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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Maybe someone can explain this phenomenon that I see when no snubbers are used:
Ringing during switch transitions is caused by oscillations set up by the inductance and capacitance within the switch topology. A second order differential equation seems to describe it quite well.
I put a scope on the gate signal to the mosfets of an induction heater. A coupling transformer is connected as the load, which transfers power to the resonant tank. At low input voltage I see ringing during the switching. As I increase the voltage the ringing gets smaller, and at high voltage input the ringing is essentially gone. Why is the ringing diminishing? I would expect it to increase.
I can only guess that the characteristic of the load current during the switch is changing, transitioning between inductive and capacitative. Can someone shed some light on this?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
You say you put a scope on the gate signal. I would say as you increase the voltage, the ringing gets damped by current through the reverse transfer capacitance of the devices.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
This behaviour is quite normal. Device capacitances are largest for small applied Vds. Capacitance then decrease quickly as Vds is increased. This is particularly noticeable for Cdg and Cds.
See figure 5 on this datasheet for an example:
MOSFETs are terribly non-linear things!
In soft-switching power circuits this voltage-dependent capacitance can be desirable. The large "low-voltage capacitance" limits the initial rate of rise of drain-source voltage right after the device turns off. This reduces turn-off losses and reduces EMI.
However, the voltage dependent capacitance can be a pain in something like a Class-E amplifier though! Since the drain-source capacitance of the device varies with applied voltage it means that an amplitude-modulated Class-E amplifier is only really operating with optimum efficiency at a single point in the modulation cycle. Typical practice here is to try to swamp the variations in device capacitance with an external drain-source shunt capacitor.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
So many figures and I missed that one. This explains why the oscillations disappear.
Snubbers add capacitance and resistance to reduce the oscillation frequency and peak. This figure shows that the capacitance decreases, which thereby reduces the oscillation amplitude. But if I add external capacitance (say, 3 times intrinsic capacitance) shouldn't the peaks remain? As I increase the voltage the intrinsic capacitance decreases, but my external capacitance stays constant.
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switching spikes on an induction heater inverter decrease with higher input voltage
