MOSFET driver blows out in half-bridge DC motor PWM controller

Question

EDIT: I've redone the MOSFET circuit diagram; it now represents what my actual layout is. I wanted to spare myself redrawing the circuit but ended up confusing everything, I hope it's clearer now! Also, I'm using IR2907 MOSFETs, in case this is useful to anyone

I'm currently trying to build a half-bridge PWM speed controller for a 24 V, ~100 A, 2 hp brushed dc motor (circuit schematic is at the end; I made it using parts of other schematics found on the net) (circuit layout pictures also in the post).

It worked fine when controlling a small 12 V toy car motor.

When I tried hooking up my final layout (2 hp motor, 12 V lithium battery for the driver Vcc, two 12 V car batteries in series for the motor), it worked fine until the driver (IR2103(S)) started smoking, then sparked and finally caught on fire (cf. following picture showing blown up driver).

[It seems like the blowup is located at pin number 6, which is vs. too much current going in the driver from the motor?]

Something I've noticed when driving both the small and large motors, is that the motor speed reaches a maximum at around ~80% of the 100 kΩ potentiometer range, then falls off when I would've expected it to keep increasing. The oscilloscope reading of the 555 output showed 100% duty cycle but with a lot of instabilities and noise when going in that region.

When I tested with the large motor, the driver blew out when I reached that "fall-off" region and stayed there for a few seconds. I also wasn't using the large 10 µF capacitor across the 24 V battery and the RC snubber.

Here are my ideas of what caused the driver failure:

• Lack of capacitor across the 24 V battery

• Lack of RC snubber across the motor

• 555 timer unstable beyond 80% of potentiometer range

• Too much current going through the driver, causing it to fail due to overheating

• Lack of MOSFET gate resistors?

Here are my questions:

• Could you please help me identify what caused this driver failure?

• Could you give me some advice to help me improve my driver circuit design (missing components, poor component choice, wires too long, parasitic inductance/capacitance, etc.)?

• Could you help me understand and solve that 555 timer "fall-off" issue?

Here are the schematics of the circuit:

Here is a picture of the layout (without the motor and the batteries), with the MOSFET circuit on the right-hand side; I used aluminium strips for the large amperage (this will be cooled by a fan in the final build); I've also added a switch for the 24 V battery (in black); larger wires are the fours wires going to the MOSFET circuit (HO, LO, Vs and GND); smaller wires are Vcc, GND and the three potentiometer wires:

ADDED:

@EdinFifić Is this the circuit you suggest I should try?:

There are a few things I don't quite get:

• Using a driver with a second MOSFET will give a lower voltage drop on the "off" part of the PWM cycle because instead of going through the body diode (in red), the back-emf current will go through a fully conducting MOSFET (which has a lower voltage drop)? Have I understood right?

• What are the advantages to using a driver (and a second MOSFET) apart from the lower voltage drop then? Maybe it is more useful for smaller motors where voltage drops matter a lot more?

• Shouldn't there be some kind of buffer between the 555 output and the MOSFET gate?

• Should the grounds of both the 12 V Vcc battery and the main motor battery be connected?

• If I choose to use a driver, which one would you recommend and/or which datasheet characteristics should I pay the most attention to?

Answer 1

Your first problem is your timing circuit.

DC motors are generally controlled with a low-frequency PWM drive of 25-100 Hz, about 400-500 Hz in some MCUs (like Arduino), and 1-2 kHz at the most. Your RC values indicate a 12.37 kHz frequency, which is too high for DC motors.
According to the formula in the below image, taken from Electronics Tutorials, your half-period at 50% setting (potentiometer in the middle) would be 40.4 us (micro-seconds), the total period 80.8 us, giving the frequency of about 12.37 kHz.

So, the first thing would be to greatly reduce the frequency by using at least a 10 nF timing capacitor (C1); I would recommend 100 nF.
The same site/page mentioned above has this schematic for PWM motor control:

As you can see, the R1 from your schematic is increased to 10 kΩ, and the timing capacitor to 100 nF. The same tutorial mentions the duty cycle range of 5-95%, so 0-100% is likely unattainable.

Your second problem with the increasing duty cycle comes from the IR2103 limitations on timing. You have indicated the motor speed dropping when you go above 80% duty cycle. That would be about 80 kΩ for the ON time, which would be about 0.8 x (80 kΩ x 1nF) = 64 us, which gives an OFF duty cycle of about a quarter of that, or 16 us.
According to its datasheet, first you have the "propagation delay" (the time it takes for the ON signal on its input to appear on its output, which could be as high as 0.8 us:

Then you have the "dead-time" (pause between turning one transistor OFF and the other one ON) of up to 0.65 us for one, 1.3 us for both combined.
This gives about 2us and if we add the 0.2 us turn-off propagation delay, it comes up to about 2.2 us in total delays of the IR2103.
Yet more delays to add to the above are the turn-on rise and turn-off fall times of about 0.2 us total at 1000 pF load (MOSFET input capacitance). Since your MOSFET is specified at 7500 pF (datasheet), these rise and fall times would be about that many times higher or up to about 1.5 us in total.
Not only that, the MOSFET itself has the turn-on delay and rise time of 0.16 us. Adding all these delays up gives you about 3.9 us, almost 4 us!

