How many MOSFETs can we safely parallel for very high currents? I had problems with a motor application at 48 V, 1600 A

Question

I tried with some configurations in which 16+16 MOSFETs of 240 A each (really they are case limited to 80-90 A because of source terminal, but I doubled this termminal with a very thick copper wire for each of them) were configured in a very symetrical arrangement, 16 MOSFETS in transistor position and 16 in synchronous rectifier configuration, and they still seems to fail at some points and I can't figure out how to avoid failure.

They were attached all with a IR21094S as driver, and each 2 transistors were driven by a MOSFET totem-pole TC4422 driver.

The motor is a 10 kW DC compound motor, that is 200 A nominal and takes probably 1600 A at start. The inductance seems to be 50 μH, the rising current speed in pulses is 1 A/µs at 50 V. Frequency chosen is 1 kHz, PWM buck with synchronous rectification configuration.

I can't figure out why, even though the circuit was carefully made, with 4 modules symetrically supplied and with separate output conductors up to the motor, and with independent snubbers, and with a motor snubber, the transistors still fail.

The circuit seems to work fine but, after some time, like tens of minutes (temperatures are normal, some 45°C) usually at accelerations, the synchronous diodes fail, followed by all the transistors.

I initially tried to sense current on the MOSFETs using a small MOSFET in parallel (drain-drain, gate/gate through a Zener, source of small MOSFET to a 22 Ω resistor and after to a voltage amplifier to activate a fast-shutdown protection circuit), but because of faster commutation time the small MOSFET entered always before the main transistor, disturbing the protection circuit and making it unusable.

There is no shot-through, I used a 2 μs gap through driver. I only suspect asymmetry in the parasitic inductances. How many MOSFETs did you guys parallel succesfully and in what conditions?

This is one of the 8 power modules:

This is the driver for two transistors, MOS or SYNCH MOS, identical:

Here is all the assembly, simplified, but detailed in the main half-bridge driver section:

One of the 8 power modules:

All power modules:

Some of the drivers:

Half of the assembly:

All stacks, without capacitors:

Output signal:

Falling edge, output yellow, 48 V supply blue. Supply is sustained only by some sporadically distributed 100 μF and 100 nF ceramic capacitors, to avoid MOSFET burns by initial tests mishandling.

Rising edge; you can see the overshoot is very small, only 5 V. Transistors have a 75 V rating

Answer 1

At 1600A, I expect that you are approaching this problem from the wrong choice of switching components. TO-220 N-FETs soldered to copper boards seems insufficient for this application and the large number of devices means that probability of component failure is high and can be cascading.

For motor drive applications, module-packaged FETs may be more appropriate, even if substantially more costly per-unit.

• Microsemi APTM20AM04FG
• Digikey's stocked single N-FETs capable of high current

These modules would allow you to reduce the total number of switching devices in your design and allow you to couple them with bus bar rather than an assortment of bare copper-clad FR4.

Even switching to a different leaded/SMD FET package might be more appropriate and enable fewer components:

• H2PAK-2 180A STH310N10F7-6
• D2PAK7 240A IRFS3107TRL7PP
• TO-247-3 209A IRFP2907PBF
• TO-247-3 400A IXFH400N075T2
• 24-BESOP 500A MMIX1F520N075T2 (special heatsink required)

Remember: your time is worth something. Rebuilding the system each time you have catastrophic failure costs you and sets you back from completing and verifying the system. Better FETs may be expensive, but not blowing tens of them up for the Nth time will save you components and time.

For the diagnosis of your presented design:

On your driver board, it looks like you have too little bootstrap hold-up capacitance. 3x100nF almost certainly needs to be supplemented by additional 1s to 10s uF to ensure that the gate driver supply remains stable.

In your testing, have you verified that the channel-to-channel gate drive delay/timing variation is acceptable, even within your generous 2us of dead time? Module-to-module shoot through is also possible, particularly if a gate driver fails, leaving a FET turned on. Additionally, checking the case temperature during operation with a thermocouple or IR camera would allow you to verify that the parts are or are not overheating.

