MW band power measurement

Thread Starter

tmarco

Joined May 31, 2018
22
Hi guys,

I have recently built a class AB power amplifier for the LW/MW band and am in the process of testing it. However I am having

some doubts with respect to the power delivered to the load (250W 50 ohm dummy load). My question is, if I measure

the RMS voltage across the (purely resistive) load, and the shape of the wave is a sine wave at 400 kHz, can I

also assume that the true rms power is given by Vrms^2 / Rload, as with lower frequencies ? Or should this be confirmed

with current measurement, taking into account power factor or something else? The waveform is attached below - the red trace is at the gate of one of

the transistors, and the yellow trace is measured across the load. Vk is the RMS

voltage. I don't know if I'm supposed to measure something else so I'm happy to hear if anybody has some advice, thanks.


18-06-08 22_40_14_296.png
 

danadak

Joined Mar 10, 2018
3,578
My question is, if I measure

the RMS voltage across the (purely resistive) load, and the shape of the wave is a sine wave at 400 kHz, can I

also assume that the true rms power is given by Vrms^2 / Rload, as with lower frequencies ?
Yes, PF does not apply as you specified load as 50 ohms resistive.

http://www.rfcafe.com/references/electrical/pwr2volts.htm

Note your load waveform not exactly sinusoidal, so the standard equations
become only approximate.

http://www.ni.com/newsletter/51446/en/


Regards, Dana.
 

Thread Starter

tmarco

Joined May 31, 2018
22
Hi Dana,

thanks for confirming that. I am actually asking this because the power indicated by measuring the Vrms voltage

doesn't seem to correspond with the current my PSU should be putting out at that power. The amplifier is a class AB amp, it is supposed to be

putting out 250W at the 137kHz band (with a PSU of 28V 10A), 3dB bandwidth approximately 60-900 kHz (schematic shown below). The driver

stage is a small TDA2030 module that should be putting out 5W for max drive, which I tested before implementing it in the amplifier. I'm using

a Basetech variable SMPS (30V 5A 150W) and a separate 12V linear PSU for setting the bias on the mosfets.


In the attached scope grab the RMS voltage is 58V, which corresponds to ~67 Watts. The PSU is set to 20V, so by a crude calculation one

would expect the PSU to be putting out about 3.5A or so. However, the SMPS indicates only ~0.5A output current. Furthermore, when

I lower the bias of both stages so that they are in cutoff, the voltage on the output goes to ~0, but the current output indicated by

the SMPS is still the same (~0.5A, maybe a 20-30 mA less). This looks like the current which is just consumed by the TDA module

to produce the gate drive for the push-pull stages. So then how is it possible that I measure this Vrms across the load, if there is no

current to sustain it? Aside from that, I have been having trouble at lower frequencies (100, 200, 300 kHz)

with large current surges once the MOSFETs hit conduction threshold, accompanied by a clicking/screeching

sound coming from the MOSFETs. Could this have to do with the common-mode choke (T2) saturating? I have

also attached some images of the amp itself. The two co-axial toroid inductors are part of a LPF - the output can be toggled to go directly

from T3 to the output BNC or through the LPF. At this moment the LPF is disconnected physically from the output because I thought it might

somehow be affecting my measurements. I realize this might be many questions at once, but I'm not sure whether I should create a new thread

for this. Thanks for helping out.


lwamp.png amp1.png amp2.png
 
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danadak

Joined Mar 10, 2018
3,578
Something is wacky. How are you measuring PSU current ? And where, input
or output. Keep in mind PSU efficiency changes significangly with load. Typically.
And whats its PF on primary side ?

Also when you zero bias output you see no V at load, right ? Its in cutoff.

Regards, Dana.
 

danadak

Joined Mar 10, 2018
3,578
Aside from that, I have been having trouble at lower frequencies (100, 200, 300 kHz)

with large current surges once the MOSFETs hit conduction threshold, accompanied by a clicking/screeching

sound coming from the MOSFETs. Could this have to do with the common-mode choke (T2) saturating?
For sure if L is saturated the demands on MOSFETs quite high. Clicking sounds sound like classic microphonics,
large currents generating large fields moving wires ? Just guessing.

Regards, Dana.
 

Thread Starter

tmarco

Joined May 31, 2018
22
I read the PSU current from whats displayed on the SMPS display. So that is what is flowing out of the SMPS

into T2 and then to the drains of the MOSFETs. Should I be using a different PSU, like a linear one? I have the

schematic from this website : g0mrf.com/lf.htm , and there also a SMPS is used (it is stated in the last paragraph

in the practical use section). When bias is off I measure 400 mVpp ripple at 400kHz, current indicated on SMPS

is 300 mA at Vcc of 20 V. So not completely zero. I have two BNC plugs - input for the signal generator and output to the load. Both are

grounded to the enclosure. All other grounded components (T1, T2, T3, capacitors, MOSFET sources, supply capacitors) are

grounded at the PSU input plugs, so I tried to go for sort of a "star" arrangement, separate grounds directly to the negative/ground

plugs. The returns of the SMPS and linear PSU for the bias setting are connected at the input plugs and both grounded. The ground plug is connected to the ground of the Basetech SMPS. I have measured

the current flowing from the linear PSU into the amplifier and in total its 150 mA (so both stages, measured at 12V input plug). I have also

measured the DC ripple both in the SMPS and linear PSU, for the linear PSU it's 15-20 mV, for the SMPS a bit less, I couldn't make out exactly

how much. I'm not sure what primary PF you refer to, do you mean the primary of the output transformer T3?
 
