistive load. Think about how you could do that: a 240 VAC wave has a peak of about 340 V, but a lot of the time, it's lower than about 100 V. In order to make any current flow when the instantaneous AC voltage is that low, you have to boost it to around 400 V. When it's above 300 V, you boost less, but still aim for the same ≈400 VDC. That relatively smooth 400 VDC is what gets chopped up at high frequency, isolated through a high frequency transformer, finally rectified and sent to the car's main battery. The pulse width modulation of the waveform to the transformer is adjusted to achieve the desired charge current, keeping in mind various limits.
So the Power Factor Correction stage is essentially a boost converter, with some smarts to make the instantaneous current drawn be proportional to the instantaneous mains voltage. You can get a single chip that does 90% of the work, except for the actual switching. The actual switching will be via an Insulated Gate Bipolar Transistor (IGBT) or perhaps a Metal Oxide Field Effect Transistor (MOSFET); at these voltages, it's more likely to be an IGBT. This device needs cooling and has a definite current limit; exceeding this current will almost certainly lead to failure. My guess is that it's this IGBT that imposes the ≈30 A limit to AC input current, even if that current does not reach the power limit for the charger. Though it could also be the input rectifier; 30 A is a common rated current limit for a bridge rectifier.
The overall power limit is usually set by the transistors driving the isolating transformer, and/or the diodes that rectify its output to DC. Obviously, the charger's firmware has to limit the input current to satisfy the limits for both the front end (the PFC stage), and the back end (the chopper and final rectifier).
[ Edit: power factor of the waveform to the transformer -> pulse width modulation of the waveform to the transformer ]
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