Hi all,
while designing a large isolated DC-DC converter (40 to 60V in, 360V out, 4kW), I ran into worries about flux walking.
The design is a full bridge of MOSFETs, one per leg, no paralleling, using MOSFETs rated at 100V and 1.4mΩ. One driver IC per MOSFET, located in its immediate vicinity, with signal coupling from the control IC to the drivers done by means of a small pulse transformer. Then one single big transformer, a bridge of SiC diodes, choke, filter capacitor. The doubts right now now are how to close the control loop.
My goal is to achieve high efficiency in this converter. Weight and size are no big concern. So I chose a large ferrite core (two E70 pairs stacked), with 3 primary and 30 secondary turns of Litz wire having a large number of strands. Secondary-primary-secondary layering. Running at just 20kHz. The result of this is well under 1% total transformer losses at all power levels, and also rather reasonable switching losses. Assuming that my calculations are right, of course... But there is a side effect: The primary inductance is pretty large, resulting in the magnetizing current being rather small in comparison to the load current. So I have doubts about whether peak current control is sufficient to rule out all possibility of flux walking trouble.
There can always be assymetries. Differences between MOSFETs, drivers, rectifier diodes, but there could also be some small undesired coupling between the power and control circuits, causing the conduction cycles of one side to be systematically longer than those of the other. When the magnetizing current is a good part of the total current, then peak current control certainly can handle that well! But when magnetizing current is small relative to load current, I fear that asymmetries and interference from the power circuit MIGHT end up causing enough flux walking, so that in one polarity the MOSFETs turn off under much higher peak load than on the other side. HOW, you ask, when peak current control is supposed to hold those peaks equal? Well, it's not instantaneous. And ferrite can saturate rather sharply. So, when the core goes into saturation, and the current starts shooting skyward, the current sensor has to detect that, the control IC has to react, its output has to switch, the signal has to propagate through the pulse transformer, the driver ICs, then those big MOSFETs have to turn off - and that whole process might take 300ns or perhaps more, and in that time the current will keep rising sharply!
Maybe peak current control is good enough to keep the MOSFETs from burning out, but two of the four might end up working with much higher switching loss than I had calculated. And that's bad for reliability, for EMI, and also for the overall efficiency of the converter.
At this relatively low input voltage and high power, I don't want to use a current sense resistor. It would dissipate something like 100W, and it might be hard to get its ESL low enough. But if I use current sense transformers, I need to use two of them, in the two half-bridges, so each core has time to reset, even when during operation at minimum input voltage the deadtime gets very short. Any differences between these two current sense transformers will directly cause flux imbalance in the big transformer, by making the control circuit extend the conduction time on one side.
I was toying with the idea of simply placing a DC-blocking capacitor in series with the primary, and then use average current control. But I would need roughly 200µF, with an ESR of well under 1mΩ, and low enough ESL, and that looks like a bank of perhaps 20 polypropylene capacitors of 10µF each in parallel. It's a bit inelegant. And I would still need peak current sensing for fast shutdown in case of trouble!
Another idea I had is deliberately increasing the magnetizing current, so that it's a more noticeable part of the total current, and the peak current controller can "see" it better. Putting a small airgap in the core would do that, and of course it would also increase the saturation current by the same proportion. But it would also cause a noticeable increase of switching loss and EMI, so I don't like that solution too much.
If you build such circuits, how do you handle this issue? Do you simply trust that peak current control will keep the flux centered well enough to prevent all trouble? Or do you put a capacitor in series to be safe? Or do you use a DC current sensor (a Hall sensor?) to sense the DC component in the primary current, and have the control circuit null it? Does that even work, in the presence of the large AC?
And a small collateral question: Is it safe and sound to use the IPTC014N10NM5 in this application? I'm a bit old-school, and have some trouble accepting the idea that such a small thing can switch 120A at 60V, but the datasheet and also my maths seem to say that it can...
