Hello,
i have a H-bridge voltage inverter that takes a 50V battery and creates a pwm sinus with about 15A, at the end there is a filter inductor between 1-10mH. My question is now, can the output filter inductance be too big? The scenario i have in mind is, when the Mosfet closes and the inductor tries to keep the current flowing and the drain voltage of the Mosfet should go down and exceed the rated drain-source voltage which destroys the Mosfet.
could this become a problem?
Hello,
i have a H-bridge voltage inverter that takes a 50V battery and creates a pwm sinus with about 15A, at the end there is a filter inductor between 1-10mH. My question is now, can the output filter inductance be too big? The scenario i have in mind is, when the Mosfet closes and the inductor tries to keep the current flowing and the drain voltage of the Mosfet should go down and exceed the rated drain-source voltage which destroys the Mosfet.
could this become a problem?
You're talking about an inverter which PWMs the DC input voltage to generate a sine wave? I don't think I've seen such a beast. The output voltage will be able 35VAC. Don't tell me it has a massive transformer to step that up to 120V or 240V?
Any energy stored in the inductors in the filter is transferred to the output, rather than absorbed in the transistor. It is not the same as switching a relay or the leakage inductance in a transformer, which must absorbed by a free-wheeling diode or a snubber network.
You're talking about an inverter which PWMs the DC input voltage to generate a sine wave? I don't think I've seen such a beast.
how does the inductor behave when his voltage source is cut off? doesnt the energy stored in the inductor create a voltage to counter the current decrease?
Hello,
i have a H-bridge voltage inverter that takes a 50V battery and creates a pwm sinus with about 15A, at the end there is a filter inductor between 1-10mH. My question is now, can the output filter inductance be too big? The scenario i have in mind is, when the Mosfet closes and the inductor tries to keep the current flowing and the drain voltage of the Mosfet should go down and exceed the rated drain-source voltage which destroys the Mosfet.
could this become a problem?You're talking about an inverter which PWMs the DC input voltage to generate a sine wave? I don't think I've seen such a beast. The output voltage will be able 35VAC. Don't tell me it has a massive transformer to step that up to 120V or 240V?
That's not normally how it's done. A DC-DC converter, consisting of a high frequency oscillator (50kHz to 500kHz), a high frequency transformer and a rectifier boosts the voltage to 170VDC or 340VDC (depending on whether it's a 120V or 240V inverter) first and an H-bridge and filter converts it to mains frequency AC.
Any energy stored in the inductors in the filter is transferred to the output, rather than absorbed in the transistor. It is not the same as switching a relay or the leakage inductance in a transformer, which must absorbed by a free-wheeling diode or a snubber network.
Back in the 80's that's the only way we did it, with PWM straight from the DC buss and possibly large output transformer depending on the power rating of the unit. Some of these transformers weight in so heavy they were very hard to move around. For a 30kWatt 3 phase unit there were three of them, a massive cabinet to hold everything.
The solar version worked right off of the million dollar solar array, converting the DC to PWM sine with some filtering.
I had actually suggested doing some of it with a front end DC to DC converter, but the votes came in negative against it with the reason that we only wanted to handle the power once. That was important for very high efficiency with the transistors that were available back then.
Back in the 80's that's the only way we did it, with PWM straight from the DC buss and possibly large output transformer depending on the power rating of the unit. Some of these transformers weight in so heavy they were very hard to move around. For a 30kWatt 3 phase unit there were three of them, a massive cabinet to hold everything.
The solar version worked right off of the million dollar solar array, converting the DC to PWM sine with some filtering.
I had actually suggested doing some of it with a front end DC to DC converter, but the votes came in negative against it with the reason that we only wanted to handle the power once. That was important for very high efficiency with the transistors that were available back then.
There was a brief period around the 70s, maybe including the late 60s or early 80s, I'm not sure -- where BJTs were the preferred output device for power switching applications. Stinkin' big triple-darlington modules, and 1us switching speed if you were lucky. But at least they don't stay stuck on like the SCRs do... (Which are still useful for some applications, but it's largely for very high power only, as IGBTs have taken over even many low power (1kW or less) cases.)
Or if you were using MOSFETs, they would've been relatively high Rds(on) for their price, and size and Qg and all that, I think? Or maybe early IGBTs too, but the ones where you need to be careful about latchup.
Tim
Too big? Sure -- if you keep increasing inductance, without adjusting anything else, your control loop goes unstable.
Your control loop is regulating inductor current, riiiiiiiight?...
Tim
how does the inductor behave when his voltage source is cut off? doesnt the energy stored in the inductor create a voltage to counter the current decrease?There's nothing to be worried about. There is always a current path for the inductor, either one of the two H bridge switches, or their (parasitic or deliberate) reverse diodes. The inductor's magnetic energy is dumped into the DC bus capacitance, which must be large enough to keep the voltage increase small enough for the circuit. But in real life, I would expect that most of that energy is delivered to the load anyway.
Too big? Sure -- if you keep increasing inductance, without adjusting anything else, your control loop goes unstable.
Really? I would say the opposite - increasing the inductance will only ever stabilise your control loop in a linear sense. Non-linearly you may hit wind-up problems if your anti-windup is not in check.
I see a few issues with increasing inductance
1. Cost and size
2. Loss - more copper loss, more core loss
3. Frequency dependence of the inductor. Larger inductors tend to roll off inductance and become capacitive at lower frequencies - this may have EMI consequences.
4. Dynamic response - larger inductor means slower transient response (assuming the PI is well tuned and reaches output saturation).
You kind of answered your question, without realizing it: slower transient response means poles closer to the right half plane. That is, while holding the controller constant.