Background
I have been considering using the VOM1271 photovoltaic gate driver with an N-channel mosfet to do simple on/off (no PWM) high-side switching of circuits up to 84V (previous question for context - optional).
The VOM data sheet states 53us turn on and 65us turn off times. However, this is only with a 200pF gate capacitance whereas most power mosfets with low Rds(on) can be up to 10nF as a reasonable upper bound, so the times would likely proportionally increase with the gate capacitance. Also, those times assume an LED forward current of 20mA, whereas I would like to use 5mA if possible to save power, as I am only switching occasionally. At 5ma the stated turn on time increases to around 160us, but the turn off time decreases to around 40us. So, for a 10nF gate capacitance with 5mA forward current, I would expect about 8ms turn on and 2ms turn off times with this gate driver.
However, the datasheet also shows that this turn on time includes the propagation delay from the start rise of the input signal to the start of the rise of the output voltage, so the actual rise time that the gate voltage spends going from 0% to 90% is likely less than 8ms.
I was aware that during the mosfet turn on period, it has to transition through a sort of half-on half-off state, and during this transition time, the mosfet can have a large transient spike in power due to the simultaneous medium Vds and medium Id.
To confirm this behaviour, I did a simple simulation on falstad.com, where I modelled the VOM as a 5V voltage source and 1MΩ resistor, based on the worst-case voltage of 5V and short circuit current of 5uA when If=5ma and Ta=100C. I used an 84Ω (minus 5.2mΩ for the mosfet's Rds(on)) resistive load to simulate drawing 1A from the 84V supply when the mosfet is fully on, as this is about the max current one of my loads would be drawing in my system.
The results confirm the large spike in power that lasts for about 0.6ms and peaks at a maximum of 21W (as opposed to the steady state value of 5.2mW), although I don't think the duration and max value are necessarily 100% accurate, so I'm mainly just confirming that the spike happens as expected.
Graphs From Left to Right: Gate Capacitor, Mosfet, Load
Voltage=Green, Current=Yellow, Power=Grey
From the simulation it also shows what region the mosfet is operating in at the current time step. First, as Vgs is rising from 0V to 5V, the mosfet first goes from cutoff region (off), to saturation region once just above the threshold gate voltage (Vth), to finally linear/ohmic region once at a certain additional voltage margin above Vth. It is during the saturation region period that the power spike occurs.
Question
Although this power spike occurs, you can see that it does not last for very long, so the total energy/heat generated in the mosfet is much smaller than if that same peak power occurred continuously/during steady state.
Therefore, I did some research to try to figure out how to calculate the maximum acceptable turn on/off time period that wouldn't overheat the mosfet. This includes articles from Analog Devices, Texas Instruments 1 & 2, and Homemade Circuits.
- From the AD article, it mentioned that for events under 10ms, the case temperature of the mosfet does not increase significantly, so Tc can be taken to be the same all the way through the transient event.
- In addition, the AD article mentioned one method of using the transient thermal impedance graph on the datasheet along with the power of the pulse to determine if Tj will rise above the safe limit (usually 150-175C), which is valid for moderate Vds values (i.e. not in thermal instability/"Spirito" region).
- Similarly, all of the articles talk about how the Safe Operating Area (SOA) graph on the datasheet can be used to determine if a certain combination of Vds and Id is safe for the mosfet with different pulse lengths from 100us to DC.
The problem I had with either the thermal impedance or the SOA methods is that in all the articles I could find so far, they talk about using a combination of a given constant Vds with a constant Ids over the whole duration of the pulse. However, for the turn on behaviour I described before, both Vds and Id are constantly changing over the course of the pulse from Vds=84V,Id=0A at the start, to Vds=Almost 0V,Id=1A at the end.
- So then how is it possible to use those start and end points and the approximate turn on time of less than 8ms to infer if the behaviour is safe?
- And if I plot those two points on the SOA graph, how should I connect them, because it may not be just a straight line/linear transition between the two?
- And then once a line is drawn, which of the timed pulse lines would you compare it against to make sure that it is safe? The 10ms line since the total period is about 8ms? But already from the simulation it appears that the mosfet is passing through the saturation/high power region for much less than the whole 8ms, even if it takes 8ms to reach 90% of max Vgs...
So it seems a bit confusing to try and use the SOA to determine if the behaviour is safe or not. So my question is:
How can you use the SOA or anything else on the datasheet to calculate the maximum acceptable duration for the turn on/off period so as not to heat up the mosfet by any significant amount and so keep it safe? Or is it not possible to use only the datasheet for this kind of complex varying behaviour, and some data from an accompanying simulation is mandatory?
