Switching a low side IGBT with a PNP transistor, will this work?

Thread Starter

Tjaart1

Joined May 7, 2018
11
Attached is a home brew schematic of a treadmill motor controller. The circuit is separated in three sections:
  1. AC switched to DC controlled with IGBT and Diode fly-back passed through resistor network off board.ac part.png
  2. 1 of the 2 secondary coils from transformer rectified and smoothed with capacitor and then passed to 12V regulator as switching voltage for IGBT. IGBT switched with NPN (TIP32) through a opto-couplar from Pic microcontroller.12v part.png
  3. 2 of the 2 coiled transformer rectified and smoothed and fed through to 5V regulator. This in turn is switched on/off of board and runs the microcontroller.5V controlle part.png

I am not a complete newbie but would like some feedback about my circuit. I am especially unsure about switching the IGBT with the TIP32.

Complete circuit : (note pot, push buttons, on/off switch, Led all off PCB) PDF attached
I don't have spice models for everything so I can't simulate the circuit.
IGBT=60n45b.... something. Its a N channel IGBT with no paralleled diode included and Q2 is a TIP32
Earthing and switching AC on/off as well as other savety trips/overloads and such are taken care of off board.
 

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Sensacell

Joined Jun 19, 2012
3,449
Looks like the gate drive is totally floating from the IGBT circuit, not gonna work.
The voltages on the Gate are RELATIVE to Emitter, so you need to connect the floating power supply ground to the Emitter circuit.
 

Thread Starter

Tjaart1

Joined May 7, 2018
11
Looks like the gate drive is totally floating from the IGBT circuit, not gonna work.
The voltages on the Gate are RELATIVE to Emitter, so you need to connect the floating power supply ground to the Emitter circuit.
I was hoping to keep the high dc motor with all its induction noise separate from the 12v side. If I understand correctly you are suggesting a pull down resistor from gate to The high voltage side emitter?
 

Sensacell

Joined Jun 19, 2012
3,449
I was hoping to keep the high dc motor with all its induction noise separate from the 12v side. If I understand correctly you are suggesting a pull down resistor from gate to The high voltage side emitter?
No,

The IGFET gate represents a capacitor from gate to emitter, to drive it, you need to charge and discharge this capacitor FAST.
To do this, you need to apply a voltage to the gate, along with a return path for the resulting current through the emitter. Then, to turn it off, you need to reverse this current to discharge the capacitance.
Where does the return current flow in your circuit? It doesn't, there is no path.
 

LowQCab

Joined Nov 6, 2012
4,075
First,
Don't use an IGBT when You don't need the particular specifications that it is capable of providing.
They tend to be more particular about how the Gate is driven,
and they produce more Heat, when compared to a MOSFET.

Second,
Use a proper Gate-Driver specifically designed for this type of application,
such as a 296-TPSI3050SDWZRCT-ND this Chip has exactly what You need to
have complete Isolation from the High-Voltage nasties, and it's only ~$5.oo.

Third,
For best results, and quiet operation,
get a big fat Choke to install in front of your main Filter-Capacitor,
and a separate Common-Mode-Choke for
the Motor-Power-Input-Wires to stop the
RFI Hash from the Motor-Brushes from blasting every Electronic-Device in you House.
.
.
.
 

Thread Starter

Tjaart1

Joined May 7, 2018
11
No,

The IGFET gate represents a capacitor from gate to emitter, to drive it, you need to charge and discharge this capacitor FAST.
To do this, you need to apply a voltage to the gate, along with a return path for the resulting current through the emitter. Then, to turn it off, you need to reverse this current to discharge the capacitance.
Where does the return current flow in your circuit? It doesn't, there is no path.
The IGBT will charge up through TIP32 and R5 (value from gate capacitance and current capacity of TIP32), then discharge to -12V through resistor network R5,R8,R7 with 12v zener dumping any voltage spikes. ( I was thinking)
 

Thread Starter

Tjaart1

Joined May 7, 2018
11
First,
Don't use an IGBT when You don't need the particular specifications that it is capable of providing.
They tend to be more particular about how the Gate is driven,
and they produce more Heat, when compared to a MOSFET.

Second,
Use a proper Gate-Driver specifically designed for this type of application,
such as a 296-TPSI3050SDWZRCT-ND this Chip has exactly what You need to
have complete Isolation from the High-Voltage nasties, and it's only ~$5.oo.

Third,
For best results, and quiet operation,
get a big fat Choke to install in front of your main Filter-Capacitor,
and a separate Common-Mode-Choke for
the Motor-Power-Input-Wires to stop the
RFI Hash from the Motor-Brushes from blasting every Electronic-Device in you House.
.
.
.
I have the IGBT's on hand and thought they switched pretty much the same as mosfets.
I am considering gate drivers but there are a sea of them, thanks for pointing me to one....
Cart Summary Digi-Key (for the one you pointed me to + in Aus at least 3 week wait)
SubtotalAU$7.15
Shipping* AU$24.00
Total* AU$31.15
Once I have it working I will use oscilloscope and see if Chokes are severely needed.
 

LowQCab

Joined Nov 6, 2012
4,075
You don't need to "check" anything,
the RF-Noise is going to be there, like it or not,
and the Power-Supply-Output will have a ton of ripple,
which makes everything "Buzz" at Mains-Frequency.
You will also possibly be setting yourself up for hard to diagnose problems.
In my opinion, the Chokes are a required item, although it will probably "work" "OK" without them.
"High-End" Treadmills come with them, ( although usually with underrated parts ),
"Cheap" Treadmills omit them completely, and quite reliably wind-up smoked,
( as can been seen by the hundreds of Threads in this Forum asking for help to repair them ).

It's kinda like having a nice Car,
but the Muffler fell-off, it leaks Oil everywhere You park it, and sometimes it overheats.
Let's go for a ride ............
.
.
.
 

Sensacell

Joined Jun 19, 2012
3,449
The IGBT will charge up through TIP32 and R5 (value from gate capacitance and current capacity of TIP32), then discharge to -12V through resistor network R5,R8,R7 with 12v zener dumping any voltage spikes. ( I was thinking)
How?

Your gate driver is powered by a totally floating transformer winding, (terminals 3-4) the circuit then connects to the IGBT circuit at ONE point only - the gate.
That's not a circuit, you need TWO connections at the minimum, the gate and a ground reference.

You cannot establish a voltage on the gate as an arbitrary single point in space, it's RELATIVE to the Emitter.
 

Thread Starter

Tjaart1

Joined May 7, 2018
11
How?

Your gate driver is powered by a totally floating transformer winding, (terminals 3-4) the circuit then connects to the IGBT circuit at ONE point only - the gate.
That's not a circuit, you need TWO connections at the minimum, the gate and a ground reference.

You cannot establish a voltage on the gate as an arbitrary single point in space, it's RELATIVE to the Emitter.
Ok, so how would you go about switching it with the given 12V from the transformer? ( I don't have a isolating gate driver ;) yet. Strongly thinking about it thou)
 
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