# Amplifying wave generator signals

Discussion in 'Analog & Mixed-Signal Design' started by kevinnas, Sep 18, 2018.

1. ### innocent.mushonga New Member

Aug 14, 2018
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I will be using 230V mains, the transformers you mentioned only give out 130V and 120V will that be enough?

2. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
Hello
I have been simulating the circuit finally but I keep frying the Mosfets. I placed a small heatsink and I am using the following mosfet: INFINEON BSC22DN20NS3GATMA1 MOSFET Transistor, N Channel, 7 A, 200 V, 0.194 ohm, 10 V, 3 V
Even when I test tge circuit using a lower input voltage.
Do u have any alternative mosfets I can use maybe something that might be better at handling the high current and voltage?

3. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
Problems arise when the task is not correct. The solution is suitable for one case is not suitable for another. I picked up the optimal transistors for the task that you set. Properly design the circuit for the correct load equivalent! Also, my circuit requires the correct establishment of the quiescent current, the power that is released on the transistors depends on it. Do you have an idea about the energy efficiency of an amplifier in the AB class? Efficiency and power dissipated in transistors depends on the amplitude of the signal. If you want to output 30 watts, then get ready to use not small heat sinks.
LTspice allows you to calculate the average power of each element and transistors in particular.

4. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
The circuit works momentarily then just dies. I was only testing it at 300mV p-p input which according to LT spice should be only 6W decipated by the transistors and that is below the 32W rating of the transistors. How can I correctly establish the quiescent current? What can I test out because at the moment I am just stuck in one position. I know the circuit before the transformer works somewhat as expected.
The reason I thinking maybe using different Mosfets like throughole that are bigger and maybe can handle more power would allows me to use a bigger heatsink compared to the small smd transistors tha I am using at the moment.
If as you mentioned that u designed the circuit for those transistors, what can I do or test out to make sure the transistors do not keep burning?

5. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
Check your load and how you connect it. I will try to find a suitable transistor in a different package (not SMD). Transistor generation is also possible. I previously described how to set the operating point. By the way, on transistors (those that I used) with an output amplitude of 55V, the power on one transistor is 15W. At 300mV input power is also about 15W. To display the power graph, use the Alt key. Clicking on it you mark your mouse on the transistor. What transistors are used in? BSZ22DN20NS3? They need fairly large radiators, heat sinks

6. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
Another issue is that the potentiometer is set to 0.56 but I am using smd pots which are not very precise but the pot if I set even just to 0.57 or 0.55 it massively changes the output and the powrr dessipation...which is a little bit worrying and durrentlY. At the moment I am just connecting the output frm the output capacitor (C2) on your diagram directly to the oscilloscope through a 50ohm BNC cable. Is that acceptable? I haven’t placed any other load at the output

Last edited: Dec 14, 2018
7. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
Potentiometer is desirable to use multiturn a small movement of the sliding contact greatly changes the current of the transistor. The desired division ratio of the potentiometer is determined by the threshold voltage of the transistor, and the threshold has a scatter.
Resting Current ~ 150mA -> P = 75V * 0.15A = 11.25W. To set the quiescent current, bypass the lower transistor with an ammeter. First set the potentiometer to the bottom position! This should be done quickly or intermittently, because P = 150V * 0.15A = 22.5W. As I said, you must have a 15W heat sink. Then, instead of the ammeter, turn on the voltmeter and use the lower potentiometer to install half the power.
The search for replacing the transistor and its testing through simulation has so far failed.

8. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
I had an idea - to put a transistor with a TO-220 case (cascode circuit) in series with each field-effect transistor. The main power will be on these additional transistors. But this will take me time. I have a question for you - have you considered that parallel capacitors are used in the circuit?
These are capacitors with a "x" multiplier.

9. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
Oh I wasn’t sure about that..I had made the circuit with single caps. Is there any way I can get away with that? Or to use bigger single capacitors instead?

10. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
Parallel connection of capacitors is a way to reduce parasitic inductance. In this case, it is very important. I did this to reduce the chances of parasitic oscillations. If there is no oscillation, then everything is in order.

11. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
Are you not afraid to burn the input amplifier of the oscilloscope?

12. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
I wouldn’t want to do that. What type of connection should I have to be able to measure safely using a scope?

13. ### Bordodynov Well-Known Member

May 20, 2015
2,176
668
You need an attenuator of three resistors: 51 Ohm, 5.1kOhm and 51 Ohm. The transfer ratio will be 100: 1, i.e. there will be a attenuation of about 100 times. The first resistor should be powerful. It can be made from 10 parallel, two watts resistors.

14. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
Thanks. Just so I can be clear about what I am doing are the attached setup images okay? The image (A) and (B) are for setting up Quiescent current. So I will connect the ameter between the output from the transformer to the emitter of the top transistor and adjust the top potentiometer until I get 150mA? Or am I connecting the ammeter between top transistor emitter to ground?
I will have to do this quickly not to damage the ameter.
After I have done this I will connect the voltmeter as shown in image (B) and adjust the bottom potentiometer until I get half the power (75V*0.15=11.25W)

And for the scope image C I connect 3 resistors to the output then to the scope.

Please advise if I have misinterpreted your suggestions. I just want to make sure that I am doing things the correct way, please excuse my lack of knowledge

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Mar 10, 2018
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16. ### Bordodynov Well-Known Member

May 20, 2015
2,176
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17. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
Hi.
I have managed to resume working on this.
• The problem I was having with blowing the chip I somehow had a feeling that maybe it was not because I was not sufficiently heat sinking it, but instead the circuit you drew has a Vgs that is too high?In your circuit you are biasing the Mosfet at Aprox 3.7V which I decided to test out.
• In the Mosfet data sheet the Gate threshold voltage (Vgs) is stated as - Min 2V and Typical 3V and Max 4V.
• The Id vs. Vgs curve Page 5 on the attached datasheet it is seen that at 25C current starts to increase around 3.7Vgs (I am assuming this is why you chose this as the bias point?).
I decided to test this out by just connecting the Mosfet - Drain to 70Vdc, Source to gnd and applying a Voltage at the gate (Vgs) by slowly increasing the Voltage from Zero.
• I could barely measure any or much current flow until around Vgs = 3V and then increasing this to 3.4V I could measure around 35mA
• Beyond this point e.g Vgs = 3.45V the current started to slowly increase even when I am not changing the Vgs and when I set to Vgs = 3.5V the Mosfet just blew up.
• Did I do the correct thing and does this prove that maybe a Vgs of 3.8V in my circuit is just going to blow the Mosfet everytime?
• There are two curves on the datasheet ID vs. Vgs curves and the other one is for 150V..maybe I do not have sufficient heat sinking and my circuit is operating on this curve or it is just one of those things where the exact biasing point varies a lot with each chip so I have to find the one that works for me?
Whichever way, I cannot get the quiescent point to be 150mA because the moment I go beyond a Vgs of 3.4V (which is around 35mA the current becomes unstable and just keeps creeping up until the mosfet blows; Do u think for my circuit I should just adjust the pots for 3.4Vgs bias point and 34mA quiescent current?

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18. ### Bordodynov Well-Known Member

May 20, 2015
2,176
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You must use a heat sink. Due to the heating (35mA * 70V = 2.45W) the threshold falls. And the maximum power without heat sink ~ 2 watts.

19. ### kevinnas Thread Starter Member

Jul 31, 2017
73
3
So are you saying that I should still try and use the Vgs = 3.7V & 150mA quiescent current? just that I have to find a heat sink good enough to make that work?
Or I can use the Vgs = 3.4V & 34mA quiescent current. In both cases yes I am aware I need a heatsink.
I have attached the heatsink I am considering using it has a thermal conductivity of 10.21C/W

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20. ### pmd34 Active Member

Feb 22, 2014
353
149
What about using a high power off the shelf amplifier and then boosting up the voltage with a transformer?