Hi,

I have a TC4428 driving a IXFH12N50F MOSFET. Supply is 12V DC.

I expected to get a pretty good rise time from this, but it seems to be at almost 300ns.

If I disconnect the driver from the gate and just measure the output of the IC, it still shows around 300ns. Looking at the data sheet, I thought it should be more like 60ns.

Am I misunderstanding the values on the data sheet, or is there some problem with my circuit?

I agree 2n3055's are no good. They were used years ago for this sort of thing because there was not much choice. Modern IGBT's or MOSFET's are much better.

Check this one out..

http://www.rmcybernetics.com/projects/DIY_Devices/homemade_pulse_controller.htm

[thought]

Oh good! I couldn't understand how the physics of what I thought I was reading could work! Thanks for clearing that up 

Hi,

I just want to check if I am understanding the SOA graphs correctly on a datasheet..

The STP30NF10 MOSFET is rated for 100V and 35A.
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/7817.pdf

The SOA graph shows that the actual current it can pass varies with Vds. Now the graph goes upto 100V, but Vds (when on) should be really small, so is this referring to the Vds voltage when off?

Looking at the graph, and taking a 24V supply as an example, it looks like it could only switch something less than 5A. Is this correct? Is this still true even when on a large heat sink?

It also shows the current handling peaking at around 3.5V. I don't really understand why lower voltages lower the current ability. Its also seems odd that a transistor rated for 100V, 35A should only work at 35A when switching from a 3.5V supply.

Where am I going wrong?

Yes, that is what I thought, and that is what the wiki says. I was hoping Lyker would explain how he thinks that a resistor can change this as he seems quite adamant that it can be done.

Specifically the part saying "...the holding current specified by the manufacturer."

This seems to contradict the idea that a resistor could change the holding current.

Ok, I reduced R2 and R4 to 4k7 as it seemed the gate voltage needed to be about -11V to have it fully on. I've also got zeners in parallel for when the voltage input gets higher. This seems to work pretty well!

Lyker Says:

A lower resistance from gate to cathode will result in a higher current required to hold the SCR latched.

How would I calculate this?

The short circuit current from the 1kV source will be 270mA, so how can I use a resistor to make the holding current say 300mA?

I'm a bit confused by this idea, I also thought it was not possible to do it like this. Here'a a quote from Wikipedia (yes I know it is not the most reliable source).

It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until both: (a) the potential VG is removed and (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer.

SCR is the same as a Thyristor right?

I just measured Vgs during the pulse to be about -5.8 V (Vth is -4V max on the datasheet).

I will reduce those resistor values slightly (R2, R4) to pull the gates a bit lower.

It makes no difference unfortunately. I've even tried just pulsing bridgeA and leaving the other off. The high side MOSFET still gets hot quickly.

For example using a 1R resistive load, 24V input (drops to 22V)  from an SLA. Pulsing to give avearge current of about 9A. After 10 secs, low side measures 25C and the high side measures 85C and rising. (I'm measuring the front of the case rather than the heat sink)

So, now it is only pulsing a resistive load and in one direction. When I've use single n-types, such heating would make me think it is not fully on, or has a high on state resistance. The datasheet says 0.06 ohms, so average power dissipation should be just 5W.

I want to try manually holding the high side on, then pulsing one of the low side ones to see if the same heating occurs. For the P-type, do I need to make it about 10V lower than the source pin rather than 10V higher as you would in an N-type?

The battery voltage drops to 22 when I connect the coil directly, so this should be fine.

I replaced the MOSFETs, and I've noticed that the circuit draws some current even when no load is connected. If I put the 4k7 resistors across the zeners, it stops this happening.

Could the values of R2/4/5/6 need changing?

(PS they are 12V zeners)

I've not removed the low side zeners yet as I think they are doing nothing right?

I really appreciate your help, I'll send you something nice when its working!

oh yes, sorry, I never worked with p-types before.

I made the changes and it did seem to work better.

When I switched to the 24V SLA though, it melted again. It did seem to take about half a second this time rather than being instant as before. The heat sink is pretty big so it can't be that.

The test coil I was using has a resistance of 2.8 ohms and an inductance of a few mH. So at most the current should be about 9A.

There are two things I can think of to check. One would be to test the circuit with a resistive load to see if it is somehow the inductance causing the problem. The other thing will be to check the voltage drop on the battery as if it goes too low, the gates wont switch properly.

I see, so those zeners being in the wrong place would be why the p-types were not switching as I expected.

I had the other excess zeners like that just as some overkill really. I can see they are not needed, but there's no harm in leaving them there is there (they are already soldered to my board)?

The attached diagram is better?

Yes, that makes sense, they have twice the resistance (0.06 vs the 0.033 of the n types). I'm using IRF5210PBF and STP40NF10L.

With the current limited to 5A, I can turn up the voltage without it blowing, but I do notice that if I feel the case of the p-types, they are very hot within seconds while the n-types are cold (this explains why the blew when using the 24V SLA).
The load (a hollow coil with a screwdriver in it so I can hear it vibrate) sounds like it is working with the pulses I send from a PIC.

So do you think the p-types are not switching on, or maybe off properly? How could I test this?

I've attached my current circuit.