Manfred
while designing a large isolated DC-DC converter (40 to 60V in, 360V out, 4kW), I ran into worries about flux walking.
The design is a full bridge of MOSFETs, one per leg, no paralleling, using MOSFETs rated at 100V and 1.4mΩ. One driver IC per MOSFET, located in its immediate vicinity, with signal coupling from the control IC to the drivers done by means of a small pulse transformer. Then one single big transformer, a bridge of SiC diodes, choke, filter capacitor. The doubts right now now are how to close the control loop.
My goal is to achieve high efficiency in this converter. Weight and size are no big concern. So I chose a large ferrite core (two E70 pairs stacked), with 3 primary and 30 secondary turns of Litz wire having a large number of strands. Secondary-primary-secondary layering. Running at just 20kHz. The result of this is well under 1% total transformer losses at all power levels, and also rather reasonable switching losses. Assuming that my calculations are right, of course... But there is a side effect: The primary inductance is pretty large, resulting in the magnetizing current being rather small in comparison to the load current. So I have doubts about whether peak current control is sufficient to rule out all possibility of flux walking trouble.
There can always be assymetries. Differences between MOSFETs, drivers, rectifier diodes, but there could also be some small undesired coupling between the power and control circuits, causing the conduction cycles of one side to be systematically longer than those of the other. When the magnetizing current is a good part of the total current, then peak current control certainly can handle that well! But when magnetizing current is small relative to load current, I fear that asymmetries and interference from the power circuit MIGHT end up causing enough flux walking, so that in one polarity the MOSFETs turn off under much higher peak load than on the other side. HOW, you ask, when peak current control is supposed to hold those peaks equal? Well, it's not instantaneous. And ferrite can saturate rather sharply. So, when the core goes into saturation, and the current starts shooting skyward, the current sensor has to detect that, the control IC has to react, its output has to switch, the signal has to propagate through the pulse transformer, the driver ICs, then those big MOSFETs have to turn off - and that whole process might take 300ns or perhaps more, and in that time the current will keep rising sharply!
Maybe peak current control is good enough to keep the MOSFETs from burning out, but two of the four might end up working with much higher switching loss than I had calculated. And that's bad for reliability, for EMI, and also for the overall efficiency of the converter.
At this relatively low input voltage and high power, I don't want to use a current sense resistor. It would dissipate something like 100W, and it might be hard to get its ESL low enough. But if I use current sense transformers, I need to use two of them, in the two half-bridges, so each core has time to reset, even when during operation at minimum input voltage the deadtime gets very short. Any differences between these two current sense transformers will directly cause flux imbalance in the big transformer, by making the control circuit extend the conduction time on one side.
I was toying with the idea of simply placing a DC-blocking capacitor in series with the primary, and then use average current control. But I would need roughly 200µF, with an ESR of well under 1mΩ, and low enough ESL, and that looks like a bank of perhaps 20 polypropylene capacitors of 10µF each in parallel. It's a bit inelegant. And I would still need peak current sensing for fast shutdown in case of trouble!
Another idea I had is deliberately increasing the magnetizing current, so that it's a more noticeable part of the total current, and the peak current controller can "see" it better. Putting a small airgap in the core would do that, and of course it would also increase the saturation current by the same proportion. But it would also cause a noticeable increase of switching loss and EMI, so I don't like that solution too much.
If you build such circuits, how do you handle this issue? Do you simply trust that peak current control will keep the flux centered well enough to prevent all trouble? Or do you put a capacitor in series to be safe? Or do you use a DC current sensor (a Hall sensor?) to sense the DC component in the primary current, and have the control circuit null it? Does that even work, in the presence of the large AC?
And a small collateral question: Is it safe and sound to use the IPTC014N10NM5 in this application? I'm a bit old-school, and have some trouble accepting the idea that such a small thing can switch 120A at 60V, but the datasheet and also my maths seem to say that it can...
Manfred