Half-bridge woes.

[HellTriX]

I setup a IRF3205 half-bridge driver driven by my single 555 PWM to the Lin and Hin pins. The output for the low works as intended. For the high output I calculated the boot strap cap close to where it should be even tried a few values up and down. I used a 4001 Diode and tried some 4183s (I think) that where in my junk box, (testing circuit with 5khz signal). The output waveform didn't even resemble a square wave. At this point I removed the mosfet from the driver and hooked ran the high output through a diode and resistor to charge a capacitor up and measure its voltage. I'm getting a 6.58v output. Ironically this is the exact same voltage that is going into the HIN and LIN pins from my 555. The boost strap effect is not working and I'm not sure why. The driver and 555 is being driven by a 12V regulated supply and I verified 12V to the inputs of each chip.

I'm probably going to be asked for a schematic? The 555 is trivial and proven with my other PWM circuit, and the half bridge driver was hooked up with the addition of a diode and cap for the boost strap and thats it.

I couldn't find a half bridge driver in Eagle or electronics workbench, so maybe you can give me suggestions on how to create a schematic if you need one?

tnx

 

[Ubergeek63]

you are missing two things that I can see from the insufficient ( yes a schematic would help ) information supplied:

A high speed diode - normal power rectifiers are pathetic when it comes to switchers.

And dead band, you have to make sure they are not on at the same time - so you need more than a single 555

 

[HellTriX]

I thought this driver handled the dead time?
Its the non-programmable one that has a defined dead-time in the
schematic?
I was looking at the table for the LIN HIN and it appeared as though
both could be driven from the same input.

I guess I could maybe something up for the input to invert the signal
and maybe even address the dead time.

 

[dknguyen]

You have not told us what gate driver IC you are using (you make it sound like you are using the 555 for a control pulse for the gate driver rather than using the 555 to drive the gates directly, right?), so we cannot determine whether or not it has deadband.

 

[picbits]

As above - start with high speed diodes.

My FET driver wouldn't work at all when I used low speed diodes (even though they were classed as switch mode rectifier diodes).

I use between 100nF and 220nF capacitors on the bridge drivers I build with these custom FET driver chips.

 

[HellTriX]

Thanks, I will probably have to go pick some up, If the case you say about your switch mode diodes not even working I doubt the ones I have would work either.

Will look around some some high speed ones.

I may test my 60Amp ultra fast diodes as a fun little test, but they probably wont work, they are far too big.

 

[HellTriX]

I tried my 60Amp ultra fast diode, and managed to get 11.44volts to my charing circuit. So it does seem more related to the diode.

Anyone have some handy part numbers for some decent fast switching diodes? I have some more projects that I plan to build that will have switching times between 40khz and 150khz or so, maybe even 400khz if I want to dink around. All this would have < 150ns switching rise/falls. Aiming for below 80ns.

 

[picbits]

Where abouts are you based ? If you're in the UK, I've got a load of 25nS 4A 200v ones for 20p each

**broken link removed**

 

[HellTriX]

USA located. I found some 4148s in my junk box, they say they are 4ns ? hope thats right. They wont flow enough current at full design spec of 1.8A peak, haven't really calculated average current yet.

But I did make a schematic. I think



Here is a schematic.
The bottom one is a duplicate of the top except I tested without mosfets to try to ascertain the max voltage being generated by the bootstrap effect. This was measured at 11.6v of my ~12 supply.
D1 = 4148
C1 = Tried values from 10nf to 47mf. (nominally 170nf).
Qx = Are not IRF540s (Only their for illustration).

The problem is, I don't think the boosted voltage is working, Or I should be seeing over 20 volts at the HO when referenced to ground?
Any clarification would be helpful. ty.
**broken link removed**

 

[picbits]

The peak is usually supplied by the capacitor, I've used 4148's before on some of the high side drivers I've built where they have specified that 1N4148 are acceptable.

I can't for the life of me remember which ICs I used though that used the 4148

 

[HellTriX]

I've been informed that I'm missing the connection from VS to the middle of the two mosfets between high side source, and low side drain for it to take the reference from.

Will correct this problem after I get back from the university.

 

[audioguru]

The correct circuit is on the first page of the datasheet for the Mosfet driver IC.

 

[HellTriX]

Got the circuit operating.
12V supply getting 18-20 volts at the high side gate (PS might be sagging below 12v). Showing that my bootstrap is working.

I put a 1ohm resistor between high and low mosfet with a center tap going to the Vs to simulate a motor and protect against shoot through for the initial test.

Which brings me to the following question:
Whats the best way to measure if I may have shoot through? I only got a single channel oscilloscope so I can measure high/low side and compare them.
I don't have any more bright ideas on what to do to check this.

 

[HellTriX]

Master of answering my own questions.
At no load I measured a current of 18 microamps of current between the two mosfets, I guess this rules out shoot through. I know the chance will go up as load goes up, but its at least currently safe to being load testing.