Current problems with an H-Bridge IC

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Hello,
I bought an H-Bridge IC called "ZXMHC 6A07", here is the datasheet.
I connected the source (S1,S4) to +3.3V to an external lab power supply. The other pair of source pins (S2,S3) are connected to gound. The gates on the "left hand side" (G1,G2) are connected to a roughly 25 kHz 3.3Vpp (1.65V offset) square wave. And G3,G4 are connected to the same, but inverted signal (25 kHz 3.3Vpp 1.65V offset, square, 180 degrees phase difference). Also both gate sides are pulled down with a 10k resistor to ground.

Right, now I have a 6.6Vpp signal. Everything as I expect it so far.

Now I connect a roughly 20 ohm resistor across the drains (D1,D2 and D3,D4) and read the differential voltage across that resistor. I connect both ground clips of my probe together and measure at both drains. I get a slight attenuation because of the internal resistance of the H-Bridge, but the signal stays at around 6Vpp.

Now I want to connect the same drains to an RLC series resonant circuit. I use an inductor with about 1.46 mH, and a capacitor with a value of 25.5nF. That would make a resonance frequency of about 26kHz. So I set my signal generator to 26kHz and connect everything in series. I expect the LC part to behave as a almost short circuit, and the 6Vpp across my 20 ohm resistor. But to my surprise I measured it to be 3.2Vpp.

After that I played a bit with the frequency, but it didn't get better. I could only get 3.2Vpp across the resistor. I double and tripple checked the inductor and capacitor values, but couldn't find the problem. What's going on here ?

Edit: I attached an image to clarify the question. In that image, R1 is the load I try to replace with a series RLC circuit.
 

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ronsimpson

Joined Oct 7, 2019
1,159
(G1,G2) are connected to a roughly 25 kHz 3.3Vpp (1.65V offset) square wave
Not certain what "1.65V offset" is but I think the signal goes from -1.65V to +1.65V.
For G2,G3 you need at least 3.3V G-S to turn on the part. 1.65V will only turn the part on to about 0.01A. A G to S resistor is fine for F2,G3.
1614778736710.png
Now lets talk G1,G4. A resistor from Gate to ground will turn the part on! Do not do this. The resistor must be G-S or Gate to +3.3 supply. The gate signal should be at supply (3.3V) to turn off the part and at ground to turn on the part. S1 an d S4 ar "P" devices and work backwards from S2,S3.

In your FPGA, do not turn both the top and bottom switches at the same time. Have a 50 to 100nS dead time where no switch is on. (use one clock time) Or delay turn on edge by one clock. No delay for turn off.

You need at least 3.3V from G to S to get the IC turned on for high current. Two graphs for cold and hot. With 3V you can only get 2A/1A.
1614779295132.png
 

Audioguru again

Joined Oct 21, 2019
2,875
The spec's for the H-bridge IC say the Vds should be 10V for it to work well or be 4.5V for it to make lots of heat.
Why do you think it will work well when your Vds is only 3.3V?

The threshold voltage is a max of 3V where some of these circuits conduct only 0.25mA.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Sorry for the confusion, I do drive the gates with 3.3V. So the signal goes from 0V to +3.3V.

I just tried setting the Vds voltage higher. That did the trick.
I guess I don't understand all the parameters of MOSFET's. Will do some homework then.

Thanks
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
@Audioguru again could you point out where in the datasheet it states that Vds should be 10V ? Because the way it looks, I should almost always get at least 1A. Looking at the attached image.

Unless I'm reading it wrong (which could very well be).
 

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Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Yes, but if he meant Vgs, I shouldn't have the problem in the first place. Because 3.3V should be enough on the gates.
 

crutschow

Joined Mar 14, 2008
26,775
Yes, but if he meant Vgs, I shouldn't have the problem in the first place. Because 3.3V should be enough on the gates.
Not if you have devices that have a Vgs(thres) near 3.3V.
Those safe-area graphs you posted show nothing about that

What are the Vgs(thres) values for the devices you are using?
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Yes, but when you look at the graph posted by @ronsimpson, it says that even at a gate voltage of 3V and a Vds of 3V I should get about 900mA.
 
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Audioguru again

Joined Oct 21, 2019
2,875
The graphs on datasheets say "TYPICAL" but some are better and many are worse. You cannot buy a typical one, you get what they have.
The datasheet says with a threshold Vgs of 3V then some devices will conduct only 0.25mA.
It also shows that Vgs should 10V for it to have a very low Rdson. It shows a worse Rdson for a Vgs of 4.5V.
 

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Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Hold on, now that I'm thinking about it more. The original problem I stated was more or less "the circuit works fine with a resistor, but when I use an RLC circuit it stops doing it's thing". Wouldn't that mean, that whenever I attach a purely resistive load there is no problem with the current. So no problem at all.

But when I put an inductor in series with a capacitor, it breaks. So wouldn't that suggest that there is something wrong with inductive/capacitive loads ?

Or am I thinking wrong here ?
 

Audioguru again

Joined Oct 21, 2019
2,875
The problem with the LC tuned circuit is that it conducts only at the sinewave of the fundamental frequency. The waveform of the 555 and H-bridge have many harmonics and their current is not passed much by the LC tuned circuit.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Yes, but I don't see how that would explain the problem. I mean the peak voltage should stay the same between the square wave and the sine wave, right ? Also, I think that's how inductive chargers work.
 

Audioguru again

Joined Oct 21, 2019
2,875
Each peak voltage of a squarewave is present for 50% of the time but the peak voltage of a sinewave (the 25kHz fundamental frequency) is present only for a moment. You probably get the 0.707 times (RMS) voltage. Plus the inductor will have some resistance.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
197
Yes the inductor has about 3.6 ohms at DC. But the current draw from my power supply is MUCH less. When I attach the resistor alone it draws about 140mA (22ohm @ 3.3V). But when I attach the RLC circuit it only draws about 40-50mA.
 
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