Royer oscillator problems when using FETs

Thread Starter

l0vot

Joined Apr 29, 2013
107
All right, I'm building a Royer oscillator, basic push/pull configuration, if I use BJTs as the drive transistors there are no problems, aside from BJTs being massively inefficent when used in 12V power electronics, so they tend to cook themselves when I try to add a load. 2 BJTs in parellel wasn't quite enough, so I would need to use at least 6 20A BJTs total to build the oscillator, and add more heat sink, obviously this is getting ridiculous, so I thought, why not use FETs instead?

Anyhow, so I found these FETs:
https://www.digikey.com/en/products/detail/vishay-siliconix/SUP70101EL-GE3/7622840

They refuse to run properly on their own when used in the same configuration as the BJTs (the BJTs always require a control winding that is a few turns larger than the primary, exactly how many extra can be kinda random), or in the configuration in this schematic:


they both turn ON at the same time whenever they are driven directly by the feedback winding when a 500 ohm bootstrap resistor is added between the gate and ground (PNP instead of NPN), the BJTs were perfectly fine with this configuration, 1Kohm has the same effect, basically they will not bootstrap properly, adding more winds to the feedback winding does not help. I stuck a BJT to the gate as an amp, the feedback winding can't shut off the gate because the BJT blocks it, so I added a 250ohm pull up resistor to the gate to shut the FETs off when not being driven by the BJTs, this finally got the FETs to behave themselves, sort of, they started heating up rapidly, even with no load attached, and the voltage is a lot lower than it should be, it should be ~24V across the primary, but it's only +-7V, less than half of what's expected, the frequency is a lot lower as well ~1.3KHz instead of the ~3.9KHz the BJTs were running at, like they are being driven in their linear region (which doesn't make any sense, the BJT is switching the gate directly to ground), or aren't shutting off all the way. The waves look square, if the gate wasn't being shut off fast enough by the pull up resistor I would expect to see it on the scope, same for a slow turn on. Reducing the input voltage to 8V only slightly reduces the voltage, and frequency, dropping to 3.7 is very odd, the frequency goes up instead of down, 2.6KHz, peak voltage is ~4V, waveform has the same spikes the circuit has when using BJTs only.

Obviously I'm missing something here.
 

Irving

Joined Jan 30, 2016
1,572
You're probably missing the fact that mosfets are voltage driven and bjt are current driven. Without schematics of all your experimentation it's hard to say much more.
 

Sensacell

Joined Jun 19, 2012
2,838
FET's are voltage driven, while BJT's are current driven.

Under most circumstances, they are not interchangeable without a redesign.
The "simplicity" of this circuit belies the true complexity of what is really going on, these circuits are very hard to design and optimize.

I find that people are very attracted to "simple" looking circuits, in reality, these are often far harder to get working than a more complex design using a more predictable architecture.
You would be better off with a "simpler" circuit with more parts.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Wait a minute, the pull up resistors are forming a 500ohm bridge between the two gates, not sure what that's doing exactly, but it may be relevant.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
oscillator.jpg
All right, maybe you can make out what's going on now, if you can get past my terrible penmanship and loose grasp on electrical symbols.

Somewhere, something in this mess is trying to clamp the voltage at 7V.
 

Irving

Joined Jan 30, 2016
1,572
Errrr... I'm speechless... where to start...

Your diodes are drawn the wrong way round (I'm assuming thats not how they are wired), and guessing at the other components I'd sy the chances of this working is about as close to zero as makes no difference. The biassing on the BJT is all wrong for a start... and P-channel FETs????
 

sparky 1

Joined Nov 3, 2018
544
Flourescent lights can be driven with royer type circuits, has an interesting history some having good efficiency.
The brightness of the CFL impressive over when current I > 450mA shown p10 below
https://iopscience.iop.org/article/10.1088/1757-899X/106/1/012013/pdf
You can use the term voltage-controlled or current-controlled Mosfet or BJT. I wanted to touch on that briefly because it can be confused.
In the case of Royer as a power converter it can use term current-driven because magnetic coupling into the active component. (also hall sense application)

