MOSFET Back EMF issue

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
In the circuit below I am generating a 1 Mhz square wave with a CMOS 555, sending to a MOSFET driver MAX4420 and then to the gate of the IRF510 MOSFET.

Two problems.
1)with the toroid NOT connected the signal is getting coupled or leaked into the drain.
2) when I connect the toroid, with or without the load, the signal is destroyed by what I think is back EMF??

Toroid1Mhz.jpg

No toroid:
DS1Z_QuickPrint20.png

With toroid. Same load or no load:
DS1Z_QuickPrint21.png
 

DickCappels

Joined Aug 21, 2008
9,092
1. There is a large capacitance between the gate and drain. You are not the first person to be annoyed by this artifact, but just a fact of life and with a sufficiently low impedance gate drive, should not be a problem (and it looks like you have taken care of that with the MAX4420

2. A 1N4007 has a very long reverse recovery time and probably will not have a chance to conduct until the drain spike is gone. You can try a Schottky or other fast diode (such as UF4004 to UF4007).

2A. I don't see any bypass capacitors in your schematic. The lack of them (one on the input voltage and the other on the output of the regulator - see the manufacturer's datasheet for a recommendation) might be part of the problem if not now, when you get a fast enough diode installed.
 

Ian0

Joined Aug 7, 2020
5,885
1. A 1N4007 is way too slow for 1MHz
2. You have a net DC voltage across the coil, so it will have saturated
What is the toroid core made of?
You need a magnetic circuit with a gap in it, and you need to make sure that the forward and reverse voltage-time-constants are equal.
 

Alec_t

Joined Sep 17, 2013
13,059
What is the current capability of the power supply?
Where are the decoupling capacitors on the supply rails?
Where are your scope probes connected?
You might need more than 5Ω on the FET gate to damp ringing.
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
1. There is a large capacitance between the gate and drain. You are not the first person to be annoyed by this artifact, but just a fact of life and with a sufficiently low impedance gate drive, should not be a problem (and it looks like you have taken care of that with the MAX4420

2. A 1N4007 has a very long reverse recovery time and probably will not have a chance to conduct until the drain spike is gone. You can try a Schottky or other fast diode (such as UF4004 to UF4007).

Ok will try another diode

2A. I don't see any bypass capacitors in your schematic. The lack of them (one on the input voltage and the other on the output of the regulator - see the manufacturer's datasheet for a recommendation) might be part of the problem if not now, when you get a fast enough diode installed.
Yes, I'm using a free online schematic tool. I do have bypass and decoupling caps throughout the circuit.
 

DickCappels

Joined Aug 21, 2008
9,092
I'm glad to hear you have bypass capacitors. Next: the diode :)

Small detail: for your oscillator circuit to have a 50% duty cycle you need to use the CMOS version of the NE555, LM555C.
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
1. A 1N4007 is way too slow for 1MHz
2. You have a net DC voltage across the coil, so it will have saturated
What is the toroid core made of?
You need a magnetic circuit with a gap in it, and you need to make sure that the forward and reverse voltage-time-constants are equal.
Duly noted. Getting a fast diode

Ferrite core. Ok
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
When running without the toroid, what is the drain load?
Not sure what you mean. without the toroid connected it routes to the connector that goes of the board(disconnected) and a trace to the diode

What is your expected drain current with the toroid?
2 amps
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
What is the current capability of the power supply?
Where are the decoupling capacitors on the supply rails?
Where are your scope probes connected?
You might need more than 5Ω on the FET gate to damp ringing.
Power supply max I is 7 amps
I have them near the voltage regulator which is on the opposite side of the board from the MOSFET
Directly on gate and drain
I'll try a higher value
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
Driving a transformer with a single-ended signal will cause an average DC current in the primary, which can lead to core saturation, as Ian0 noted.
To prevent that you can drive the transformer using a push-pull driver with a series capacitor at the driver output to block any DC component.
That might be my next step if the fast Diode doesn't help
 

Ian0

Joined Aug 7, 2020
5,885
You can fiddle about with the capacitors and gate resistors as much as you like, but the core is saturated and that is the problem.
You can drive the transformer single ended if you allow the drain voltage to swing above Vcc by putting a zener in series with the diode.
 

Thread Starter

cv43coeralsea

Joined Mar 17, 2021
22
You can fiddle about with the capacitors and gate resistors as much as you like, but the core is saturated and that is the problem.
You can drive the transformer single ended if you allow the drain voltage to swing above Vcc by putting a zener in series with the diode.
OK, will try that.
 

crutschow

Joined Mar 14, 2008
30,455
Driving a transformer with a single-ended signal will cause an average DC current in the primary, which can lead to core saturation, as Ian0 noted.
To prevent that you can drive the transformer using a push-pull driver with a series capacitor at the driver output to block any DC component.

(Edit: Replaced deleted post)
 

BobTPH

Joined Jun 5, 2013
5,489
Di/Dt = V/L = 36 / 70 x 1M = 514K A/sec

So, in an on period of 1/2 usec, your current in the toroid can only rise by 250 mA. That would be without a load. A load would increase this.
 

crutschow

Joined Mar 14, 2008
30,455
Here is a LTspice simulation showing the primary current of the transformer (red trace).
Note how the DC level is rapidly rising (until the real transformer would saturate).

1659987640253.png
 
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