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

Question

I tried with some configurations in which 16+16 MOSfets of 240A each (really they are case limited to 80-90A 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 attacked all with a IR21094S as driver, and each 2 transistors were driven by a MOSFET totem-pole TC4422 driver. The motor is a 10kW dc compound motor, that is 200A nominal and take probably 1600A at start. The inductance seems to be 50uH, the rising current speed in pulses is = 1 A/µs at 50V Frequency chosen is 1kHz, PWM buck with synchronous rectification configuration

I can't figure out why, even the circuit was carefully made, with 4 modules symetrically supplied and with separate output conductors up to motor, and with independent snubbers, and with a motor snubber, transistors still fails. The circuit seems to work fine but, after some time, like tens of minutes (temperatures are normal, some 45 C) usually at accelerations, usually synchronous diodes fails, followed by all the transistors

I initially tried to sense current on MOSfets using a small mosfet in parallel (drain-drain, gate/gate through a zenner, source of small mos to a 22 ohms 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 not shot-through, i used 2us gap through driver, i only suspect assimetry in the parasitic inductances . How many MOSFETs did you guys paralleled succesfully and in what conditions?

One of the 8 power modules

All power modules

Some of the drivers

Half of the assembly

All stack, without capacitors

Output signal

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

Rising edge; you can see the overshoot is very small, only 5 volts. transistors are at 75v 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 halfbridge 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 (VDS) So for 24v system , use 75v mosfet , for 36v system use 100v mosfet and for 48v system use 150v mosfet.

2. Current

Current rate your mosfets properly for steady state and overcurrent condition , use 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 donot need 16 mosfet , for example This infineon mosfet is rating 7.5mohm at 150v in to220 package . So for 200a 8 of these in parallel should work if heatsinked properly. Power loss in each transistor is (200/8)x(200/8)x7.5= 4.6w which is realistic. and pushing 25a 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 STRES More on overrating mosfet voltage rating and I will backup my answer by automotive standards which use 40v transistors in 12v car systems , and 75v for 24v trucks electrical systems . I think the reason is load dump and such spikes . this will prove important in field testing in harsh enviroments not on your test bench . So the least you can do is using IRFP4468PBF mosfet (100v rated not 75v or 60v like the) remember 48v system is not actually 48v , because batteries fully charged whether lithium or lead acid is around 55 to 60v so you need to keep some margin.

2- Add gate resistors around 3-5ohm for each transistor (they wont slow down the turn On ) remember 15/3=5A per transistor which can charge the gate of Qg=500nC in : dt=q/I= 100ns which is more than enough for 20khz 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 HEATINK , i cannot beleaive that you are pushing that amount of current from mosfet and just using that tiny peice of metal to remove heat, especially if the board is working for sometime them 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 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.