Capacitor choice for 200KHz

[akis]

I need a capacitor at 20uF and 200KHz and 4V peak AC voltage. Ideally I should use a ceramic or film capacitor, but do not have one.

So I used a pair of electrolytics at 1000uF each. I know that the electrolytics are not good at 200KHz so hopefully the 2*1000uF will suffice.

However I am thinking of buying non-electrolytics at 20uF, except I am not sure what I need and they do cost a lot, so I'd rather ask before.

At 20uF, it seems that there are all made for high voltages (200V, 300V) and have class qualifiers like "X2"...

All I want is a 20uF as described above, what type/quality should I buy?

[budm]

Re: Capacitor choice for 200KHz

X/Y cap is safety cap for hooking up across the line,or Line to Ground, and it will fail open circuit and also not catching on fire they are CE/VDE/UL approved cap to use in that mode.
I do not understand what you are trying to and the function of this 20uF, and do not see how you can be using a pair of 1000uF in place of 20uF cap.
Any diagram?

[akis]

Re: Capacitor choice for 200KHz

Hello

I want to couple the output of a push-pull power amplifier (sinusoidal 4V peak, 200KHz) into a transformer and want to make sure there is no DC so I used a capacitor. Because the "load" maybe as low or lower than 4R the capacitor should not add to that, and at 200 KHz a 20uF capacitor is roughly 1/10 of 4R [ Zc = 1 / (2 * pi * F * C) ] - hence the 20uF.

As I did not have a ceramic 20uF, while testing on the prototype, I used 2 * 1000 uF cans, and it works OK under load, although I feel I am mis-using the electrolytics for this application, and should be using a non-electrolytic capacitor.

Ceramics and Film capacitors do not generally go up to 20uF except for those X2 funny ones at very high voltages (and very high prices).

Are there any other capacitors I could use ?

Thanks

[budm]

Re: Capacitor choice for 200KHz

At only 4V peak, you can easily use parallel MLCC rated at 25v or more. They use MLCC in half bridge drive system for switch mode power supply all the time. How high is the power supply voltage, can't be that much if you only need 4V peak.
I.E.

[akis]

Re: Capacitor choice for 200KHz

The power supply of the amplifier is +/-6 Volts. I cannot pretend I understand what the MLCC is.

[budm]

Re: Capacitor choice for 200KHz

MLCC is the Multi-LayeR-Ceramic-Capacitor.

[akis]

Re: Capacitor choice for 200KHz

ahh thank you. so Farnell UK does not sell them (the through hole ones) and mouser are so expensive. mouser have them but demand a minimum order of 30 pounds and delievery of 12 pounds unless your order is over 50 pounds... why are capacitors are so expensive...

[budm]

Re: Capacitor choice for 200KHz

They use a lot of them on computer mother board if you can use the surface mount type ( you can carefully solder the wires to the end caps). Do you have cap meter? Since your power supply is Bi-polar, so you are correct to use Ceramic type since they are not polarized and also has lower ESR and inductance.

[999999999]

Re: Capacitor choice for 200KHz

Can you restate what your circuit is? Push pull power amplifier of what signal at 200KHz?

Ceramic can work for digital signals but degrades analog.

You can get film caps at 20uF, not sure of prices there but they are used as motor start capacitors, commonly found at HVAC supply houses for about $20 USD. Naturally they are large, near the size of a 16 oz soft drink can.

[akis]

Re: Capacitor choice for 200KHz

ah that is a clever idea, buy the cheap surface mount and attach two terminals to it. sounds fiddly but possible.

[akis]

Re: Capacitor choice for 200KHz

The circuit is an op-amp oscillator at 200KHz pure sinusoidal at +/-4V. This is then fed to a current amplification stage to drive a transformer. The current amplification stage is push-pull using complementary pairs at the output. The output of this stage, which is +/-4V, is fed to the transformer, whose primary resistance is expected to be as low as 2.5R. I have used a capacitor to isolate the transformer's primary from the output. I started by using 2 * 680nF capacitors, but then I got slightly better output with 2 * 1000 uF and 2 * 470uF all in parallel.

[akis]

Re: Capacitor choice for 200KHz

This is the prototype which I am using, except the capacitor I used is as I said a combination of 2 * 1000uF and 2*470u

[budm]

Re: Capacitor choice for 200KHz

Very simple Complementary pair output with no bias adjustment or temperature compensation circuits to take care of any drift in components and to reduce cross-over distortion or DC offset adjustment to set the center point to be at 0V. Distortion is not any concern? Not sure what the receiving end input signal (generated by this circuits) requirements are. It may do its job just find.

[akis]

Re: Capacitor choice for 200KHz

As you suggested I am looking to buy MLCC 10uF/25V and 20uF/25V X7R from Mouser and try them to see how they look on the scope. They are not that expensive at 25V.

Regarding biasing and overheating, the circuit works well at +/-6V which is the expected rail voltage. On the test circuit the transistors have tiny local heatsinks and they do not even get warm. The final device will have a proper larger heat sink.

From the theory that I know I can increase the 0.15R to something larger, maybe even 1R, to stop any single transistors from getting too hot, as long as I do not see a reduction of the amplitude of the output.

As an experiment, at +/- 7 V, individual transistors do get hot, reach 45-50C, and they stabilise there, while some others are at 30C. This is because of the fixed biasing as you said.

[budm]

Re: Capacitor choice for 200KHz

You should mount the 2 Bias Diode on the heat sink (just have the bodies touch the heat sink and use heat sink compound) to help reduce the thermal run away and at least maintain the bias current when the output transistors get heat up.

[Uranium-235]

Re: Capacitor choice for 200KHz

Very simple Complementary pair output with no bias adjustment or temperature compensation circuits to take care of any drift in components and to reduce cross-over distortion or DC offset adjustment to set the center point to be at 0V. Distortion is not any concern? Not sure what the receiving end input signal (generated by this circuits) requirements are. It may do its job just find.

You should mount the 2 Bias Diode on the heat sink (just have the bodies touch the heat sink and use heat sink compound) to help reduce the thermal run away and at least maintain the bias current when the output transistors get heat up.

I notice you have 'knowledge seeker' selected in your profile. We should probably add 'Guru' or 'Circuit Designer' as selections just for people like you.

[budm]

Re: Capacitor choice for 200KHz

Oh, I am still learning in as many field of electronics as possible, part of my day time job to keep up with the technology.
Actually, I do design works from the what they call from napkin concept to final product down to every nuts and bolts, Sch & PCB layout (Altium Designer CAD program) , mechanical (Autodesk and Solidworks CAD), BOM.
The only thing I will not touch is the software/firmware stuff, we have the guys to do that part.

[Uranium-235]

Re: Capacitor choice for 200KHz

that still puts you ahead in the fundamentals of most of the people here, but than again, fundamentals is what its all about

[akis]

Re: Capacitor choice for 200KHz

Hi guys, been a while. I have gone through many iterations since I last posted here. Rial supply is now increased to +/-12V DC. There are two biasing diodes one of which is bypassed with 1KOhm trimmer so I can adjust the bias. My capacitor is a 10uF/25V surface mount on which I have soldered two wires. It is a tiny little thing but works really well. http://uk.farnell.com/jsp/search/pro...sp?SKU=2070524

[budm]

Re: Capacitor choice for 200KHz

So can we how you use this amplifier with the rest of your setup?