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Please be gentle, as I am a bit of a noob when it comes to these things.

I am attempting to use an IRS2186 to control 2x IRF3205s in a half bridge configuration (datasheets below). I am using a known good atmega168 to deliver 50% PWM at 32 kHz to the high side of the previous components. It is currently not turning on the high side.

On this application note, at the bottom of page 5, it says that in a standard PWM configuration, one need only use a bootstrap diode, and capacitor to make this setup work, so that is all I am using currently, everything else is in the configuration in the "typical configuration" section of the datasheet for the driver. My second question is is this true? The previously mentioned sheet is for a different driver IC, but they are very similar.

I have calculated using this page. That I need around a 500 nF cap as a bootstrap capacitor. My first question is am I correct here? I am unsure if this is the right calculation to use. I am also using a MBR160GS diode as a bootstrap diode, will this work?

My last question is does anyone have any idea what I'm doing wrong here? Any guess at all would help.

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  • \$\begingroup\$ Have you checked to see if the problem with the high side FET not turning on is due to the upper side UV (under voltage) detect in the driver kicking in and cutting off the high side output? \$\endgroup\$ – Michael Karas Jan 5 '13 at 2:35
  • \$\begingroup\$ How would I check this? \$\endgroup\$ – Patrick Jan 5 '13 at 2:43
  • \$\begingroup\$ I've read in the specsheet for the driver that the low side and logic supply should be between 10 and 20 volts, which seems high to me. It also doesnt work when I hook it up that way. \$\endgroup\$ – Patrick Jan 5 '13 at 2:53
  • \$\begingroup\$ I should also mention that I have one 5v supply available (for logic, etc) and one 12-15v supply available (that I am trying to switch with the mosfets), I am not sure if this will even work with my available supplies. \$\endgroup\$ – Patrick Jan 5 '13 at 2:54
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With the IRS2186 PWM control inputs have to be provided for both the high side and the low side. This means you have to worry about things like proper phase control and dead time relations for the top and bottom FETs. If not done properly, the FETs could cross conduct causing their destruction. It is up to you to provide complimentary PWM high side and low side inputs with adequate dead time to the IRS2186 for half bridge control. Usually using a part like a IRS2104 would be a better choice for half bridge, because it would take care of all the dead time and alignment issues for you. IRS2186 is fine, but there is just more that you have to control.

In a half bridge the source of the top FET is connected to the drain of the bottom FET. During operation the IRS2186 bootstrap cap is charged through the bootstrap diode with a return path through the bottom FET. So, to charge the bootstrap cap, the bottom FET has to be on. If the return path through the bottom FET is not made (bottom FET not turned on or something) there will be no bias for the floating driver because the bootstrap cap will never charge. Make sure that during each 32kHz switching cycle the lower FET turns on. Also, since this is a half bridge make sure that the maximum PWM duty cycle for each FET is less than 50%.

A 0.5uF cap for the bootstrap cap should be enough. You will also want a decoupling cap of at least 1uF at Vcc of IRS2186.

Schottky diodes are not usually used for bootstrap diodes, since they don't have high enough reverse voltage for the application. It is common to operate a half bridge off line with an input voltage of close to 300V. \$V_{\text{RRM}}\$ of the diode will need to be \$1.5 \left(V_{\text{cc}}+V_{\text{in}}\right)\$ . But, if your input voltage is low enough a schottky could be used. For example for \$V_{\text{cc}}\$ and \$V_{\text{in}}\$ of 15V, \$V_{\text{RRM}}\$ of 1.5(15+15) = 45V would be fine. So, in that case a MBR160GS would work.

\$V_{\text{cc}}\$ of 12V is a bit low, after subtracting about 1V for \$V_{\text{B}}\$ ripple and forward diode drop of 0.7V you're down to 10.3V which is really close to the UV lockout. It would be better to have a volt higher \$V_{\text{cc}}\$, or a larger bootstrap cap, like 1uF (less ripple), to have more margin for UV lockout.

You could check operation of the floating drive by temporarily connecting \$V_{\text{S}}\$ of the IRS2186 to return and driving HIN with 50% PWM. You wouldn't want to drive the transformer during the test, but you could see if the bootstrap voltage was good and that the FET drive worked. Of course, you will need a scope.

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  • \$\begingroup\$ This is very interesting stuff, I did not know that the low side FET had to be on for the bootstrap cap to charge. I eventually would like to use 3 of these as a brushless motor control, does the 50% limitation mean that I will only ever be able to run this thing at 50% power? Alternatively, I have been considering using a part like this: micrel.com/_PDF/mic5011.pdf, for each of the high and low side FETs. Correct me if I am wrong, but this part eliminates needing to alternate the high and low sides, no? \$\endgroup\$ – Patrick Jan 5 '13 at 20:40
  • \$\begingroup\$ The 50% limit applies to the half bridge when driving a transformer for isolation. For 3 phase BLDC motor control duty cycle could be up to 95% for IRS2186 type of driver. The micrel part has a charge pump built in to bias the drive, but these parts are quite slow (25uSec turn on), you wouldn't be able to use 32kHz PWM ... 10kHz would be about the limit. Half bridge always alternates the on time of the high and low switch, regardless of bias needs. \$\endgroup\$ – gsills Jan 5 '13 at 21:36
  • \$\begingroup\$ I see. I am a little confused about how I would implement brushless motor control if the low side of each half bridge needs to be on when the high side is off. I am using this page: blog.spingarage.com/58108473 for a reference on the pattern of signals to use, and it looks like the low sides arent always on when their respective high side is off. Does it matter which low side is on in a 3 phase bldc driver for the bootstrap caps to charge? Let me know if that question makes no sense, Im not sure if I worded it right. \$\endgroup\$ – Patrick Jan 5 '13 at 23:27
  • \$\begingroup\$ I think I've got it figured out now, sleeping on it helped. Essentially, I believe I am going to have to pulse both the power and ground on the 2 bridges that it says in that chart at the same time, and then in the off pulses, turn on the low FET of the half bridge that had its high FET on to charge the bootstrap. Also, alternating the high and low side appears to have worked. I believe I am going to go with the 2104 as well, it simplifies my code and makes the thing more robust. \$\endgroup\$ – Patrick Jan 6 '13 at 11:45
  • \$\begingroup\$ Here is a link to a good description of BLDC motor operation ( ww1.microchip.com/downloads/en/appnotes/00857a.pdf ). Strictly speaking, until the H bridges are added as commutators, the motor is AC synchronous. After commutation and control are added it becomes BLDC. \$\endgroup\$ – gsills Jan 7 '13 at 21:28

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