Your mention of 'enhancing' the lead of the transistor seems like it won't help too much, given the 246A silicon / 196A package rated limits of the IRFS7730. This is also additional work required to assemble the system, increasing the labor costs and potential unreliability.

Additionally, your rising and falling images indicate severe problems with bypass capacitance. You are dropping your bus voltage by ~50%! You MUST have sufficient bypass capacitance in both total value (100+ uF, likely) and in ripple current rating (>100Arms steady state, more during startup) to successfully implement your system. The supply "browning out" extremely hard may be part of the reason for your complete system failures. These capacitors will be expensive. Parts along the lines of these film capacitors may be appropriate, depending on your construction method and requirements.

Additional link: Infineon's app note on Current Ratings of Power Semiconductors and Thermal Design

Answer 2

You could post your schematic for more info, gate resistors play a role in the speed of turn on/off (not only the current supplied by the totem pole).

1. Voltage

I have worked with power MOSFETs in half-bridge and full bridge topologies and one most of the causes for failure seems to be voltage spikes. TVS diodes across lower side switch can help. But the real solution is to rely on the avalanche rating of the MOSFET and overrate MOSFET voltage (\$V_{DS}\$). So for 24 V system, use 75 V MOSFET, for 36 V system use 100 V MOSFET and for 48 V system use 150 V MOSFET.

2. Current

Current rate your MOSFETs properly for steady state and overcurrent condition, use a number of MOSFETs that can handle safely (thermal limit) handle the continuous rating of the motor and the spikes are manged by MOSFETs themselves because the can handle overcurrent easily, You do not need 16 MOSFETs, for example This Infineon MOSFET is rating 7.5 mOhm at 150 V in TO220 package . So for 200 A 8 of these in parallel should work if heatsinked properly. Power loss in each transistor is (200/8)x(200/8)x7.5= 4.6 W which is realistic.
And pushing 25 A per transistor is well under max wirebond limit, which leaves space for current spikes.

3. Current limiting

Adding a current sensor, hall effect or a 1 milli Ohm shunt with current sense amplifier should work in limiting acceleration deceleration, and preventing over current condition if you sample current and control PWM fast enough (cycle by cycle current limit)

4. Gate Drive and Layout

On of the most important factors is the layout of you power and gate drive circuit since you are switching high current at few kilohertz, any stray inductance in the circuit will create huge voltage spikes, especially at MOSFET gate and source. For 16 MOSFET I can imagine the length of the gate driver trace or wire! Look for some app notes regarding minimizing gate drive ringing an-937 and APT0402.

EDIT:

After seeing your schematic:
I recommend:

1- I WILL STRESS More on overrating MOSFET voltage rating and I will backup my answer by automotive standards which use 40 V transistors in 12 V car systems, and 75 V for 24 V trucks electrical systems. I think the reason is load dump and such spikes. This will prove important in field testing in harsh environments not on your test bench.
So the least you can do is using IRFP4468PBF MOSFET (100 V rated not 75 V or 60 V like the ?IRFB7730?). Remember 48 V system is not actually 48 V, because batteries fully charged whether lithium or lead acid is around 55 to 60 V so you need to keep some margin.

2- Add gate resistors around 3-5 Ohm for each transistor (they wont slow down the turn on) remember 15/3=5 A per transistor which can charge the gate of Qg=500 nC in: dt=q/I= 100 ns which is more than enough for 20 kHz switching frequency.

3- fast turn off circuit is not needed, just use a Schottky diode anti parallel to gate resistor, since the TC4422 will turn off the MOSFET quickly.

4- USE BETTER HEATSINK, I cannot believe that you are pushing that amount of current from MOSFET and just using that tiny piece of metal to remove heat, especially if the board is working for some time then failing, that means the failure is due to overheat. If you have thermal imager that would be great in detecting such the heat stress concentration. Attach the MOSFETs to aluminum of copper thick bars and use fans if necessary something used in welding machine

By the way there are posts on this websites that would tell you how to calculated thermal resistance and how much heat will build up from the transistor at the specified power loss.