In general the base of a BJT is less dependent on voltage in the active region where more gain is mostly more current into base.
This can be seen on VI trace chart example below shows a narrower voltage range with an increase in current.
and with a mosfet gate the opposite where effect is mostly a change in voltage.
reference: https://toshiba.semicon-storage.com...transistors-driven-by-current-or-voltage.html

The term mosfet driver is well accepted today. Since 1954 Royer oscillator variations and patents grew in number
so that today we find integrated circuits developed from Royer some are relatively simple to impliment.
The camping lantern circuits with cfl now mostly leds but what applications were lost in it's obselescene?
 
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Irving

Joined Jan 30, 2016
1,572
OK, in the interests of passing on knowledge, I think the circuit below is what was probably intended...

edit: just realised its missing the 1000uH inductor from +12v to the MOSFET sources. I'll add it as soon as I can get back on PC.

1622453266045.png
 
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DickCappels

Joined Aug 21, 2008
7,449
And there is your 7 volt clamp -just guessing - the reverse Veb breakdown voltage is about six volts on many bipolar transistors. Add that to the forward Vbe of the other transistor and you have a 7V peak clamp (or 14V P-P).
 

ronsimpson

Joined Oct 7, 2019
1,242
BJTs being massively inefficent when used in 12V power electronics
So why do you feel BJT are massively inefficient? Using the circuit in post #1 I can see why you think so. In the circuit there is a 5 watt resistor that makes the Base current. (Base current from +12V) In high power circuits the Base current comes directly form a low voltage winding on the transformer. (Base current from +1.2V supply)
Anyhow, so I found these FETs:
This is a "P-FET" and you want a "N". Also at 100A there will likely be 1V from D-S which a good BJT will have less of this type of loss. 60V 200A N-fet
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
inefficient
The BJTs drop at least 1.5V across them even when lightly loaded, this can rapidly climb to >2V. My rule of thumb is electronic componets can only realistically do half their rated continuous current, so whenever I shop for componets I try to get something that can do at least double what's being asked of it, the FETs I got should have a 6mOhm on-state resistance, the transformer is only built for maybe 40A, and that's pushing it, so the two FETs should be dissipating 9.6W with a 50% duty cycle when the transformer is under heavy load, in contrast the BJTs are going to need to get rid of ~60-80W.

The BJTs:
https://octopart.com/mjh6287-on+semiconductor-92916

Irving: that's pretty close, except all the transistors are the type that end up grounded to their heatsink, and sink current to the heatsink, so they all must have a positive load, or independent heat sinks, these are all mounted on the same heat sink, and R3 must connect to ground in order to bias the BJTs ON, otherwise there's no way for the circuit to bootstrap.

I think I fixed the schematic so it more accurately represents what's going on:

oscillator-modded-mk2.png

q1 and q2 probably need to be flipped, but you get the point
 

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Irving

Joined Jan 30, 2016
1,572
The 500ohm resistor to +12v is the bias to q1 & q2 and it's unbalanced to get the oscillation started. The way youve drawn it to ground provides no bias current so not sure how thats going to work...
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
The 500ohm resistor to +12v is the bias to q1 & q2 and it's unbalanced to get the oscillation started. The way youve drawn it to ground provides no bias current so not sure how thats going to work...
q1 and q2 turn ON when switched to the negative rail, they are probably backwards in the schematic, all the transistors are N channel, all of the transistors switch ON when the gate/base is pulled low relative to the load, i'll flip them, see if that helps:
oscillator-modded-mk2.png
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
And there is your 7 volt clamp -just guessing - the reverse Veb breakdown voltage is about six volts on many bipolar transistors. Add that to the forward Vbe of the other transistor and you have a 7V peak clamp (or 14V P-P).
any way to check if that's the problem/fix it?
 
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