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Thread Starter

tmarco

Joined May 31, 2018
22
The thing is, I haven't been able to find the exact core specified in the schematic for T2. I had two spare ferrite toroids

that I assumed to be adequately sized for doing what they need to. But I don't have any permeability values

so I can't calculate the saturation current or anything like that - I have tried looking up info but the cores are ebay generics

from china, no luck so far. Although last night I've found a similar 3C85 core on ebay that I've ordered (2 pieces). These have a ui of ~2000. I've tried pinpointing the exact source of the clicking/screeching noise and it seems to come from

the corner where the MOSFETs are, so it could be that. Also if I let it struggle for a bit longer (10-15 seconds) the heatsink

gets a bit hot. This is what I concluded so far, but I'm wary of testing it any more under those conditions for fear of blowing

something up.
 

danadak

Joined Mar 10, 2018
3,578

Thread Starter

tmarco

Joined May 31, 2018
22
I don't know if the SMPS is going into discontinuous mode, but the clicking/screeching comes from within the amplifier.

I don't have any high voltage probes and proper equipment to measure mains PF into SMPS, and things like 3-voltmeter

method only work with linear loads so I guess that's gonna be a bit difficult to carry out.
 

danadak

Joined Mar 10, 2018
3,578
Look at your probes rating, and this -

Yes, you can do this on almost any real multichannel oscilloscope. While the Rigols are inexpensive, they're definitely real.

A few things to keep in mind:
The CAT rating of your probe. Do not exceed this.
I pull the ground leads off when I'm doing differential measurements to prevent accidents.
Know roughly your voltage to ground so you do not exceed the maximum input voltage on your 'scope. For example you cannot measure something that's at 400v to ground with a 1x probe.
Avoid switchable probes (1x / 10x switchable). Accidents happen. They wouldn't be called accidents if they didn't happen.
If you want to measure a small signal on top of a very large voltage, use AC input coupling on both probes. That way you're not wasting your ADC's resolution & input amplifier's range measuring the DC offset.
Think three times about your measurement before you make it. If at all possible make the measurement hands free, or at least single handed.
https://www.eevblog.com/forum/testgear/oscilloscope-operation-differential-voltage-measurement/

Regards, Dana.
 

Thread Starter

tmarco

Joined May 31, 2018
22
Hi Dana,

just to confirm - you suggest to measure the PF on the input (mains) side of the SMPS while it is operating

in a state similar to what was shown on my initial scope image? So while I'm measuring the 50 V "Vrms"

on the output? I have a dual channel oscilloscope - if I use them to make a differential voltage measurement

across the SMPS primary I have nothing left to observe the current with. Is there a way around this that I'm

not aware of? I'm also not sure what good this will exactly do - the SMPS is obviously not putting out the current to provide me with the Vrms that I'm measuring. I assume something is wrong within the circuit itself - could it have to do with the grounding? Maybe I should isolate the input/output BNC plugs from the enclosure?
 
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danadak

Joined Mar 10, 2018
3,578
The purpose of measuring primary side PSU was simply to correlate the power
numbers you are seeing, and account for efficiency losses.

Regards, Dana.
 

Thread Starter

tmarco

Joined May 31, 2018
22
Thanks for helping out guys. I felt like I was getting a bit lost so I decided to start from the beginning with some input measurements
(as BR-549 suggested), and have concluded that there is something fundamentally wrong with my setup.

When the TDA module is disconnected, no input signal is applied, and transistors are in cutoff (biased a bit below cutoff, at 3.6-3.8V),
I measure no noise or spurious signals at the gate of the mosfets (I'm always measuring the red trace at the gate of MOSFET 1,
taken from the top of the schematic). But the same applies to the other MOSFETs, I have compared it several times. The drain voltage is also 20 V (as set on PSU), no ripple or anything like that. SMPS shows 10mA
current consumption (excluding bias current). When I connect the TDA module and everything else is the same, current consumption
increases to about 60 mA, but still everything is ok.

Now I disconnect the TDA module again, and I bias both MOSFET stages into conduction (it goes gradually first, from 60 mA at 3.6V bias
until it jumps to 600-700 mA around 3.9-4V. This is where I encounter the first problem (img 1, left to right). For some reason, at this point a 37-38 MHz signal appears at the drain (yellow trace, about 8Vpp on centered on 20V offset from PSU) and also this same signal appears at the gate, 9Vpp centered at the threshold bias voltage (~4V). There is no hissing/clicking sound and both the waveform and current consumption are quite stable. However, the 37MHz signal is also present at the input BNC plug (yellow trace, img 2 - the red probe was disconnected here). This is weird since the BNC plug is disconnected both from the signal generator and the TDA, it's only connection is the plug outer shield to ground.

When I now connect the TDA and use same settings, the waveform becomes very unstable (img 3), almost like it is modulated with something else, and the current consumption goes to max (in this case I limited it at 1.5A), and there is the hissing/squealing sound again - i still can't make out whether it's coming from the mosfets, or maybe the capacitors. I don't think it's the coils since I tried moving/steadying them while the sound was on and there was no change at all.

At one point, I captured waveform 4 (img 4) - the red trace is again at the gate of mosfet 1. I think here the timebase was set differently, and so 5 MHz square pulses could be identified - is this some kind of coupling from the SMPS?

This is what I have so far. There is the 37MHz coupling from somewhere, then there is the TDA effect which seems to worsen
things when it's powered up, and the 5Mhz square pulses that to me suggest maybe SMPS signal leakage from the switches. I really hope somebody with more experience can suggest further steps to take. Thanks

dcbiasonly_yellow_draintop_TDAdisconnected.png dcbiasonly_yellow_bncinput_TDAdisconnected.jpg dcbiasonly-TDApowerconnected.jpg dcbiasonly_yellow_bncoutput.jpg
 
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