5- sorry for mistake on current sensor, I meant the shunt should be 100micro Ohm (not 1milli). Better is to use contact less isolated hall sensor around the wire like these.
Remember Bi-directional current sensors are very important in motor drive because you can attach them to motor wire (not before ground) to sense current supply and regenerative current during braking so you can limit both currents.

Answer 3

We use 4 x 100A (8 including the reverse-blocking FETs), and tested ok with 400Amp.

We had trouble with inductive spikes, even though the MOSFETs were rated for breakdown power (NOT ALL MOSFETS ARE RATED TO SURVIVE VOLTAGE BREAKDOWN). The breakdown voltage wasn't balanced, and one MOSFET took most of the inductive power on turn-off. And the breakdown voltage did not increase with temperature.

In our case, we did not exceed the rated current in our voltage-breakdown test, because we could get voltage-breakdown failure just by using a bigger inductor. But in your case you could have peak-current failure during voltage-breakdown even if you don't have thermal failure.

Also, it's not clear what you mean by "case-limited because of the source terminal". I've not personally used a MOSFET where I could increase the current rating by using a larger conductor.

Note: MOSFETs current share naturally, Rds increases with current.

Other note: You have to turn the FETs all the way on. They will each have different threshold voltage. This is not a problem if your turn-on is faster than your inductive ramp-up.

Answer 4

Mosfet Modules are the best way to go. An example is this one from Digikey which costs about $28usd(each). I wouldnt attempt a TO220 package with your project. https://www.littelfuse.com/~/media/electronics/datasheets/discrete_mosfets/littelfuse_discrete_mosfets_n-channel_trench_gate_ixfn180n25t_datasheet.pdf.pdf

Modules have very robust connectors. Terminals can also be a concern. Every connection needs to be tight and checked. Wiring must be suitable to handle the pulsed current so you are looking at substantial cabling for your operation. I suggest use an IR/laser sensor gun at minimum - easy, cheap and accurate enough.

Even modules need correct heatsinking - and it doesnt end there. Either water cooling piping or forced air cooling will be necessary. Static cooling doesnt work and won't work for your application.

In the datasheet look for the PD or power dissipation of the fet you choose then the derating in W/degC and you will see that a fet at say 900PD and running at say 80degC won't be 900W. If you try to push it to 900W, it will self destruct long before reaching that. A serious cooling solution is always required for power switching. High power SCRs use distilled water irrigation (yes, showers! distilled water doesnt conduct), but must be filtered and checked for conductance due to impurity washings. Your TO220's have no heatsinks at all which is one reason they are burning.

Paralleling fets is always a gamble. Every fet ( as mentioned) has slightly different characteristics and it's a juggling act to set up multiples and think they will all behave the same. Again as already mentioned, a temp sensor such as IR/laser gun will quickly establish if any fet is out of sync.

Buy the biggest fet module available. Suggest strongly not to use TO220 or any of the 3pin fets for high power - eventually they will burn. Yes, it will be expensive but as long as you use the correct heatsinking and calculate the derated Power Dissipation, well oversize Vdss, use a very low RdsON, it won't blow.

Don't buy fets just based on Current handling. Calculate the I2R losses, apply Power dissipation derating and pay some money for the biggest fet you can find.

Answer 5

Have a look at this circuit..It handles around 800 A pulse with 3 paralell MOSFETs. The load dump/backemf was the biggest problem. I used 12 V automotive lamps to burn off the load dump. The lamps can burn brightly consuming about 24 W of load dump. After that was done MOSFET failures went away. Has the usual 4.7 Ω gate resistors with additional capacitive isolation to reduce inductive ringing on the gates. https://hackaday.io/project/25741/gallery#b30eef72ef37fe0fc4e83efad00298c8