The third problem and finally the reason your IC burned out: your assumption about the pin 6 (V_S) is correct.
The datasheet mentions the absolute maximum ratings for this pin: V_B - 25 V to V_B + 0.3 V.
The internal schematic shows details, but is not showing a Schottky diode which is likely placed between V_S and V_B (RED diode in the diagram) OR two output MOSFETs body diodes (BLUE on the diagram) which would explain the V_B+0.3 V as its maximum potential:
My thinking as to why it failed:

• when the lower transistor switches off, the motor coil acts as an inductor which produces a powerful high voltage spike; the longer the duty cycle, the more accumulated energy in its magnetic field and the more powerful the spike - that's why it burns when you go above 80%;
• the spike voltage adds up to the positive supply potential (if one end of the motor is wired directly to V+ supply line) and it can easily exceed the V+ + V_B (almost +24 V in your case with 12 V supply, and almost +36 V with 24 V supply). This, combined with the fact that the boost potential V_B is actually lower (possibly less than V+) when your duty cycle approaches 100%, and it becomes even easier for its current to go through the IC's output diode(s) on V_S towards the V_B pin;
• if your motor is connected to the lower power supply rail (V- or 0V), then its negative voltage spike can be absorbed by your lower MOSFET's body diode, so it's harder to see how the IC fails in that case.

SOLUTIONS:

1. Decrease the 555 frequency, in other words, increase the timing period by increasing R1 to 10 kΩ and C1 to 100 nF;
2. Get rid of the IR2103, as you're not getting anything more from it than from the NE555 alone - both have about 200 mA current sourcing and sinking capability, so you're not improving anything by adding IR2103;
3. Use a single MOSFET as a current sink for the motor, meaning connect one end of the motor to V+ supply rail and the other to the MOSFET's drain electrode. Now you should be able to go up to almost a 100% duty cycle (as much as the 555 can go);
4. Definitely place an RC snubber across the motor, or even a diode;
5. Place a 10 Ω resistor between the 555 output and the MOSFET gate to dampen ringing/oscillations.

ADDED ANSWERS (answering more questions afterwards is not practiced here, but I will just give short quick answers):

1. Yes, you understood the MOSFET vs diode drop right.
2. Advantages are lower energy loss, less waste heat and thus smaller components/heatsinks.
3. 555 doesn't need buffer to drive a MOSFET at low frequencies (up to 10 kHz).
4. The grounds should be connected for the MOSFET source to be at same ground as the 555.
5. Almost any that suits your needs, too much to go over right now.

I have come up with a better schematic:

^{simulate this circuit – Schematic created using CircuitLab}

This is just a rough idea, some values/components need to be changed, but it's a start.
Across the motor, you can use either a diode (D1) or the RC snubber to prevent high voltage spikes from destroying the MOSFET. The direction of the current from inductive kickback is the same as the current running the motor, which means its voltage adds up to the supply voltage (the dot end becomes negative and the opposite end positive).
D2 is the braking diode. As the DC motor continues spinning in the same direction after the power is disconnected, it starts generating current in the opposite direction, meaning its dot end is positive.
D2 in that case allows the current to flow from the dot end of the motor through the battery and back into the motor from the opposite end (conventional current flow), completing the circuit.
You should use a Schottky diode because it has a lower voltage drop than the MOSFET body diode (usually).
C2 and C3 are needed to absorb all the transients and peak currents: C2 would be a non-polarized, film capacitor of at least 470nF and up to 10uF and C3 would be at least 1000uF low-ESR type and should be a few of them in parallel to absorb such large currents (up to 100A).
A film capacitor could accompany each parallel electrolytic low-ESR capacitor.
You would also need to parallel a couple MOSFETS (while providing a larger current driver for all of them) and use at least AWG 4 (or 25mm²) copper wire...

There are quite a few caveats of circuit design involving large current/power, and it is not meant for a beginner. I have spent months searching for larger power solutions in couple specific cases because the current involved was over 40 amperes.
Anything over 10 A starts becoming a more serious business, growing exponentially (as the loss is squared with the current). It is the reason you will rarely see hobby kits running more than 10 A, because it needs more serious/hefty wiring/traces/circuits/boards/cooling, and things heat up and burn rather quickly.

Answer 2

Here's a major error in your design: -

The high-side drive and low-side drive don't match the polarities of the MOSFETs.

However, your hand-drawn schematic indicates you may have got this right?

But, the other side of the story is that you may have got your MOSFETs upside down: -

Anyway, something is screwed up in the diagram you posted. Any engineering saying: the devil is in the detail.

Working on breadboard is also going to throw up many problems.

Answer 3

To anyone still wandering around here, I think I've finally found out why everything failed : when taking my circuit apart, I noticed that both mosfets were dead (gate-drain short on the low-side mosfet and gate-drain-source short for the high-side one). When searching for causes on the internet, I found that it's commonly caused by voltage transients and motor brushes noise and will often kill the driving circuits also (driver blew up and the 555 timer was later found dead as well).

So in the end : one 555, one mosfet driver and two power mosfets dead only because I didn't bother with transients and noise reduction when setting up my circuit for testing.

An (expensive) error I will remember.